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. 

 no). 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- 



FIG. no. LONGITUDINAL SECTION OF THE EYE. 



aq, Aqueous humor ; c, cornea ; cp, capsule of the lens ; ch, choroid ; ci, 

 ciliary process ; *, iris ; Ig, 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 middle 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- 



