Ocular Pathology Study Guide

Where is the ciliary ganglion?

2006-03-30T18:01:00.000-08:00

The ciliary ganglion (arrow 4) is an important anatomic structure in the posterior orbit because sensory innervation to the anterior eye, parasympathetic axons for the iris to control pupil constriction, and sympathetic fibers pass that innervate the iris dilators and blood vessels. Located about 1 cm in front of the annulus of Zinn, the ciliary ganglion lies interposed between the optic nerve (shown in gray in the image) and lateral rectus (arrow 9) at the lateral aspect of the ophthalmic artery with its attendant innervation from the abducens nerve. Only the parasympathetic fibers are thought to synapse in the ganglion. The input to the ciliary ganglion includes the nasociliary nerve (arrowhead 6), sympathetic fibers from the internal carotid (arrow 7), and a motor root from the inferior division of the 3rd nerve (3 in the image) that eventually innervates the inferior oblique muscle (arrow 5). The superior division of the 3rd nerve (2 in the image) supplies the superior rectus and then courses around its medial border (arrow 10 shows the course of the nerve) to supply the levator palpebrae. The inferior division of the 3rd nerve also supplies the inferior rectus (arrow 11). The ciliary ganglion provides short ciliary nerves that travel on both sides of the optic nerve (arrow 8). Despite this maze of inputs, only parasympathetic fibers synapse in the ciliary ganglion. A low power photomicrograph of a section of a dissected ciliary ganglion (number 1) shows the nerve trunks (arrows 2 that emanate from the ganglion. At higher magnification one can see ganglion cells with central nuclei (arrow 3) and prominent nucleoli.

What are the landmarks of the limbus?

2006-03-21T11:14:00.000-08:00

The limbus is a transition zone where the cornea meets with elements of the sclera; both contribute. The transition zone begins at a line (the green arrow in the figure) designated by the termination of Bowman's layer and Descemet's membrane. The transition ends at a line (the red line in the figure)designated by the scleral spur drawn perpendicular to the ocular surface. The transition zone with these limits is then a donut that is about 1.5 mm in width. The surgical limbus has been stated in many books to have clear landmarks that include Bowman's layer. Since Bowman's layer is not readily visible in the slit lamp consider the limbus as the blue region overlying clear cornea and extending back to the scleral spur. The limbus is an important landmark for surgical procedures for cataract extraction, glaucoma filtration surgery etc. . Because the trabecular meshwork is encompassed in this region, the limbus has implications in the laser treatment for glaucoma.

What is the SQ5R method of studying?

2006-03-04T10:23:00.000-08:00

Human memory has an intrinsic decay rate that is staggeringly exponential. Retention of learned material from reading drops to about 20-30% after 1 hour for most people The SQ5R, SQ4RW, SQ3R are all study methods that if used properly with textbooks and readings can increase your retention to 80% on a long term basis. The basis of the method is that you are engaged in active processing of material driven by 'curiosity', the most powerful stimulus for learning.
Here is the method:
SURVEY - Before reading the actual chapter, read the introduction and summary (if given). Skim through the chapter paying attention to topic headings, bold-faced words, pictures, charts, and graphs. These can give you an idea of the general structure and content before you begin reading. This should stimulate curiosity for the material and questions will begin to arise. You might also simply jot down everything you know about the subject before you start reading and then ask what you would think is important to learn that you don't know.
QUESTION - Set a purpose for your reading by developing questions about the material. Use the topic and heading information you gathered in the survey step to create questions to be answered. Begin asking yourself who, what, where, when, why, and how questions. Questions are most beneficial when they are general, covering main topics and important points. However, remember that the purpose of the questions is to stimulate curiosity.
READ - Read to answer your questions. Some say divide the reading material into sections that will take about 20 minutes to read (often the chapter is already broken into sections which will work just fine). Read the material section by section. Look for answers to your questions, key concepts, and supporting details. Study charts, graphs, tables, and pictures. These can serve to present new information as well as tie together concepts from the reading.
RECITE - Ask your question and try to recite the answer incorporating key information and ideas. Put the material in your own words and go back and re-read until you feel comfortable with it. This may be frustrating at first, but it will lead to better understanding and save you review time in the long run. (Do this after each section.)
RECORD (WRITE) - Here I would strongly recommend putting your questions on the computer or writing them in a notebook. Then in a brief note format with abbreviations you develop, write the answers without looking at the material. Check it after to make sure it is right. Chances are this process will lead to other questions and you may wish to jot those down as well and try to answer them.
REVIEW - After completing each section review the prior sections simply by answering your questions. You will be amazed how much you retain compared to just reading a chapter or underlining. An hour later review your notes again. Review will need to be done again in one day and then in a week to maintain your 80% retention.

We have tried to promote this method in the material presented in anatomy here by asking you questions that you search for the answers in links. We have purposely added more material than the answer to be complete. The material is redundant in areas to encourage review.

For ophthalmology residents, putting the notes on your laptop is a great resource for clinic. You can link your answers to photographs that you take later, or ones available on the web. You can link to original articles and to medrounds for the diagrams and answers. Take your laptop everywhere so you can continually organize and review the material. Material that you don't wish to memorize such as specific drug therapy for relatively rare problems can be found in your notes. You can add to your information and update it at rounds and conferences. You can also keep patient photographs and information organized so you can show specialists later to help you with the diagnosis as you follow their case.

What are the layers of the tear film and their sources?

2006-03-04T09:39:00.000-08:00

The basic structure of tears includes oil (shown in yellow) floating near the air interface, aqueous mixed with proteins and soluble mucins (blue) beneath the oil, and a dense membrane bound mucin layer (the glyocalyx) on the cornea surface with its numerous microvillous processes.
(Click on figure to the left to enlarge).

The aqueous layer of tears is produced by the lacrimal glands at the upper outer of the eyelid and within the eyelid. The lacrimal gland produce proteins and aqueous. To the left is a photomicrograph of the main lacrimal gland with its lumen (1) acinar structures lined by cells with red granules (2) and an interstitium (3) containing scattered lymphocytes and plasma cells.

The oil layer of the tear film is produced by the Meibomian glands in the upper and lower eyelids. These glands are specialized sebaceous glands, composed of lobules of clear cells that contain a variety of lipids. In the sagittal section to the left one can see quite nicely the Meibomian gland of the upper tarsus all along the conjunctiva surface (6). The duct that will empty the lipid is shown adjacent to the marginal conjunctiva (4). By the way the accessory lacrimal gland of Krauss (5) is shown in the fornix (5). The soluble mucins are largely produced by specialized Goblet cells that are scattered within the conjunctiva among epithelial cells. The are most numerous in the fornix (3 in the photo to the left).
For a detailed look at conjunctival anatomy with clinical, histologic and ultrastructural photos click on this LINK. All components act in concert to provide adequate lubrication and protection to the cornea.

From the Mebomian gland the oil is secreted from the small orifices that dot the inside edge of both eyelids (Figure left, Click to Enlarge). In the slit lamp photograph a human lower eyelid shown to the left one can see the Meibomian orifices located inside the lash line. At one of the orifices there is a droplet of oil emanting from the surface. Click on the photograph to enlarge.

What are the pulleys of the recti extraocular muscles?

2006-03-04T08:28:00.000-08:00

The human orbit contains specialized musculofibroelastic tissues in and just posterior to Tenon's fascia that act as compliant pulleys and determine the pulling directions of recti EOMs. In this sense, the pulleys are the functional origins of the recti EOMs and are determinants of ocular motility. Because the muscle bellies were fixed in the orbit by MRI studies even during wide ocular excursions, it seemed apparent to Joseph Demer, who described the pulley system, that the muscles must be coupled to the orbit by a pulley system in which the pulleys are located in a position such that the insertions of the muscle are allowed to move with the eye. Magnetic resonance imaging demonstrated dense connective tissue structures within posterior Tenon's fascia near the equator of the globe adjacent to the recti EOMs. Histochemistry showed these structures to be fibroelastic EOM sleeves consisting of dense bands of collagen and elastin, suspended from the orbit. There are adjacent bands that are suspended to EOM sleeves of similar composition. Substantial smooth muscle is present in the pulley suspensions and in posterior Tenon's fascia. Tenon's fascia itself was likened to a drum head suspended at its periphery by the orbital walls.

Reference:
Demer JL, Miller JM, Poukens V, Vinters HV, Glasgow BJ. Evidence for fibromuscular pulleys of the recti extraocular muscles. Invest Ophthalmol Vis Sci. 1995 May;36(6):1125-36.

What is the blood supply and drainage for the choroid?

2006-03-03T08:03:00.000-08:00

The blood supply to the choroid comes ultimately from the ophthalmic artery (#1 in Figure). There are variations but quite posterior branches that will become the central retinal artery (#3 in Figure), and ciliary arteries on each side of the optic nerve. These vessels divide 2 long posterior ciliary arteries and about ~20 short posterior ciliary arteries that enter the eye immediately adjacent and around the optic nerve. The short posterior ciliary arteries directly supply the choroid and the long posterior ciliary arteries travel in the suprachoroidal space anteriorly (#6 in Figure) then supply the choroid anteriorly via recurrent branches. The ophthalmic artery (#4 in Figure) continues to provide branches for the posterior (#7 in Figure) and anterior (#8 in Figure) ethmoidal vessels. The superior oblique muscle is shown for orientation ( #9 in Figure).

Blood in the choroid circulates through the choriocapillaries and larger vessels of the choroid to drain into the 4-6 vortex veins (#1 in the figure). These emerge just posterior to the equator in quadrants (#2 in the figure). The superotemporal and superonasal vortex veins will drain into the superior ophthalmic vein. The inferonasal and inferotemporal (#3 in the figure) vortex veins will drain into the inferior ophthalmic vein. These vessels will eventually exit via the cavernous sinus (# 5 in the figure).
The superotemporal ophthalmic vein usually exits the eye directly adjacent to or underneath the superior oblique tendon. This has clinical implications for approaches to surgery in which the superior oblique tendon is recessed.

What are the origins, insertions, attachments, actions and blood supply of the extraocular muscles?

2006-03-02T14:06:00.000-08:00

The Four Recti Muscles
The four recti muscles arises from a short funnel-shaped tendinous ring called the annulus of Zinn (#11 in Figure). The annulus of Zinn is encloses the optic foramen and a part of the medial end of the superior orbital fissure. There are 2 tendons.
The Lower Tendon (of Zinn) is attached to the inferior root of the lesser wing of the sphenoid between the optic foramen and the superior orbital fissure (#8 in Figure). The lower tendon gives origin to part of the medial and lateral recti and all of the inferior rectus.
The Upper Tendon (of Lockwood) arises from the body of the sphenoid, and gives origin to part of the medial and lateral recti and all of the superior rectus muscle
The superior and medial recti muscles are much more closely attached to the dural sheath of the optic nerve (#10 in Figure). This fact may be responsible for the characteristic pain which accompanies extreme eye movements in retro-bulbar neuritis.

Medial Rectus (#4 in Figure): The medial rectus is the largest of the ocular muscles and stronger than the lateral.
Origin- The medial rectus muscle arises from the annulus of Zinn. It has a wide origin to the medial side of and below the optic foramen from both parts of the common tendon, and from the sheath of the optic nerve.
Insertion – The medial rectus inserts medially, in the horizontal meridian about 5.5 mm from the limbus.
Blood supply – The medial rectus is supplied by the inferior muscular branch of ophthalmic artery and 2 anterior ciliary arteries.
Size – The medial rectus muscle is 40.8 mm long; tendon is 3.7 mm long and 10.3 mm wide.
Relations– Above the medial rectus lies the superior oblique. The ophthalmic artery and its anterior and posterior ethmoidal branches and the posterior ethmoidal, anterior ethmoidal and infratrochlear nerves run between the medial rectus and superior oblique muscles. Below the medial rectus is the orbital floor. Medial to the rectus is orbital fat, separating it from the orbital plate of the ethmoid (ethmoid air cells). Laterally is the central orbital fat.
Innervation– The inferior division of the 3rd nerve innervates the medial rectus on its lateral surface at about the junction of its middle and posterior thirds.
Action. – The medial rectus is a pure adductor.

Inferior Rectus (#7 in Figure): The inferior rectus is the shortest of the recti muscles.
Origin–It arises below the optic foramen, from the middle slip of the lower common tendon of the annulus of Zinn at the apex of the orbit.
Insertion– inserted inferiorly, in vertical meridian about 6.5 mm from the limbus. The inferior rectus is also attached to the lower lid by means of the fascial expansion of its sheath.
Blood supply – the inferior muscular branch of ophthalmic artery and infraorbital artery, 2 anterior ciliary vessels
Size – 40 mm long; tendon is 5.5 mm long and 9.8 mm wide
Relations– Inferior division of the 3rd nerve lies above the muscle, and the optic nerve is separated by orbital fat, and the globe of the eye.
Lateral. – The nerve to the inferior oblique runs in front of the lateral border of the inferior rectus between it and the lateral rectus. Below is the floor of the orbit, roofing the maxillary sinus. The muscle is in contact with the orbital process of the palatine bone, but more anteriorly it is separated by orbital fat from the orbital plate of the maxilla.
Innervation– The inferior rectus is supplied by the inferior division of the 3rd nerve, which enters it on its upper aspect at about the junction of the middle and posterior thirds.
Actions – The inferior rectus makes the eye look downwards or medially or wheel-rotates it laterally (extorsion). By means of its fascial expansion it also depresses the lower lid.
The principal action is depression which increases as the eye is turned out and is nil when the eye is adducted. The inferior rectus is the only depressor in the abducted position of the eye.

Lateral Rectus (#5 in Figure):
Origin – arises from the annulus of Zinn and spans the superior orbital fissure.
Insertion – inserted laterally, in horizontal meridian 6.9 mm from the limbus
Blood supply – the lacrimal artery (the only rectus muscle with a single blood supply a common board question!)
Size – 40.6 mm long; tendon is 8 mm long and 9.2 mm wide
The lateral or external rectus arises from both the lower and upper parts of the common tendon from those portions which bridge the superior orbital (sphenoidal) fissure.
The origin is said to assume form of the letter U placed so that the opening faces the optic foramen, the limbs of the U being referred to as the upper and lower heads of the muscle.
Relations– The structures which go through the two heads of the lateral rectus, within the cone of muscles or within the annulus of Zinn, have been referred to as the oculomotor foramen.
These structures from above downwards are the upper division of the 3rd nerve, the naso-ciliary, and a branch from the sympathetic, then the lower division of the 3rd, then the 6th, and then sometimes the ophthalmic vein or veins.
The 6th nerve is actually passing from being below the lower division of the 3rd to lie lateral and in between the two divisions.
Innervation– The 6th nerve (abducens) enters it on its medial aspect, just behind its middle.
Actions– The lateral rectus is a pure abductor – that is, makes the eye look directly laterally in the horizontal plane.

Superior Rectus (#2 in Figure):
Origin – The superior rectus arises from the upper part of the annulus of Zinn above and to the lateral side of the optic foramen and from the sheath of the optic nerve. This origin lies below that of the levator, and is continuous on the medial side with the medial rectus and on the lateral with the lateral rectus.
Insertion – inserted superiorly, in vertical meridian 7.7 mm from limbus
Blood supply – Superior muscular branch of ophthalmic artery and 2 anterior ciliary a.
Size – 41.8 mm long; tendon is 5.8 mm long and 10.6 mm wide
Relations– Above the superior rectus is the levator and the frontal nerve, which separate it from the roof of the orbit. Below is the optic nerve, but separated by orbital fat, the ophthalmic artery, and the naso-ciliary nerve. Farther forwards the reflected tendon of the superior oblique passes beneath the superior rectus to reach its insertion. Laterally, in the angle between superior and lateral recti, are found the lacrimal artery and nerve.
Medially,the ophthalmic artery and naso-ciliary nerve lie in the angle between the superior rectus and the medial rectus and superior oblique muscles.
Innervation – The superior rectus is supplied by the superior division of the oculomotor (3rd cranial), which enters the under-surface of the muscle at the junction of the middle and posterior thirds.
Actions – The superior rectus makes the eye look upwards or medially or wheel-rotates it medially (intorts). It also helps the levator to lift the upper lid.

Superior Oblique (#3 in Figure): The superior oblique is the longest and thinnest eye muscle.
Origin – arises above and medial to the optic foramen by a narrow tendon which partially overlaps the origin of the levator. The origin is just outside the medial rectus and blends with the periosteum of the sphenoid bone.
Insertion – inserted to trochlea at orbital rim, then inferior and under superior rectus posterior to center of rotation
Blood supply – the superior muscular branch of ophthalmic artery supply blood
Size – 40 mm long; tendon is 20 mm long and 10.8 mm wide
The trochlea consists of a U-shaped piece of fibro-cartilage, which is closed above by fibrous tissue, and is attached to the fovea or spina trochlearis on the under-aspect of the frontal bone a few millimeters behind the orbital margin. Through the pulley the tendon is enclosed in a synovial sheath, beyond which a strong fibrous sheath accompanies the tendon to the eyeball.
Actions – The superior oblique moves the eye downwards or laterally or (wheel-) rotates it inwards (i.e. makes twelve o’clock on the cornea move towards the nose).
The principal is the depression, and this increases as the eye is adducted. The superior oblique is the only muscle which can depress in the adducted position. Its action is practically nil when the eye is abducted.
The abduction and intorsion are the subsidiary actions, and increase as the eye turns out.
The superior oblique acts with the inferior rectus to make the eye look directly down. The abductor component of the action of the oblique muscles is due to their being inserted behind the equator of the globe.
Innervation – The superior oblique is supplied by the 4th or trochlear nerve which, having divided into three or four branches, enters the muscle on the upper-surface near its lateral border; the most anterior branch at the junction of the posterior and middle thirds, the most posterior about 8 mm. from its origin.
Blood-supply comes from the superior muscular branch of the ophthalmic artery.

Inferior Oblique (#6 in Figure):
Origin – The inferior oblique is the only extrinsic muscle to take origin from the front of the orbit; arises from a rounded tendon in adepression on orbital floor near orbital rim (maxilla), ust behind the orbtial margin and lateral to orifice of the naso-lacrimal duct. Some of its fibres may, in fact, arise from the fascia covering the lacrimal sac.
Insertion – inserted posterior inferior temporal quadrant at level of macula
Blood supply – the inferior branch of ophthalmic artery and infraorbital artery
Size – 37 mm long; the shortest tendon of insertion ( essentially no tendon) and it is 9.6 mm wide at insertion.
Relations – Near its origin the lower surface of the muscle contacts the periosteum of the orbital floor, laterally it is separated from the floor by fat. Just before the insertion of the muscle, this surface which now faces laterally is covered by the lateral rectus and Tenon's capsule.
The upper aspect contacts fat, then the inferior rectus, then finally spreading out and becoming concave it moulds itself on the globe.
Innervation– the inferior division of the oculomotor nerve, crosses above the posterior border to enter the muscle on its upper-surface at about the middle of the muscle.
Blood-supply comes from the infraorbital artery and the inferior muscular branch of the ophthalmic a.
Actions– The inferior oblique makes the eye look upwards or laterally or wheel-rotates it laterally (extorts). The principal action is the elevation which increases as the eye is turned in and is nil in abduction. The inferior oblique is the only elevator in the adducted position.

Levator Palpebrae Superioris Muscle (#1 in Figure): striated muscle to elevate the eyelid.
The levator palpebrae superioris arises from the under-surface of the lesser wing of the sphenoid above and in front of the optic foramen by a short tendon which is blended with the underlying origin of the superior rectus.
The flat ribbon-like muscle belly (about 40 mm in length) passes forwards below the roof of the orbit and on the superior rectus to about 1 cm. behind the orbital septum (at the upper fornix or a few millimeters in front of the equator of the eye), where it ends in a membranous expansion or aponeurosis. From the equator forward about 10-15 mm of levator tendon spreads out in a fan-shaped manner, so as to occupy the whole breadth of the orbit and thus gives the whole muscle the rough form of an isosceles triangle.
Attachments. – (a) The main insertion of the levator is to the skin of the upper lid at and below the upper palpebral sulcus. It reaches this by intercalating with the fibres of the orbicularis.
(b) To the Tarsal Plate. – Some of the fibres of the aponeurosis are attached to the front and lower part of the tarsal plate, but the main attachment of the levator here is via the smooth superior palpebral muscle of Muller. This is continuous with the fleshy part of the levator, and is attached to the upper border of the tarsus.
Relations– Above the levator and between it and the roof of the orbit are the 4th and frontal nerves and the supraorbital vessels. The 4th nerve crosses the muscle close to its origin from lateral to medial to reach the superior oblique. The supraorbital artery is above the muscle in its anterior half only. The frontal nerve crosses the muscle obliquely from the lateral to the medial side. Below the levator is the medial part of the superior rectus.
Innervation– The Superior Division of the 3rd, which reaches the muscle either by piercing the medial edge of the superior rectus.
Action. – The levator raises the upper eyelid, thus uncovering the cornea and a portion of the sclera, and deepens the superior palpebral fold. Its antagonist is the palpebral portion of the orbicularis.

Muller's muscle- The superior palpebral muscle is a smooth muscle that acts a an eyelid elevator.
Origin- arises from the inferior or bulbar aspect of the levator palpebrae behind the fornix.
Insertion-upper edge of the tarsal plate, between the levator and conjunctiva.
Action- eyelid elevator
Size- 15-20 mm at its origin, 10 mm in vertical length, slightly sider at its insertion
Relations-lies between the tendon of the levator and the conjunctiva in the eyelid
Innervation- sympathetic fibers

What is the anatomy of the human eyelid?

2006-03-02T10:16:00.000-08:00

The human eyelids protect, nourish and sustain the cornea. The basic structure is divided arbitrarily by surgeons into 2 lamellae, anterior and posterior. It is more realistic to consider the individual structures that comprise the lids and their function. In the photograph to the left, a low magnification, to show the cornea (1) and the lens (2) for orientation. Click on the photo to enlarge it and see the structures more clearly. The inner lining of the eyelids is conjunctival epithelium, which contains a variable number of goblet cells. The goblet cells are high in density in the fornix (3) and less so near the marginal conjunctiva (4). The upper eyelids also contain accessory lacrimal glands (5) known as the glands of Krauss that are scattered over the entire measure of the lids and supply constant lubrication. They empty directly into the fornix. The eyelids get much of their form from the tarsus, a specialized fibrous layer (6) that contains numerous clear holocrine sebaceous glands called Meibomian glands. The lower lid is similar to the upper lid but notice the shortened tarsal plate. There are more Meibomian glands and ducts in the upper lid than the lower eyelid. This has implications in terms of the frequency of malignant tumors (sebaceous carcinoma is more frequent on the upper eyelid) that arise from these sites.

A pictorial rendition form Grey's anatomy is more graphic as to the numerous structures in a cross section of the eyelid even though the resolution is low. The levator aponeurosis(1 Fig left) , one of the elevators of the eyelid enters between the striated orbicularis oculi muscle and the conjunctival surface. Accessory lacrimal glands of Wolfring are shown (2 Fig left ). The meibomian glands (3 Fig left) of the tarsal plate produce the lipid that will line the layer of the tear film. The Meibomian lids empty into ducts that dot the marginal surface of the eyelid and can be seen emanating droplets of oil for the tears. The orbicularis muscle (4 Fig left ) is a striated muscle that is responsible for blinking and squeezing eyelids shut. The levator muscle is also striated but only its tendinous fibers are seen in eyelid sections. Muller's muscle, an elevator of the eyelid is composed of smooth muscle. It is located between the conjunctival epithelium and levator aponeurosis. At the base of the tarsus it can be picked up as a fold of conjunctiva clamped, and removed. This is the basis of the Mullerectomy operation to elevate the eyelid when it droops after cataract surgery. The cilia or eyelashes (6) emanate from the lid immediately adjacent to apocrine glands of Moll (7). The skin surface (8) of the eyelid is the thinnest epidermis in the body and contains hair and adnexa.

The color image of the eyelid section provides an excellent opportunity to test knowledge of the eyelid structures. Click to enlarge the photograph and then see if you have correctly identified by the structures by using the back function key and correlating your answers with the numbers below.
1- skin surface; 2 fat and fascia; 3 orbicularis oculi; 4 levator aponeurosis; 5 tarsal plate; 6 conjunctival surface; 7 Meibomian ducts; 8 glands of Moll; 9 accessory lacrimal gland of Wolfring

What is the structure of the sclera, episclera and Tenons capsule?

2006-03-02T10:00:00.000-08:00

The Fibrous Tunic (tunica fibrosa oculi).—The sclera and cornea form the fibrous tunic of the bulb of the eye; the sclera is opaque, and constitutes the posterior five-sixths of the tunic; the cornea is transparent, and forms the anterior sixth.

Tenon's capsule is a tenuous tissue layer composed of dense collagen (a fascia of sorts)that lies between the episclera and substantia propria. It extends forward from the rectus muscle insertions becoming thinner as it moves anteriorly. It is thicker in children than adults. It forms a surgical plane that in histologic sections is quite ill-defined.

The Episclera is composed of loose vascularized layers of collagen that lie immediately beneath Tenon's capsule. The collagen fibrils in the episclera are larger than they are in the substantia propria and they are circumferentially arranged.

The Sclera.—The sclera has received its name from its extreme density and hardness; it serves to maintain the form of the eye. It is much thicker behind than in front; the thickness of its posterior part at the macula is 1 mm. Its external surface is of white color, and is in contact with the inner surface of the fascia of the bulb; it is quite smooth, except at the points where the Recti and Obliqui are inserted into it. The sclera thins to 0.3 mm just behind the rectus muscle insertions and this area is extremely vulnerable to traumatic rupture. In fact this is the most common site of a ruptured globe due to blunt trauma. At the equator the sclera measures 0.4-0.5 mm in thickness. The anterior sclera is covered by the conjunctiva.
The inner surface of the sclera is brown in color and marked by grooves, in which the ciliary nerves and vessels are lodged. Behind the sclera is pierced by the optic nerve, and is continuous through the fibrous sheath of this nerve with the dura mater. Where the optic nerve passes through the sclera, the latter forms a thin the lamina cribrosa. Small holes in this lamina transmit nerve fibers of the optic nerve, and the fibrous septa dividing the lamina are continuous with the membranous processes which separate the bundles of nerve fibers. One of these openings, larger than the rest, occupies the center of the lamina; it transmits the central artery and vein of the retina. Around the entrance of the optic nerve are numerous small apertures for the transmission of the ciliary vessels and nerves, and about midway between this entrance and the sclerocorneal junction are four or five large apertures for the transmission of the vortex veins . In front, the sclera is directly continuous with the cornea, the line of union being termed the sclero-corneal junction. In the inner part of the sclera close to this junction is a circular canal, (canal of Schlemm). In a meridional section of this region this sinus presents the appearance of a cleft, the outer wall of which consists of the firm tissue of the sclera, while its inner wall is formed by a triangular mass of trabecular tissue the apex of the mass is directed forward and is continuous with the posterior elastic lamina of the cornea. The sinus is lined by endothelium and communicates externally with the anterior ciliary veins.
The sclera is formed of white fibrous tissue intermixed with fine elastic fibers; flattened connective-tissue corpuscles, some of which are pigmented, are contained in cell spaces between the fibers. The fibers are aggregated into bundles, which are arranged chiefly in a longitudinal direction. Its vessels are not numerous, the capillaries being of small size, uniting at long and wide intervals. Its nerves are derived from the ciliary nerves.

What are the pathways of the cranial nerves?

2006-03-01T19:28:00.000-08:00

The Optic Nerve (II cranial) consists chiefly of coarse fibers which arise from the ganglionic layer of the retina. They constitute the third neuron in the series composing the visual path and are supposed to convey only visual impressions. A number of fine fibers also pass in the optic nerve from the retina to the primary centers and are supposed to be concerned in the pupillary reflexes. There are in addition a few fibers which pass from the brain to the retina; they are supposed to control chemical changes in the retina and the movements of the pigment cells and cones. Each optic nerve has about 1.0 - 1.2 million fibers. In the optic chiasm, the nerves from the medial half of each retina cross to enter the opposite optic tract, while the nerves from the lateral half of each retina pass into the optic tract of the same side. The crossed fibers tend to occupy the medial side of each optic nerve, but in the chiasma and in the optic tract they are more intermingled. The optic tract is attached to the tuber cinereum and lamina terminalis and also to the cerebral peduncle as it crosses obliquely over its under surface. These are not functional connections. A small band of fibers from the medial geniculate body joins the optic tract as the latter passes over it and crosses to the opposite tract and medial geniculate body in the posterior part of the chiasma. This is the commissure of Gudden and is probably connected with the auditory system. Most of the fibers of the optic tract terminate in the lateral geniculate body, some pass through the superior brachium to the superior colliculus, and others either pass over or through the lateral geniculate body to the pulvinar of the thalamus. These end-stations are often called the primary visual centers. The lateral geniculate body consists of medium-sized pigmented nerve cells arranged in several layers by the penetrating fibers of the optic tract. Their axons pass upward beneath the longer fibers of the optic tract, the tænia semicircularis, the caudate nucleus and the posterior horn of the lateral ventricle where they join the optic radiation of Gratiolet. They pass backward and medially to terminate in the visuo-sensory cortex in the immediate neighborhood of the calcarine fissure of the occipital lobe. This center is connected with the one in the opposite side by commissural fibers which course in the optic radiation and the splenium of the corpus callosum. Association fibers connect it with other regions of the cortex of the same side. Scheme showing central connections of the optic nerves and optic tracts. The region of the pulvinar in which optic tract fibers terminate resembles in structure the lateral geniculate body. Its axons also have a similar course though in a somewhat more dorsal plane. The superior colliculus receives fibers from the optic tract through the superior brachium. Some enter by the superficial white layer (stratum zonale), others appear to dip down into the gray cap (stratum cinereum) while others probably decussate across the midline to the opposite colliculus. Other fibers from the superior brachium pass into the stratum opticum (upper gray-white layer). Some of these turn upward into the gray cap while others terminate among the cells of this layer. Since the superior colliculi appear to be the central organs concerned in the control of eye-muscle movements and eye-muscle reflexes we should expect to find them receiving fibers from other sensory paths. Many fibers pass to the superior colliculus from the medial fillet as the latter passes through the tegmentum bringing the superior colliculus into relation with the sensory fibers of the spinal cord. Fibers from the central sensory path of the trigeminal probably pass with these. Part of the ventral spinocerebellar tract (Gowers) is said to pass up through the reticular formation of the pons and mid-brain toward the superior colliculus and the thalamus. The superior colliculus is intimately connected with the central auditory path (the lateral lemniscus), as part of its fibers pass the inferior colliculus and terminate in the superior colliculus. They are probably concerned with reflex movements of the eyes depending on auditory stimuli. The superior colliculus is said to receive fibers from the stria medullaris thalamis of the opposite side which pass through the commissura habenulæ and turn back to the roof of the mid-brain, especially to the superior colliculus. By this path both the primary and cortical olfactory centers are brought into relation with the eye-muscle reflex apparatus. The fibers which pass to the nuclei of the eye muscles arise from large cells in the stratum opticum and stratum lemnisci and pass around the ventral aspect of the central gray matter where most of them cross the midline in the fountain decussation of Meynert, and then turn downward to form the ventral longitudinal bundle. This bundle runs down partly through the red nucleus, in the formatio reticularis, ventral to the posterior longitudinal bundle of the mid-brain, pons and medulla oblongata into the ventral funiculus of the spinal cord where it is known as the tectospinal fasciculus. Some of the fibers are said to pass down with the rubrospinal tract in the lateral funiculus. Some fibers do not decussate but pass down in the ventral longitudinal bundle of the same side on which they arise unless possibly they come from the opposite colliculus over the aqueduct. From the ventral longitudinal bundle collaterals are given off to the nuclei of the eye muscles, the oculomotor, the trochlear and the abducens. Many collaterals pass to the red nucleus, and are probably concerned with the reflexes of the rubrospinal tract. The fibers of the tectospinal tract end by collaterals and terminals either directly or indirectly among the motor cells in the anterior column of the spinal cord. The superior colliculus receives fibers from the visual sensory area of the occipital cortex; they pass in the optic radiation. Probably no fibers pass from the superior colliculus to the visual sensory cortex.

The Oculomotor Nerve (III cranial) contains somatic motor fibers to the Obliquus inferior, Rectus inferior, Rectus superior, Levator palpebræ superioris and Rectus medialis muscles and sympathetic efferent fibers (preganglionic fibers) to the ciliary ganglion. The postganglionic fibers connected with these supply the ciliary muscle and the sphincter of the iris. The axons arise from the nucleus of the oculomotor nerve and pass in bundles through the posterior longitudinal bundle, the tegmentum, the red nucleus and the medial margin of the substantia nigra in a series of curves and finally emerge from the oculomotor sulcus on the medial side of the cerebral peduncle.
The oculomotor nucleus lies in the gray substance of the floor of the cerebral aqueduct subjacent to the superior colliculus and extends in front of the aqueduct a short distance into the floor of the third ventricle. The inferior end is continuous with the trochlear nucleus. It is from 6 to 10 mm. in length. It is intimately related to the posterior longitudinal bundle which lies against its ventro-lateral aspect and many of its cells lie among the fibers of the posterior longitudinal bundle. The nucleus of the oculomotor nerve contains several distinct groups of cells which differ in size and appearance from each other and are supposed to send their axons each to a separate muscle. Much uncertainty still exists as to which group supplies which muscle. There are seven of these groups or nuclei on either side of the midline and one medial nucleus. The cells of the anterior nuclei are smaller and are supposed to give off the sympathetic efferent axons. The majority of fibers arise from the nucleus of the same side some, however, cross from the opposite side and are supposed to supply the Rectus medialis muscle. Since oculomotor and abducens nuclei are intimately connected by the posterior longitudinal bundle this decussation of fibers to the Medial rectus may facilitate the conjugate movements of the eyes in which the Medial and Lateral recti are especially involved. Many collaterals and terminals are given off to the oculomotor nucleus from the posterior longitudinal bundle and thus connect it with the vestibular nucleus, the trochlear and abducens nuclei and probably with other cranial nuclei. Fibers from the visual reflex center in the superior colliculus pass to the nucleus. It is also connected with the cortex of the occipital lobe of the cerebrum by fibers which pass through the optic radiation. The pathway for voluntary motor impulses is probably similar to that for the abducent nerve.

The Trochlear Nerve (IV cranial) contains somatic motor fibers only. It supplies the superior oblique muscle of the eye. Its nucleus of origin, trochlear nucleus, is a small, oval mass situated in the ventral part of the central gray matter of the cerebral aqueduct at the level of the upper part of the inferior colliculus. The axons from the nucleus pass downward in the tegmentum toward the pons, but turn abruptly dorsalward before reaching it, and pass into the superior medullary velum, in which they cross horizontally, to decussate with the nerve of the opposite side, and emerges from the surface of the velum, immediately behind the inferior colliculus. The cells of the trochlear nucleus are large, irregular and yellowish in color. The nuclei of the two sides are separated by the raphé through which dendrites extend from one nucleus to the other. They receive many collaterals and terminals from the posterior longitudinal bundle which lies on the ventral side of the nucleus. There are no branches from the fibers of the pyramidal tracts to these nuclei; the volitional pathway must be an indirect one, as is the case with other motor nuclei.

The Trigeminal Nerve (V cranial) contains somatic motor and somatic sensory fibers. The motor fibers arise in the motor nucleus of the trigeminal and pass ventro-laterally through the pons to supply the muscles of mastication. The sensory fibers arise from the unipolar cells of the semilunar ganglion; the peripheral branches of the T-shaped fibers are distributed to the face and anterior two-thirds of the head; the central fibers pass into the pons with the motor root and bifurcate into ascending and descending branches which terminate in the sensory nuclei of the trigeminal.
The motor nucleus of the trigeminal is situated in the upper part of the pons beneath the lateral angle of the fourth ventricle. It is serially homologous with the facial nucleus and the nucleus ambiguus (motor nucleus of the vagus and glossopharyngeal) which belong to the motor nuclei of the lateral somatic group. The axons arise from large pigmented multipolar cells. The motor nucleus receives reflex collaterals and terminals, (1) from the terminal nucleus of the trigeminal of the same and a few from the opposite side, via the central sensory tract (trigeminothalamic tract); (2) from the mesencephalic root of the trigeminal; (3) from the posterior longitudinal bundle; (4) and probably from fibers in the formatio reticularis. It also receives collaterals and terminals from the opposite pyramidal tract (corticopontine fibers) for voluntary movements. There is probably a connecting or association neuron interposed between these fibers and the motor neurons.
The terminal sensory nucleus consists of an enlarged upper end, the main sensory nucleus, and a long more slender descending portion which passes down through the pons and medulla to become continuous with the dorsal part of the posterior column of the gray matter especially the substantia gelatinosa of the spinal cord. This descending portion consists mainly of substantia gelatinosa and is called the nucleus of the spinal tract of the trigeminal nerve. 41 The main sensory nucleus lies lateral to the motor nucleus beneath the superior peduncle. It receives the short ascending branches of the sensory root. The descending branches which form the tractus spinalis, pass down through the pons and medulla on the lateral side of the nucleus of the tractus spinalis, in which they end by collaterals and terminals, into the spinal cord on the level of the second cervical segment. It decreases rapidly in size as it descends. At first it is located between the emergent part of the facial nerve and the vestibular nerve, then between the nucleus of the facial nerve and the inferior peduncle. Lower down in the upper part of the medulla it lies beneath the inferior peduncle and is broken up into bundles by the olivocerebellar fibers and the roots of the ninth and tenth cranial nerves. Finally it comes to the surface of the medulla under the tubercle of Rolando and continues in this position lateral to the fasciculus cuneatus as far as the upper part of the cervical region where it disappears.
The cells of the sensory nucleus are of large and medium size and send their axons into the formatio reticularis where they form a distinct bundle, the central path of the trigeminal (trigeminothalamic tract), which passes upward through the formatio reticularis and tegmentum to the ventro-lateral part of the thalamus. Most of the fibers cross to the trigeminothalamic tract of the opposite side. This tract lies dorsal to the medial fillet; approaches close to it in the tegmentum and terminates in a distinct part of the thalamus. From the thalamus impulses are conveyed to the somatic sensory area of the cortex by axons of cells in the thalamus through the internal capsule and corona radiata. Many collaterals are given off in the medulla and pass from the trigeminothalamic tract to the motor nuclei, especially to the nucleus ambiguus, the facial nucleus and the motor nucleus of the trigeminal. 43 The somatic sensory fibers of the vagus, the glossopharyngeal and the facial nerves probably end in the nucleus of the descending tract of the trigeminal and their cortical impulses are probably carried up in the central sensory path of the trigeminal. 44 The mesencephalic root (descending root of the trigeminal) arises from unipolar cells arranged in scattered groups in a column at the lateral edge of the central gray matter surrounding the upper end of the fourth ventricle and the cerebral aqueduct. They have usually been considered as motor fibers that join the motor root, but Johnston claims that they join the sensory root of the trigeminal, that they develop in the alar, not in the basal lamina, and that the pear-shaped unipolar cells are sensory in type.

The Abducens Nerve (VI cranial) contains somatic motor fibers only which supply the lateral rectus muscle of the eye. The fibers arise from the nucleus of the abducens nerve and pass ventrally through the formatio reticularis of the pons to emerge in the transverse groove between the caudal edge of the pons and the pyramid. The nucleus is serially homologous with the nuclei of the trochlear and oculomotor above and with the hypoglossal and medial part of the anterior column of the spinal cord below. It is situated close to the floor of the fourth ventricle, just above the level of the striæ medullares. Voluntary impulses from the cerebral cortex are conducted by the pyramidal tract fibers (corticopontine fibers). These fibers probably terminate in relation with association neurons which control the coördinated action of all the eye muscles. This association and coördination mechanism is interposed between the terminals and collaterals of the voluntary fibers and the neurons within the nuclei of origin of the motor fibers to the eye muscles. The fibers of the posterior longitudinal bundle are supposed to play an important role in the coördination of the movements of the eyeball. Whether it is concerned only with coördinations between the vestibular apparatus and the eye or with more extensive coördinations is unknown. Many fibers of the posterior longitudinal bundle have their origin in the terminal nuclei of the vestibular nerve and from the posterior longitudinal bundle many collaterals and terminals are given off to the abducent nucleus as well as to the trochlear and oculomotor nuclei. The abducens nucleus probably receives collaterals and terminals from the ventral longitudinal bundle (tectospinal fasciculus); fibers which have their origin in the superior colliculus, the primary visual center, and are concerned with visual reflexes. Others probably come from the reflex auditory center in the inferior colliculus and from other sensory nuclei of the brain-stem.

The Facial Nerve (VII cranial) consists of somatic sensory, sympathetic afferent, taste, somatic motor and sympathetic efferent fibers. The afferent or sensory fibers arise from cells in the geniculate ganglion. This portion of the nerve is often described as the nervus intermedius.

(1) The somatic sensory fibers are few in number and convey sensory impulses from the middle ear region. Their existence has not been fully confirmed. Their central termination is likewise uncertain, it is possible that they join the spinal tract of the trigeminal as do the somatic sensory fibers of the vagus and glossopharyngeal.

(2) The sympathetic afferent fibers are likewise few in number and of unknown termination.

(3) Taste fibers convey impulses from the anterior two-thirds of the tongue via the chorda tympani. They are supposed to join the tractus solitarius and terminate in its nucleus. The central connections of this nucleus have already been considered.
(4) Somatic motor fibers, supplying the muscles derived from the hyoid arch, arise from the large multipolar cells of the nucleus of the facial nerve. This nucleus is serially homologous with the nucleus ambiguus and lateral part of the anterior column of the spinal cord. Voluntary impulses from the cerebral cortex are conveyed by terminals and collaterals of the pyramidal tract of the opposite side, indirectly, that is with the interpolation of a connecting neuron, to the facial nucleus. This nucleus undoubtedly receives many reflex fibers from various sources, i. e., from the superior colliculus via the ventral longitudinal bundle (tectospinal fasciculus) for optic reflexes; from the inferior colliculus via the auditory reflex path; and indirectly from the terminal sensory nuclei of the brain-stem. Through the posterior longitudinal bundle it is intimately connected with other motor nuclei of the brain-stem.
(5) Sympathetic efferent fibers (preganglionic fibers) arise according to some authors from the small cells of the facial nucleus, or according to others from a special nucleus of cells scattered in the reticular formation, dorso-medial to the facial nucleus. This is sometimes called the superior salivatory nucleus. These preganglionic fibers are distributed partly via the chorda tympani and lingual nerves to the submaxillary ganglion, thence by postganglionic (vasodilator) fibers to the submaxillary and sublingual glands. Some of the preganglionic fibers pass to the sphenopalatine ganglion via the great superficial petrosal nerve.

What are the contents of orbital foramina and fissures?

2006-03-01T18:59:00.000-08:00

Identify the contents of the superior orbital fissure, inferior orbital fissure, and optic canal and the muscles surrounding these structures. Check the numbers below.
Superior Orbital Fissure
-lacrimal (1)
-frontal of V1 (2)
-cranial nerve IV (3)
-cranial nerve III (superior div) (4)
-nasociliary branch of cranial nerve V (5)
-sympathetic roots of ciliary ganglion (arrow)
-cranial nerve VI (6)
-cranial nerve III inferior div. (7)
-superior ophthalmic vein (8)

Inferior Orbital Fissure
-maxillary (9)
-pterygopalantine ganglion nerve (10)
-pterygoid nerves (11)
-inferior ophthalmic vein (12)
The inferior ophthalmic vein's exit from the orbit may be more complicated than show here. [See the link]

Optic Canal
-Optic nerve (13)
-Ophthalmic Artery (14)
-Sympathetic fibers from carotid plexus (14a)

Muscles:
-Levator (15)
-Superior rectus (16)
-Superior Oblique (17)
-Medial rectus (18)
-Inferior rectus (19)
-Lateral rectus (20)

Supraorbital foramen- supraorbital nerve (CN V1)

Anterior ethmoidal foramen- anterior ethmoidal vessels and nerves

Posterior ethmoidal foramen- posterior ethmoidal nerves

Zygomatic foramen- zygomaticofacial and zygomaticotemporal branches of the zygomatic nerve and artery.

Nasolacrimal duct - drainage apparatus from the lacrimal fossa to the inferior meatus of the nose

What are the paranasal sinuses?

2006-03-01T18:56:00.000-08:00

The paraorbital sinuses include the frontal sinus above the orbit (orange), the ethmoid sinuses (green) in the medial wall of the orbit, the sphenoid sinus (blue) that is posteriorly located and the maxillary sinus (purple-pink).
Below, the sinuses are shown in a sagittal view. The maxillary sinus (red-pink) and the ethmoid sinuses would not be visible in the midline plane that shows the drainage from the sinuses (arrows). It is evident that the maxillary sinus is close to the teeth. As such pain from the sinus may be referred to teeth. This occurs in maxillary sinus infections. Occasionally normal teeth have been removed from patients with life threatening fungal sinus infections because this source of referred pain was unrecognized.

Which orbital bones are prone to infection and fracture?

2006-03-01T16:54:00.000-08:00

The medial wall of the orbit is in areas paper thin (0.2-0.4 mm in thickness). This area over the ethmoidal air cells (ethmoid bone shown in green #3 ) and green in the figure below is prone to infection and blow out fractures. Interestingly, the honeycomb structure of the ethmoid air cells seems to provide more protection than would be expected because the floor of the orbit, although thicker, is much more prone to the blow-out fracture. The blow-out fracture can occur by a mechanism in which intraorbital pressure is increased from blunt trauma. The floor of the orbit is composed of the maxillary bone, part of the zygoma and the palantine bone. In a blow out fracture the contents of the inferior orbital fissure are susceptible to entrapment and injury. The medial wall of the orbit at the lesser wing of the sphenoid is the thickest area; forming the optic canal (nerve).

What are the bones of the orbit?

2006-03-01T16:48:00.000-08:00

Seven bones make up the orbit. Click on the photograph to enlarge and test yourself by naming the numbered bones. The roof of the orbit is composed of 2 bones, the frontal bone and the sphenoid bone. The frontal bone (#1 in blue) comprises the anterior part of the roof of the orbit and the lesser wing of the sphenoid (#2 in tan) surrounds the optic canal and forms the posterior part of the roof.

The medial wall of the orbit is composed of 4 bones: sphenoid, ethmoid, lacrimal and maxillary bone. The lesser wing of the sphenoid (#2 in tan) is most posterior and is joined to the ethmoid bone (#3 in dark green), moving anteriorly to the lacrimal bone (#4 in light red) and then to the maxillary bone (#5 in light green).

The floor of the orbit is composed of 3 bones: maxillary bone (#5 light green); zygoma (#6 in pink) and posteriorly, the palantine bone (#8 in bright red). The palantine bone borders on the inferior orbital fissure, which narrows posteriorly. In radiologic studies the medial direction and narrowing are key identifiers for this fissure.

The lateral wall of the orbit is composed of the zygoma(#6 in pink) and the greater wing of the sphenoid (#7 in tan).

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Lacrimal Excretory System- Human

2006-02-13T15:46:00.000-08:00

Tears are produced in the lacrimal gland (#1 in diagram to the left)exit the ocular surface via the puncta at the medial portion of the eyelids (#2 Diagram left). Each punctum is a small, round, or transversely oval aperture situated on a slight elevation. The puncta can be seen to be roughly in line with the openings of the Meibomian glands, the nearest of which is only 0.5 to 1 mm away.
Each puncta empties into a canaliculus (Diagram left #3)which has at first a vertical and then a horizontal portion.
In the photo below, the puncta and canaliculi are lined by stratified squamous epithelium (
Photomicrograph below #'s 1 and 2). Click to enlarge the photograph. The puncta are positioned between conjunctiva (Photo below 3) and skin (Photo below #4) at the border of the eyelid.

The lacrimal sac is placed in the lacrimal fossa (formed by the lacrimal bone and the frontal process of the maxilla) which lies in the anterior part of the medial wall of the orbit. The sac is closed above(Diagram above #4) and open below, where it is continuous with the naso-lacrimal duct (Diagram above #5). The lacrimal sac and duct are both lined by two layers of epithelium, the superficial of which is columnar, the deeper flattened. The bases of the columnar cells pass through the deeper layer to reach the basement membrane. Unlike the pseudostratied columnar respiratory epithelium of the sinuses and nose, the superficial layer is not ciliated, but contains goblet cells and mucous glands.

Lacrimal Gland Histology

2006-02-13T11:22:00.000-08:00

Photomicrograph (above) shows a low power image of an exenteration specimen that has the eye intact with the adnexal structures. The lobules of the orbital portion of the lacrimal gland (1) are near the orbital septum but lie under the levator muscle (4). The fornix of the upper eyelid (2) lies immediately adjacent to the accessory lacrimal gland of Krause(3). The glands of Krause are accessory lacrimal glands having the same structure as the main gland. They are placed deeply in the subconjunctival connective tissue (mainly) of the upper fornix between the tarsus and the inferior lacrimal gland, of which they are offshoots. There are some 42 in the upper and 6 to 8 in the lower fornix. They are thus found largely on the lateral side. Their ducts unite into a rather long duct or sinus which opens into the fornix. Similar glands are found in the caruncle. The Glands of Wolfring or Ciaccio are also accessory lacrimal glands, but larger than the glands of Krause. There are 2 to 5 in the upper lid situated actually in the upper border of the tarsus about its middle between the extremities of the tarsal glands or just above the tarsus. The excretory duct is large and short and lined by a basal layer of cubical cells and a superficial layer of cylindrical cells like the conjunctiva on which it opens. The sclera (5), ciliary body(6) and iris (7) are all identifiable.

Photomicrograph (above) shows a higher magnfication of a human lacrimal gland complete with acinar structures that contain lumens (1) and protein rich acinar cells that secrete lysozyme, tear lipocalin, lactoferrin and IgA. The reddish granules are secretory vesicles replete with protein. Some lumens are filled with prtotein that is being secreted. Lymphocytes and plasma cells are scattered in the interstitium.
The lacrimal gland situated above and lateral to the eye in the orbit secretes the tears and into ducts in upper fornix. The lacrimal gland and its tears exist in animals which live in air. Fish do not have lacrimal gland. The lacrimal gland consists of an orbital or superior portion; and a small palpebral or inferior portion; which are continuous. The orbital portion is lodged in its fossa on the anterior and lateral part of the roof of the orbit. It is shaped like an almond.
The lacrimal gland consists of a lobules and is a tubulo-racemose gland with short branched gland tubules somewhat similar to the parotid. The acini consist of two layers of cells placed on a thin hyaline basement membrane and surrounding a central lumen. The basal cells are myoepithelial in character while the acinar cells are cylindrical, and secrete fluid into a series of ducts of increasing size until becoming the excretory duct.

Projection of the eye onto the brain

2005-11-22T08:52:00.000-08:00

The sensory homunculus is the topographical representation of body parts on the brain. In the figure one can see that the hands have a disproportionately large representation compare to the eyes with respect to sensation but the perception of vision of course is in area 17 of the calcarine cortex.

The overview of the visual pathway is shown in the figure below. Axons from the ganglion cells in the retina)( number 1) pass through the optic chiasm (arrow 2). The temporal fibers from these axons are uncrossed and project to the lateral geniculate body in the thalamus (3). The fibers sweep downward and laterally (shown in fainter blue so that you don't think the optic tract enters the pons) into the temporal lobe (4) and eventually ascend and travel more medially to synapse in area 17 or the calcarine cortex. Macular fibers are represented more posteriorly in the calcarine cortex whereas nasal fibers are projected more anteriorly. The individual fibers can be related to the visual field and its representation on the calcarine cortex. In the figure it is evident that the projection from nasal retinal fibers shown in yellow project to the contralateral anterior calcarine cortex with synapses in the lateral geniculate body. The left side of the brain views the right visual field and the images are projected as upright compared to the visual field.

Interruption of the visual pathway results in defects in vision that are location specific. Ophthalmologists will need to know the anatomic correlate of each pattern in the visual field (see link).

Sensory Pathway of V.

2005-11-20T06:59:00.000-08:00

Trace the pathway of sensory input from the iris or cornea to the brain. In summary sensory nerves from the cornea and iris travel as the long posterior ciliary nerves (combined of course with sympathetic fibers) exit the eye posteriorly and pass thru the ciliary ganglion to join the nasociliary nerve as the long sensory root. The nasociliary nerve exits the orbit thru the superior orbital fissure and enters the cavernous sinus lateral to the internal carotid artery. The nerve has its cell body in trigeminal ganglion (also called semilunar or Gasserian ganglion) which is present in Meckel's cave. From the the trigeminal ganglion the fibers enter the pons and descend in the ipsilateral spinal trigeminal tract, synapsing in the most ventral portion (synapse 1). The fibers ascend and cross as the trigeminothalamic tract to the ventroposteromedial nucleus of the thalamus (2nd synapse) and onward to the internal capsule to the postcentral gyrus of somatosensory cortex (3rd synapse).

Hutchinson's sign is the accompaniment of herpetic lesions on the tip of the nose with corneal involvement. This is possible because both are branches of V1, the nasociliary nerve. The nasociliary nerve fibers enters the nose through branches that pass through the anterior ethmoidal foramen across the cribriform plate of the ethmoid bone, through a slit at the side of the crista galli, into the nasal cavity. The nasociliary nerve fibers supply internal nasal branches to the mucous membrane of the front part of the septum, lateral wall of the nasal cavity and as the external nasal branch, supplies the skin of the ala and apex of the nose.

The detailed explanation for the entire Cranial Nerve V is given from Gray's Anatomy as seen below with stunning diagrams in links.

Sensory Root.—The fibers of the sensory root arise from the cells of the semilunar ganglion which lies in a cavity of the dura mater near the apex of the petrous part of the temporal bone. They pass backward below the superior petrosal sinus and tentorium cerebelli, and, entering the pons, divide into upper and lower roots. The upper root ends partly in a nucleus which is situated in the pons lateral to the lower motor nucleus, and partly in the locus cæruleus; the lower root descends through the pons and medulla oblongata, and ends in the upper part of the substantia gelatinosa of Rolando. This lower root is sometimes named the spinal root of the nerve. Medullation of the fibers of the sensory root begins about the fifth month of fetal life, but the whole of its fibers are not medullated until the third month after birth.
4
The Semilunar Ganglion (ganglion semilunare [Gasseri]; Gasserian ganglion) occupies a cavity (cavum Meckelii) in the dura mater covering the trigeminal impression near the apex of the petrous part of the temporal bone. It is somewhat crescentic in shape, with its convexity directed forward: medially, it is in relation with the internal carotid artery and the posterior part of the cavernous sinus. The motor root runs in front of and medial to the sensory root, and passes beneath the ganglion; it leaves the skull through the foramen ovale, and, immediately below this foramen, joins the mandibular nerve. The greater superficial petrosal nerve lies also underneath the ganglion.
5
The ganglion receives, on its medial side, filaments from the carotid plexus of the sympathetic. It give off minute branches to the tentorium cerebelli, and to the dura mater in the middle fossa of the cranium. From its convex border, which is directed forward and lateralward, three large nerves proceed, viz., the ophthalmic, maxillary, and mandibular. The ophthalmic and maxillary consist exclusively of sensory fibers; the mandibular is joined outside the cranium by the motor root.
6
Associated with the three divisions of the trigeminal nerve are four small ganglia. The ciliary ganglion is connected with the ophthalmic nerve; the sphenopalatine ganglion with the maxillary nerve; and the otic and submaxillary ganglia with the mandibular nerve. All four receive sensory filaments from the trigeminal, and motor and sympathetic filaments from various sources; these filaments are called the roots of the ganglia.
7
The Ophthalmic Nerve (n. ophthalmicus) (Figs. 776, 777), or first division of the trigeminal, is a sensory nerve. It supplies branches to the cornea, ciliary body, and iris; to the lacrimal gland and conjunctiva; to the part of the mucous membrane of the nasal cavity; and to the skin of the eyelids, eyebrow, forehead, and nose. It is the smallest of the three divisions of the trigeminal, and arises from the upper part of the semilunar ganglion as a short, flattened band, about 2.5 cm. long, which passes forward along the lateral wall of the cavernous sinus, below the oculomotor and trochlear nerves; just before entering the orbit, through the superior orbital fissure, it divides into three branches, lacrimal, frontal, and nasociliary.
8
The ophthalmic nerve is joined by filaments from the cavernous plexus of the sympathetic, and communicates with the oculomotor, trochlear, and abducent nerves; it gives off a recurrent filament which passes between the layers of the tentorium.
9

FIG. 777– Nerves of the orbit, and the ciliary ganglion. Side view. (See enlarged image)

The Lacrimal Nerve (n. lacrimalis) is the smallest of the three branches of the ophthalmic. It sometimes receives a filament from the trochlear nerve, but this is possibly derived from the branch which goes from the ophthalmic to the trochlear nerve. It passes forward in a separate tube of dura mater, and enters the orbit through the narrowest part of the superior orbital fissure. In the orbit it runs along the upper border of the Rectus lateralis, with the lacrimal artery, and communicates with the zygomatic branch of the maxillary nerve. It enters the lacrimal gland and gives off several filaments, which supply the gland and the conjunctiva. Finally it pierces the orbital septum, and ends in the skin of the upper eyelid, joining with filaments of the facial nerve. The lacrimal nerve is occasionally absent, and its place is then taken by the zygomaticotemporal branch of the maxillary. Sometimes the latter branch is absent, and a continuation of the lacrimal is substituted for it.
10
The Frontal Nerve (n. frontalis) is the largest branch of the ophthalmic, and may be regarded, both from its size and direction, as the continuation of the nerve. It enters the orbit through the superior orbital fissure, and runs forward between the Levator palpebræ superioris and the periosteum. Midway between the apex and base of the orbit it divides into two branches, supratrochlear and supraorbital.
11
The supratrochlear nerve (n. supratrochlearis), the smaller of the two, passes above the pulley of the Obliquus superior, and gives off a descending filament, to join the infratrochlear branch of the nasociliary nerve. It then escapes from the orbit between the pulley of the Obliquus superior and the supraorbital foramen, curves up on to the forehead close to the bone, ascends beneath the Corrugator and Frontalis, and dividing into branches which pierce these muscles, it supplies the skin of the lower part of the forehead close to the middle line and sends filaments to the conjunctiva and skin of the upper eyelid.
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The supraorbital nerve (n. supraorbitalis) passes through the supraorbital foramen, and gives off, in this situation, palpebral filaments to the upper eyelid. It then ascends upon the forehead, and ends in two branches, a medial and a lateral, which supply the integument of the scalp, reaching nearly as far back as the lambdoidal suture; they are at first situated beneath the Frontalis, the medial branch perforating the muscle, the lateral branch the galea aponeurotica. Both branches supply small twigs to the pericranium.
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The Nasociliary Nerve (n. nasociliaris; nasal nerve) is intermediate in size between the frontal and lacrimal, and is more deeply placed. It enters the orbit between the two heads of the Rectus lateralis, and between the superior and inferior rami of the oculomotor nerve. It passes across the optic nerve and runs obliquely beneath the Rectus superior and Obliquus superior, to the medial wall of the orbital cavity. Here it passes through the anterior ethmoidal foramen, and, entering the cavity of the cranium, traverses a shallow groove on the lateral margin of the front part of the cribriform plate of the ethmoid bone, and runs down, through a slit at the side of the crista galli, into the nasal cavity. It supplies internal nasal branches to the mucous membrane of the front part of the septum and lateral wall of the nasal cavity. Finally, it emerges, as the external nasal branch, between the lower border of the nasal bone and the lateral nasal cartilage, and, passing down beneath the Nasalis muscle, supplies the skin of the ala and apex of the nose.
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The nasociliary nerve gives off the following branches, viz.: the long root of the ciliary ganglion, the long ciliary, and the ethmoidal nerves.
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The long root of the ciliary ganglion (radix longa ganglii ciliaris) usually arises from the nasociliary between the two heads of the Rectus lateralis. It passes forward on the lateral side of the optic nerve, and enters the postero-superior angle of the ciliary ganglion; it is sometimes joined by a filament from the cavernous plexus of the sympathetic, or from the superior ramus of the trochlear nerve.
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The long ciliary nerves (nn. ciliares longi), two or three in number, are given off from the nasociliary, as it crosses the optic nerve. They accompany the short ciliary nerves from the ciliary ganglion, pierce the posterior part of the sclera, and running forward between it and the choroid, are distributed to the iris and cornea. The long ciliary nerves are supposed to contain sympathetic fibers from the superior cervical ganglion to the Dilator pupillæ muscle.
17
The infratrochlear nerve (n. infratrochlearis) is given off from the nasociliary just before it enters the anterior ethmoidal foramen. It runs forward along the upper border of the Rectus medialis, and is joined, near the pulley of the Obliquus superior, by a filament from the supratrochlear nerve. It then passes to the medial angle of the eye, and supplies the skin of the eyelids and side of the nose, the conjunctiva, lacrimal sac, and caruncula lacrimalis.

Pupillary Reflex Pathway

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The pupillary reflex can be thought of as an afferent limb, a parasympathetic efferent limb, and a sympathetic efferent limb. A summary is shown in this interactive link.
Afferent limb- Light stimulates photoreceptors and the signal is conveyed to a special set of ganglion cells that send their nerve impulses thru the axons in a similar topographic distribution as those carrying other signals in the optic nerve (arrow 1 in the figure, please enlarge). Decussation occurs at the optic chiasm (arrow 2 in the figure) for the nasal fibers (arrow 4) . The big difference is that the afferent fibers do not enter the lateral geniculate body but instead exit and pass thru the brachium of the superior colliculus (arrow 5) where they synapse on the pretectal olivary nuclei (arrow 7) (pontine olivary and sublentiform nuclei). These nuclei project bilaterally to the Edinger Westphal nuclei (arrows 6 and 7) via internuncial neurons by a process that has not been fully elucidated.

Efferent parasympathetic response- The Edinger Westphal nuclei send fibers to join the oculomotor CNIII (arrow 8) and follow that course on the dorsomedial surface of the nerve (arrow 9). This is important because this is an outer surface which lies adjacent to the posterior communicating artery and is exposed to forces of herniation. After coursing thru the cavernous sinus these fibers emerge to enter the orbit with the inferior oblique branch of CNIII. These fibers synapse at the ciliary ganglion (arrow 10) and then enter the eye thru short posterior ciliary nerves to distribute fibers to the choroid, iris (arrow 11), and ciliary body. The red nucleus is shown in red and substantia nigra in black. The medial geniculate body is dark gray and medial to the brachium of the superior colliculus. The lateral geniculate body where the fibers responsible for vision will synapse lie lateral to the pupillary fiber pathway in another plane of section.

Efferent sympathetic response- This is believed to start in the hypothalamus and project in an uncrossed fashion with synapses in the mesencephalon and pons. These neurons project to and synapse upon the intermediolateral cell column from C8-T2 in the spinal cord. These exit the spinal cord and pass thru the stellate ganglion to synapse in the superior cervical ganglion. These fibers travel with the internal carotid artery, enter the cavernous sinus and travel with CN VI in the cavernous sinus to enter the superior orbital fissue with cranial nerve V. The fibers travel with the nasociliary branch of V, and pass thru the ciliary ganglion without synapsing. The fibers pass thru the long ciliary nerves to terminate on the dilator muscle. Some fibers diverge in the superior orbital fissue to innervate Muller's muscle.

These pathways are important for the clinician to understand the basis for a blown pupil with a third nerve palsy. The pupillary fibers are compressed and this is most likely due to an aneurysm that affects the 3rd nerve at the posterior communicating artery.

Reference: Kourouyan and Horton J Comp Neurol. 1997 Apr 28;381(1):68-80.

Cranial Nerve IV- Trochlear Nerve

2005-11-13T13:27:00.000-08:00

The trochlear nerve (IV cranial) innervates the superior oblique muscle and contains only somatic motor fibers. CN IV originates from the trochlear nucleus, which is a small, oval mass (yellow in the pseudocolored section below) situated in the ventral part of the central gray matter of the cerebral aqueduct at the level of the upper part of the inferior colliculus. In the figure, the cerebral aqueduct is near the top which is the posterior part of the section.
The cells of the trochlear nucleus are large, irregular and yellowish in color. The nuclei of the two sides are separated by the raphé through which dendrites extend from one nucleus to the other. They receive many collaterals and terminals from the posterior longitudinal bundle which lies on the ventral side of the nucleus.
The nucleus then is located at the level of the pons but posteriorly. The axons from the nucleus pass downward in the tegmentum toward the pons, but turn abruptly dorsalward before reaching it, and pass around the cerebral aqueduct into the superior medullary velum, in which they cross horizontally, to decussate with the nerve of the opposite side, and emerges from the surface of the velum, immediately beneath the inferior colliculus. The nerve descends to the base of the brainstem, crosses the superior cerebellar peduncle and turns forward against the cerebral peduncle. The nerve passes between the posterior cerebral and superior cerebellar arteries but more lateral than the third cranial nerve (see Figure). It pierces the dura in the free border of the tentorium cerebelli just behind the posterior clinoid process and passes forward to the cavernous sinus. The course of this nerve in the cranium is long~about 75 mm .
CN IV travels in the lateral wall of the cavernous sinus. The nerve is initially below the 3rd cranial nerve but then crosses the oculomotor nerve and enters the orbit through the superior orbital fissure as the most superior of all nerves entering the orbit. In the orbit the trochlear nerve is medial to the frontal nerve. CN IV crosses over the levator muscle and enters the superior lateral border of the superior oblique muscle in its posterior third where it provides innervation (arrow 1).
The function of the superior oblique muscle is to rotate and depress the eye. Paralysis of the trochlear nerve causes vertical diplopia which is increased when the eye is directed downward and inward such as when going down stairs. The patient compensates by tilting the head to the side opposite the paresis. This relieves them of the need to move the eye downward in the field of action of the paretic muscle. On examination there is a hyperdeviation that increases on contralateral gaze and with ipsilateral head tilt. Congenital fourth nerve palsies may be evident with a head tilt seen in old photographs. The most common cause of an acquired 4th nerve palsy is head trauma. Other causes of 4th nerve palsy include tumors of the pineal gland, A-V malformation, demyelination, meningitis, carotid-cavernous fistula, and iatrogenic causes (neurosurgical cut at the edge of the tentorium).

Optic Canal, Optic Chiasm

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The opening to the optic canal is the optic foramen at the apex of the orbit in the less wing of the sphenoid bone. From its location at the junction of the roof and medial wall of the orbit the optic canal is directed medially, upward moving posteriorly to the middle cranial fossa. This direction is useful to distinguish the optic canal from the superior orbital fissure on CT scan. The optic canal is on the same horizontal plane as the upper portion of the superior orbital fissure so they will often be seen in sections of the CT scan together. The optic canal houses the optic nerve, ophthalmic artery and sympathetic fibers from the cartoid plexus. A line drawn from the top of the nose to the auditory canal will approximately form the inferior border of the optic foramen. The dura of the optic nerve merges with the periosteum as the nerve enters the optic canal. The optic canal measures about 12 mm in length and about 7 mm in width but there are quite a few variations (Akdemir et al).

Upon exiting the optic canal, the optic nerve now lies medial to the internal carotid artery as seen in the picture below. Identify the structures in the photograph: arrow 1. optic nerve
2. superior oblique muscle transected and removed 3. superior rectus muscle transected and removed
4. ophthalmic artery
5. posterior ciliary arteries
6. posterior ethmoidal artery
7. anterior ethmoidal artery
8. supraorbital artery
9. lacrimal gland
10. ophthamic-internal carotid
As th optic nerve exits the orbit, it lies above the ophthalmic arteries but below the anterior cerebral arteries and anterior communicating arteries, which join to complete the anterior circle of Willis (see figure). The optic nerve passes over the cavernous sinus extending medially to join the fellow optic nerve and form the optic chiasm. The optic nerve and chiasm is therefore medial and superior to the cavernous sinuses. The optic chiasm lies anterior to the pituitary gland stalk (see figure link). These anatomic relationships become important when discussing the effect of aneurysms and tumors on the visual pathway.

The general and simplistic projection of visual fields in relation to the organization of optic nerve fibers as nasal fibers (temporal fields) cross through the optic chiasm. Therefore the right visual field is projected to the left side of the brain (see link). However, there are some important details of these anatomic relationships that the ophthalmologist needs to remember:
1. extramacular nasal and inferior retinal fibers (superotemporal fields) cross in the anterior portion of the chiasm (Wilbrand's knee red fiber in Figure). Hence the relationship to pituitary tumors.
2. Nasal macular fibers cross in the posterior part of the chiasm.
3. Temporal fibers remain uncrossed.
4. Macular projections are located centrally in the nerve and chiasm and account for most of the fibers!!!