THE TARPON 171 



situated in the extreme upper and anterior part of the head, 

 and are slightly smaller than are those of the horse. The eye 

 is so encased in the orbit that its size is not apparent until it 

 has been removed. 



The orbit is an oval bony cavity opening forward and out- 

 ward. The long diameter is from the front back pointing 

 slightly upward and inward from the median line of the head. 

 The top of the eye is almost level with the slanting ridge of 

 the snout. The optic nerve joints at the chiasm and the posi- 

 tion of the e^^es in the head would indicate the presence of bi- 

 nocular fixation. The measurements of the eyeball are laterally 

 fifty millimeters in the widest part and forty millimeters in 

 height. 



The anterior part of the eyeball is of bone and cartilage and 

 is bluish black. The cornea is almost flat, having normally a 

 slightly curved surface. At the ecjuator the color disappears, 

 the black suddenly merging into an ivory or yellowish white. 

 The back of the ball is covered by a tiat and soft, but dense 

 membrane through which the optic nerve passes. A space of 

 more than an inch between the ball and the back of the orbit 

 is filled with heavy muscles, vessels and fat. On the outer 

 side of the optic nerve entrance to the eyeball is a white body 

 about the size of the optic nerve and entering the cranial cav- 



ity. 



The cornea is oval in its horizontal diameter having a width 

 of 50 m.m. and a height of 40 m.m. The central part is circular 

 and has a diameter equal to that of the pupil (20 m.m.). The 

 transparency extends from the pupillary margin to the periph- 

 ery where its junction with the sclera is marked by a black 

 band 1 m.m. wide. From the center of the cornea which is 1 m. 

 m. in thickness to a point midway between this and the margin 

 the thickness increases to 3 m.m. after which it again decreases 

 at the circumferential border to 1 m.m. where it joins the 

 sclera in which true bone is incorporated with the sclera fibres. 



