38 Practicum VI. The Eye of the Cat. 



EYEBALL ; (^)the UPPER and LOWER LIDS, hairy on the ectal surface, but 

 with no lashes at the margin ; (c) near the mesal angle, on the margin of 

 either lid, a slight elevation, in which is the orifice of the LACHRYMAL 

 CANAL. 



a. With the forceps grasp the skin at the mesal angle and pull 

 laterad. With the scissors cut through into the orbit, keeping close to 

 the bone. This will transect the two lachrymal canals. 



b. Under favorable conditions as to time, patience and skill the canals may be traced 

 to the LACHRYMAL SACK, which receives also the canal from the other lid, and is contin- 

 ued into the cavity of the nose. Through this duct is carried into the nose any superflu- 

 ous moisture on the surface of the eye ; the smallness of the orifices at the margins of the 

 lids prevents the entrance of dust which might clog the passage. 



6. The Third Eyelid. At the mesal or nasal angle of the eye be- 

 tween the lids, is a fold of mucosa, the PLICA or nictating membrane or 

 third lid. Grasp the free margin and draw it laterad over the ball. The 

 plica is easily seen in birds ; in man it is rudimentary and represented 

 by a slight fold of mucosa. 



7. Removal of the eye. Grasp either lid with the fingers or forceps 

 and with the scissors cut about the ball close to the margin of the orbit. 

 Cut deeper and deeper, dividing the fat and muscles, and lastly the firm, 

 white, cylindrical OPTIC NERVE, at the bottom or apex of the orbit where 

 it passes through a foramen in the cranium to join the chiasma ; PI. XIX. 



8. The Orbit. The margin of the ORBIT, the cavity containing the 

 eyeball may be felt through the skin and its form and limits should be 

 noted on the skull. 



Compare the margin of the orbit with that of the prepared skull, 

 and note that in the latter it is incomplete for a short distance at the lateral 

 side, but that in the head under examination the gap is closed by a fibrous 

 band which may be cut with the scissors ; PI. XVI, /, 2. Ascertain that 

 the mesal wall of the orbit, its roof and part of its floor are bony, but that 

 the rest of the floor and its lateral wall are fleshy. In man and monkeys 

 these parts of the wall are also bony, so that the orbit is completely cir- 

 cumscribed in the prepared skull. 



9. The Muscles of the Lower Jaw. On Fig. i the word cranium 

 corresponds nearly with a line across the side of the head about 5 mm. 

 dorsad of the orbit and auditory meatus ; cut along this line, not too 

 deeply at first, until the bone is reached. This is the CRANIUM, the bony 

 case for the brain. The transected muscle is the TEMPORAL, one of the 

 FLEXORS of the MANDIBLE, or lower jaw. 



a. Dissect it up from the bone, and note that it arises partly from 

 the smooth, convex surface of the cranium, partly from ridges or crests 

 of bone at the margins of the muscle. 



b. With young cats the crest at the dorsal margin is widely separate from its opposite, 

 but with age they approach, especially caudad, and for a greater or less distance may unite 

 to form a single, mesal crest. In some animals (e. g., lion, hyena, gorilla) this mesal crest 

 is considerably elevated, and the temporal muscles are correspondingly thick and pow- 

 erful. 



c. In man the area covered by the temporal muscle is comparatively slight. Its 

 extent and the action of the muscle may be felt if the fingers are pressed upon the tem- 

 ple while the mouth is opened and closed. 



10. A line from the middle of the eye to the auditory meatus cor- 

 responds nearly with the ZYGOMA or cheek bone ; it may be felt under the 

 skin in ourselves and is prominent in certain races and emaciated per- 

 sons. From it arises the MASSETER, the second great mandibtilar flexor. 

 Determine its ventral border with the finger and tracer, and cut along it 

 carefully so as not to injure the blade. 



