THE CAT 243 



surface of the eyelids, the exposed surface of the eye, and both 

 sides of the nictitating membrane. 



Observe the exposed portion of the eyeball. The convex, trans- 

 parent cornea forms its outer portion. The yellow iris will be seen, 

 in the middle of which is the pupil, the opening through which 

 light is admitted into the interior of the eye; the iris contains 

 delicate muscle fibers by the contraction of which the pupil is 

 made smaller. 



Exercise 34. Draw a sketch showing the exposed portion of the eye 

 and the lids. 



Carefully cut away the eyelids and the bony arch which forms 

 the lateral wall of the orbit. Observe the lachrymal gland ; it is 

 a large, flattened, reddish body, which often has the appearance 

 of a broad muscle, and lies close to the hinder side of the eyeball 

 at the outer angle of the eye, in contact with the bony arch. The 

 fluid furnished by the gland bathes the cornea and is then drained 

 off through the two lachrymal canals, each of which opens at the 

 edge of one of the lids near the inner angle of the eye. These two 

 canals join to form the nasolachrymal duct, which passes through 

 the bony wall of the orbit and opens into the nasal cavity. 



The Muscles of the Orbit. Ten small muscles control the move- 

 ments of the eyeball. Eight of these are muscles which pass 

 from the inner portion of the orbit, near the entrance of the optic 

 nerve, straight to the sides of the eyeball. Of these the four 

 larger are called the rectus muscles ; the four smaller, which lie 

 between the rectus muscles, are the four heads of a single muscle, 

 the retractor. The other two muscles are the inferior and superior 

 oblique muscles, which have an oblique position. Besides these an 

 eleventh muscle is present, which moves the upper lid. 



The lateral and posterior portion of the bony orbit being re- 

 moved and the lachrymal gland identified, note the muscle which 

 is the most anterior in position of those exposed. It lies on the 

 outer side of the eyeball, parallel to the margin of the cornea, 

 and is the inferior oblique. It is inserted in the outer surface of 

 the eyeball near the cornea and passes ventrally beneath the 

 eyeball to its origin in the anterior surface of the orbit. 



