222 ELEMENTS OF MAMMALIAN ANATOMY. 
walls of the orbital cavity. The retractor oculi originates 
on the boundary of the optic foramen and is inserted into 
the sclerotic around the entrance of the optic nerve. This 
muscle is completely hidden by the recti muscles. 
The eyeball is composed of three membranes and three 
humors. The outer coat, the sclerotic, consists of the 
opaque portion forming the caudal two-thirds, and the 
transparent portion, or cornea, forming the remainder (Fig. 
110). The optic nerve pierces the sclerotic a little mediad 
of the longitudinal axis of the eye. The surface of the 
cornea is more strongly curved than that of the opaque por- 
Fic. 110. LonceitupInaL SECTION oF THE EYE. 
aq, Aqueous humor; c, cornea; cp, capsule of the lens; ch, choroid; ct, 
ciliary process; 1, iris; /g, ligament of lens; m, ciliary muscle; opn, 
optic nerve: rt, retina; sc, sclerotic coat; os, ora serrata. 
tion of the sclerotic, and contains no blood-vessels. The 
second or muddle coat of the eye is formed by the choroid 
membrane and its extension, the iris. This coat is incom- 
plete, as there is an aperture, the pupil, through the iris for 
the admission of light. The choroid appears as a jet-black 
membrane, less than half as thick as the sclerotic, lying 
closely appressed to the latter. The choroid is lined in- 
