i 4 VERTEBRATE ZOOLOGY 



muscles at their insertion on the eyeball and pull it gently forward ; 

 the inferior rectus muscle will be seen, which passes from the lower 

 side of the eyeball to the posterior wall of the orbit. Between the 

 rectus muscles in the spiny (but not in the smooth) dogfish will 

 be seen a cartilaginous rod which passes from the eyeball to the 

 posterior wall of the orbit ; this is the ophthalmic peduncle. 



Find the optic nerve ; it will be seen to enter the eyeball. Cut 

 the nerve and remove the eyeball from the orbit. Note the origins 

 of the six eye muscles in the orbit : the two oblique muscles will be 

 seen to take their origin in its anterior wall, and the four rectus 

 muscles in its posterior wall. 



Study the structure of the eyeball. Its outer wall is made up 

 of three layers, which are called the sclera, or sclerotic coat, the 

 choroid coat, and the retina. The sclera is the outer coating ; it 

 covers the entire eye except in front, where its place is taken by 

 the cornea. 



Just within the sclera is the choroid coat. Cut the eyeball in 

 two by an incision parallel to the cornea, so that you can look 

 into the interior. The soft vitreous humor which forms the inner 

 portion of the eye will exude, and the crystalline lens will be seen. 

 The choroid coat will be recognized by its dark color; it con- 

 tains both the pigment and the blood vessels of the eye. On the 

 inner surface of the choroid, next to the retina, is a layer called 

 the tapetum lucidum, which reflects light and causes the eye to 

 shine in the dark. 



Just within the choroid and forming the inner lining of the 

 eye is the retina ; it is light in color and is easily separated from 

 the choroid. 



The retina represents an expansion of the optic nerve on the 

 inner surface of the eye, and is the portion of it which is sensi- 

 tive to light. Note the point where the optic nerve pierces the 

 sclera and choroid ; this point is called the blind spot, because it 

 is nonsensitive. 



The eye has in it two principal chambers,— an anterior cham- 

 ber between the iris and the cornea, which is filled with a watery 

 fluid called the aqueous humor, and a large posterior chamber, 

 which is filled with the gelatinous vitreous humor. 



