BIOL 347 General Physiology Lab
The Exterior of the Globe:
In dissecting the eye, it is important to note the aspect in which the structures are located (anterior, posterior, superior, inferior, lateral, medial etc). Examine the external appearance of the eye. Some of the structures, which may remain intact on the globe, are the extrinsic muscles, orbital fat root of the optic nerve, the eyelids, (including the third eyelid known as the nictitating membrane) and the conjunctiva. Locate the positions of the eye with reference to the six aforementioned aspects. Some clues, which may help in this identification, are the shape of the pupil or cornea, the position of the optic nerve and the location of the nictitating membrane. The pupil, when contracted, and the cornea are pear shaped, with the broader end situated medially or nasally. On the posterior aspect of the globe, the root of the optic nerve is located somewhat ventral to the medial horizontal plane and curves nasally. The nicitating membrane is located in the medial and ventral quadrant when viewed from the front. In the human, a vestige of the nicitating membrane (the plica semilunaris) can be seen in the medial corner of the eye, situated behind the pink pad of tissue (the caruncle).
The specific extrinsic muscles need not be identified by name on your specimen, since some of them have been torn away at the insertion. However, their functions will be briefly described. There are six muscles concerned with the orientation of the eye in space: 1.) the superior rectus, 2.) the inferior rectus, 3.) the lateral rectus, 4.) the medial rectus, 5.) the superior oblique and 6.) the inferior oblique. The first two muscles rotate the eye upward and downward, the second two muscles rotate the eye nasally and temporally, and the third two are responsible for tilting ht eye. Of course, any rotation of the eye is due to an intricate coordination of all of these muscles responding simultaneously. Three cranial nerves innervate these muscles- the oculomotor, trochlear and abducens. There are two other muscles attached to the eye. The levator is located above ht superior rectus, which upon contraction raises the upper eyelid. Both the levator and the superior rectus are served by the oculomotor nerve and they act conjointly. Thus when the superior rectus contracts, rotating the eye upwards, the levator also contracts to withdraw the upper eyelid from the field of vision. Rhythmic inhibition of the levator causes blinking. The other aforementioned muscle is the retractor surrounding the optic nerve and inserted around the equator of the globe (the equator is defined with reference to the anterior and posterior poles). The retractor, which is absent in humans, pulls the entire eye deeper within the orbit. As a protective mechanism this action as results in the passive partial closure of the nictitating membrane over the sclera and cornea. The third eyelid has no muscle of its own, but is served indirectly by the retractor. The abducens nerve innervates the retractor. The lower eyelid has no separate muscle either, but a branch of the inferior rectus is attached to it, providing some degree of movement.
The yellowish orbital fat in which these muscles are embedded functions as a cushion absorbing any mechanical shocks to the eye and provides support to the globe within the orbit.
Around the root of the optical nerve is the dural sheath. This sheath is continuous with the dura matter of the brain, consistent with the embryological development of the optic nerve and retina as an extension of the central nervous system.
The conjunctiva is a delicate, folded membrane arising at the margin of the cornea and sclera (called the limbus region), covering the sclera for a short distance before folding backward to line the inner surface of the eyelids. The conjunctiva secretes a watery fluid lubricates the area between the eyelids and the cornea.
Now with the aid of a scalpel, scrape away the extrinsic muscles, orbital fat and the eyelids, but preserve the root of the optic nerve. These structures as well as any others surrounding the eye arte collectively termed the ocular adnexa. After the adnexa is removed, the sclera will be completely exposed over the entire surface of the globe. With a light pressure applies by the fingers and thumb, note any differences in scleral thickness at different points on the globe. Is the globe spherical or is one axis relatively short? Note any pigmentation at the corneal margin and examine the corneal surface for abrasions due to the loss of epithelium. If abrasions are not present, make a few by scraping with the scalpel. If the cornea is not too cloudy, the position and shape of the pupil should be noted. How does the cow pupil differ from the human pupil? The innervation of the cornea is standard in most mammals. Fine corneal nerve fibers emanate from a pericorneal plexus deriving from the ciliary nerves. The latter are both sensory and motor in function, supplying fibers to the ciliary muscle, iris, and cornea, and constitute two twigs of the ophthalmic branch of the trigeminal nerve. In the cornea both free nerve endings and terminals with rounded end bulbs have been identified histologically. All of the cutaneous sensations can be elicited from the cornea, but it is interesting that this sensitivity is reduced in the infant during the first year of life.
After the cleaning of the globe, immerse it in water and observe the episcleral fibers. These fibers connect the membrane loosely surrounding the globe and are commonly known as Teronıs capsule.
Mark the top and bottom by means of a notch and bisect the globe around the equator. Hold the globe between the thumb and the forefingers of the left hand and make an incision through the sclera with the scalpel, using a sawing action. Upon penetration, registered by an escape of fluid, insert the scalpel or a pair of scissors and cut all around the equator. Without letting the two halves separate, take the scalpel and gently cut through the vitreous humor with a sawing motion. Unless the vitreous humor is cut first, it is liable to pull away as a whole, bringing the lens and retina with it. This method of bisection avoids the lens, which is ordinarily hard to achieve.
The Interior of the Globe:
Place the two halves in water to avoid drying out and to float out some of the more delicate structures. Note the inequities in scleral thickness. The appearance of the posterior half is similar to the view the ophthalmologist receives when examining the eye. This hemispherical bowl is called the fundus. Look for the optic nerve head or disc (devoid of receptor elements), the filmy retina and vessels (folds in the retina are artifacts), the choroid and any surface coloration it may have (the tapetum). Except in primate mammals, no central retinal area, macula or fovea, is macroscopically distinguishable.
The choroid is a heavily pigmented layer behind the retina, providing a dark chamber for the eye and absorbing incident light before it can be reflected. On the inner surface of the choroid is a brilliant patch of colors, the tapetum, varying from yellow to blue to violet. It is responsible for the glow so readily seen in the pupil of certain animals at night when they face a light source. The tapetum is not found in the human eye. Although its function is uncertain, it may act as a back reflector, increasing the amount of light passing through the retina, and hence its sensitivity at low levels of illumination. The receptor elements of the retina are located adjacent to the inner surface of the choroid, with the bipolars, ganglion cells and blood vessels lying over them. Light passing through the dioptric apparatus (cornea, aqueous humor, lens and vitreous humor) must therefore pass through these retinal layers before stimulating the photoreceptors or being absorbed by the choroid. Gently pull apart of the retina away from the choroid and note its transparency.
Now examine the anterior half of the globe. Notice the sharp termination of the retina along the circle of attachment called the ora ciliaris retinae (ora serrata in humans). Beyond this point the choroid thickens to form the ciliary body, which is roughly triangular in cross section. The ciliary body tapers down from the iris and from it emanate the sensory ligaments of the lens. The deeply pigmented sphere including the choroid, ciliary body and iris is termed the uvea or middle coat of the eye. A number of meridional folds, the ciliary processes, can be easily distinguished on the inner surface of the ciliary body. The lens is located behind the iris. Through it one may obtain ³the cowıs eye view² by holding the anterior half towards the light.
After making these observations on the interior of the globe, the dissection of the two halves may begin. First it will be necessary to remove the vitreous humor, a gel-like substance having the consistency of raw eye white. With a brushing motion, pull the vitreous from the posterior half. Of done properly, the retina and choroid will remain intact and in position. Place a small piece of the vitreous humor on a paper towel. Notice the gradual loss of water content leaving the small solid residue. A watery fluid called the aqueous humor fills the chamber between the lens and cornea. The vitreous is similar to, if not identical to the aqueous humor. The major difference between the two is that the vitreous contains a tenuous molecular sponge or network of immense capacity for water absorption.
Dissection of the Posterior Half:
After removal of the vitreous replace the posterior half under water and note how the retina comes away readily from the underlying choroid but remains attached at the optic disc. The greater strength of the retina in this area is due to the increased density of neuroglial cells around the optic disc. As in the glial cells of the retina are supportive and connective in function, as well as serving to insulate one neuron from another. The blood vessels of the retina are visible only insofar as they retain some blood in them, but tend to follow retinal folds.
With a scalpel, bisect the posterior half through the optic nerve root and the optic disc. Note that it is narrowest where it transverses the scleral foramen. With blunt forceps strip away the retina and examine the choroid and tapetum. Scrape away some of the black pigment of the choroid near the margin of the tapetum. How far does the tapetum extend beneath this pigment layer? Again with forceps peel off the choroid. This tears away from the sclera, the fibers of the suprachoroid, the outer cellular layer of the choroid composed of pigmented connective tissue, and exposes a pigment layer on the inner surface of the sclera upon which the imprint of ciliary nerves and vessels are seen as the lightest tracery. As described above, the ciliary nerves innervate the cornea, iris and ciliary muscle. In the eye, nerves and blood vessels tend to follow similar routes, frequently located adjacent to one another. Where this occurs they are often named similarly, thus ciliary nerves and ciliary vessels. The arterial circulatory system of the eye arises, in humans, from the internal carotid artery. From this artery branches the ophthalmic artery, which subsequently divides into the ciliary arteries and central retinal arteries. Venous blood leaves the globe primarily via the large vorticose veins and smaller ciliary veins between the choroid and sclera. These converge upward and downward into superior and inferior ophthalmic vein that empty into the cavernous sinus located in the floor of the cranium. The sinus is drained by the internal jugular vein. The vascular system of the cow eye differs considerably from the human eye. One of the major differences is that arterial influx and venous efflux are provided primarily by the external carotid and external jugular, respectively, rather then their respective internal branches as in humans.
Dissection of the Anterior Half:
Invert the anterior half over a dish of water. If the lens and vitreous do not fall away, it will be necessary to cut through the suspensory ligaments of the lens (attached to the ciliary body) by working a scalpel around, between the lens equator and the ciliary processes, taking care not to damage either. Detach the lens and note any differences in curvature between the anterior and posterior surfaces. Then bisect the lens along the antero-posterior axis. Note the laminated structure and harder central nucleus. Using your fingers, break one of the lens halves into quarters. The laminations may be more apparent now. A thin, transparent membrane, called the lens capsule, should be visible at the point of fracture. The suspensory ligaments are continuous with the lens capsule. The lens is the mechanism for ocular accommodation. When the ciliary muscle, located on the broad anterior aspect of the ciliary body, contracts in a sphincter like fashion, tension on the suspensory ligaments is relieved, resulting in an increased curvature of the lens. Objects near to the eye are thus brought into focus on the retina. The innervation for the accommodation reflex is not well understood. It is thought to be primarily a parasympathetic effect, initiated by a change in the focus of the light incident on the fovea. Pathways to the visual cortex and superior colliculus via the optic nerve and lateral geniculate bodies return to the ciliary muscle is very small and probably has little significance. In considering the role of this muscle in accommodation, how does its small size relate to the location of the eyes on the head?
Now inspect the inner aspect of the remaining portion of the anterior half. With blunt forceps test the attachment of the retina at the margin of the ciliary body. Remove any vitreous that may remain. Strip off the anterior uvea in one piece with the forceps and place it aside. Examine the inner surface of the outer coat and then bisect it. At the edge of the cut, look for differences in corneal thickness between the center and periphery. Note the overlap the sclera over the cornea.
Now examine the anterior uvea. Note the white band denoting position of the ciliary body (and muscle), the iris pattern and pigment, and the size and shape of the pupil. Observe closely for traces of ciliary nerves and vessels. When contracted the pupil assumes a somewhat pear shape shaped horizontal slit with the broader end situated medially in conformity with the shape of the cornea, but upon dilation, the pupil becomes more circular. Changes in pupil size are a classical example of the antagonism between the sympathetic and parasympathetic divisions of the autonomic nervous system. The dilator muscles of the iris, oriented radially and innervated by the sympathetic system, produce an increase in pupil size upon contraction. Fibers from the superior cervical ganglion reach the trigeminal nerve via the internal carotid nerve. The ciliary nerves of the ophthalmic branch of the trigeminal then terminate on the dilator muscles. In opposite fashion the sphincter muscles of the iris, oriented concentrically and innervated by the parasympathetic system, cause the pupil to shrink upon contraction. The sphincters are dominant over the dilators. Like the ciliary muscle sphincters, the iris sphincters are innervated by the oculomotor nerve. Afferents from the cornea, the ciliary muscle and the iris project into the trigeminal nerve. Although the iris muscles respond to a wide variety of stimuli of psychological origin, the simple and involuntary papillary light reflex is relatively straightforward. In the optic nerve a discrete bundle of fibers, the papillary fibers, are believed to be specialized for the papillary reflex arc. These fibers semi decussate at the optic chiasm and terminate in the pretectal nuclei of the midbrain. At this point additional crossing of fibers occurs via the posterior commissure, other fibers from the petectal nuclei synapse in the nuclei of the oculomotor nerves. As mentioned previously, the oculomotor nerves then innervate the sphincters of the iris (but after one synapse in the ciliary ganglion). The dilators of the iris remain in relatively tonic contraction, exerting their influence when the sphincter reflex arc is inactive. Finally, on the upper and lower edges of the iris are located several deeply pigmented masses, the corpora nigra. These bodies are arranged in such a way that when the iris sphincters contract strongly the corpora interlace with one another, providing an additional degree or closure of the pupil beyond that provided by the muscular response of the iris.