These are lecture notes listed in sequence:
Dissection of the Eye and its Orbit in the Cat
Follow protocol Notebook Illustrations
The brain should have previously been removed from your cat (see Removal and Study of the Cat Brain). You should identify the upper and lower eyelids, and the nictitating membrane which comes up from below the eye. Note the conjunctival surfaces, and the inferior fornix.
REMOVAL OF THE EYE
Locate the optic nerves where they enter the cranium through the optic foramina.
Draw lines from each optic nerve to the medial and lateral limits of the orbits.
The accompanying image shows the location of the cut to make.
Cut on these lines with the end of a hack saw. Be certain to cut laterally enough to prevent part of the frontal from overhanging, and all the way to the optic foramina.
lift the triangle Lift off the triangular section of the roof of the orbit, cutting it free from any underlying tissue which may adhere to it, preserving features attached to the eye. Mark the superior-most muscle with an indelible “X” to keep track of it later in the dissection. (Can you find the superior oblique and its trochlea, on the medial superior border of the orbit? This is a challenge.)
Cut the anterior portion of the eye loose from the socket by cutting closely along the bone to free up the eye itself. Lift up the entire structure, cutting any peripheral tissues which may hold it down. After you have lifted it in the front, slide the scalpel under the rear-most portion to free it from the skull. Note that the nictitating membrane marks the inferior portion of the dissected portion.
Note the membrane which surrounds the entire orbit (periorbita). The lacrimal gland is under it on the lateral superior side. Work the periorbita open with a blunt probe, remove the adipose tissue which served as padding. The most superficial superior muscle will be the levator palpebrae superioris, which raises the eyelid. It does not insert into the bulb.
DISSECTION OF THE EYE
The fat has been removed from the excised eye to reveal the muscles (upper picture).
Find the four rectus muscles:
lateral, superior, medial and inferior rectus
Use a blunt probe to free them from the optic nerve. The inferior obliquecrosses over the inferior rectus. The superior oblique should be present, but this dissection may not retain the trochlear loop. Its tendon of insertion is medial to the insertion of the superior rectus. Here is a labeled version of the right hand image. (Do not confuse the deeper four retractor bulbi muscles which are under the rectus muscles.)
Deep to the four rectus muscles (peeled back in this image) are four portions of the retractor bulbi. Note the optic nerve which exits the eye at the center of these four parts. The four retractor bulbi have been reflected to show the rear of the eye ball. The sclera is opalescent bluish white, the optic nerve at the center.
With the fine scissors, make a small vertical snip through the rear wall of the bulb just medial to the optic nerve (do not squeeze the bulb during this cut…).
Hold the eye over a petri dish lid, extend the cut (keep it shallow) sagittally through the center of the cornea. Note that aqueous humor drains out as the cornea is being cut.
As you lift the cut anterior piece, note that the lens is attached to it by suspensory ligaments. Cut through the iris, and lift off the cut portion, cutting its ligaments.
Note that the lens is opaque-white, an artifact of preservation. The posterior cavity is filled with clear jelly-like vitreous humor. Note the three tunics of the wall of the eye:
fibrous tunic (sclera plus cornea)
vascular tunic (iris, ciliary body and choroid)
Here is a labeled image of the components of the eye (upper image in left frame).
The retina will appear pearly yellow. Note the optic disk identifiable by the vessels may be seen to emerge from it, and pass under the retina. The fovea centralis will be towards the center and seen as a yellowish depression. (Here is a labeled image.) The retina peels away easily from the underlying choroid. This, a detached retina is a serious condition requiring immediate attention in patients to prevent death of the retina and blindness in the patient.
In the anterior portion of the eye, note that the lens is supported by suspensory ligaments. Posterior to it is the ruffled surface of the black pigmented ciliary processes which produce the aqueous humor. The ora serrata is the anterior boundary of the retina. The iris is golden on the exterior surface, black on the posterior. The cornea is tough and relatively thick.
Make three illustrations:
1) Cuts made to remove eye
2) muscles teased out
3) cross section of the eye
A visual reflex mediated by semicircular cancals
Tracking objects is a complex activity under normal circumstances, but the complexity is markedly increased during movement of the head. There is a system within the brain located in the superior colliculi which receives information on rotation of the head from the semicircular canals. The superior colliculi then directs eye movements via the extrinsic muscles of the eye to compensate for these head movements.
The effects of this system can be seen when it over compensates from prolonged rotation of the body. The semicircular canals become accommodated to the rotation, and when the rotation is halted, the endolymph continues to move, triggering impulses interpreted in the superior colliculi as strong rotational acceleration signals. It therefore directs the compensating jerking of the eyes called nystagmus (from the Greek, meaning to nod). Nystagmus may also be a sign of malfunction in one of the stages of the system responsible for this reflex.
ANATOMY OF THE EXTERNAL EYE:
Familiarize your self with the external features of the eye:
Medial and lateral canthus (corners of the eye
Caruncle (little mound of flesh in the medial canthus. Can you spot the tiny puncti?)
Superior and inferior palpebrae (eyelids)
Conjunctiva (covers the sclera)
Iris and pupil
1. Place student on rotating stool, away from all furniture, sharp corners, etc.
2. Rotate student a determined number of times (5, 10, 15, etc) Take care to stop before nausea is induced…
3. Stop rotation abruptly with subject’s eyes facing observers so that the eyes may be observed at close range (about a foot away).
4. Observe the following:
a) Is the movement large or small
b) How frequent is the movement (jerks/second or second between jerks)
c) How long do the movements persist.
Does the strength or persistence relate to the length of time rotation was performed?
Is there a minimum length of time in order to accommodate to the rotation?
Use of a video camera with slow motion playback may allow this reflex to be quantified.
Histology of the Eye
Follow protocol Notebook Illustrations
This is another slide in which, because of the size of the specimen, you will have to move around to see the entire structure, even at the lowest power. First look at the slide with the naked eye to orient yourself to its gross features: you should be able to see the cornea, lens (often fragmented on the interior), cavities, and ciliary bodies. Note that some slides are oriented R and L, others up and down…
With the 4x objective, find the sclera and follow it around the entire perimeter, identifying the regions as you come to them and noting the layers in each region. Again, identify all features listed before you begin an illustration. You will make three illustrations:
- The gross anatomy of the anterior portion
- A low power view of the rear wall
- A high power view of the retina:
Slide 8: Eye, Monkey, general structure, sagittal section, general features H 1064 (MF 4th, page 343, 345)
At 25x: Gross anatomy of section through the eye: (Here is a labeled version of the eye section.)
cornea (anterior 1/6th)
sclera (posterior 5/6ths)
corneal limbus (cornea & sclera join)
canal of Schlemm (poss. not visible)
lens (It is often fragmented during slide preparation, and only the outline remains.)
suspensory ligaments (not visible?)
At 40x: Gross anatomy of anterior portion of the eye
Here is a labeled view of the anterior portion of the eye.
VASCULAR TUNIC, 60x:
VASCULAR TUNIC, 100x: (Here is a labeled version of the anterior vascular tunic and fibrous tunic.)
400x view of cornea, left to right:
posterior epithelium (low cuboidal cells)
Corneal stroma withkeratocytes embedded in collagen fibers
anterior epithelium (stratified squamous epithelium)
At 100x: cross section through rear wall of eye with optic nerve: Here is a labeled view of the rear wall.
left to right:
optic nerve with parallel axons from ganglion cells
optic disc (note photoreceptors are absent in retina where the optic nerve is attached) adventitia (orbital fatty tissue)
choroid, with melanocytes
cell bodies of
Here is an extraordinarily good image of the blind spot where the optic nerve exits the rear of the eye.
300x: view through the rear wall of the eye showing, top to bottom:
ganglion cell bodies
bipolar cell bodies
photoreceptor cell bodies
pigmented cells of choroid
400x view of cross section of retina: (Here is a labeled version of the cross section of the retina.)
Deep to superficial tissues (left to right):
rods (finer and longer)
cones (thicker and shorter)
outer limiting membrane
nuclei of cones (closer to choroid)
nuclei of rods (further from choroid)
horizontal cells (closest to receptors)
amacrine cells (closest to ganglion)
ganglion cell bodiesMullers fibers (vertical fr ganglion cells)
Other images related to the eye: