THE OPTIC NERVE . 337 



therefore when they contract they draw upon the margin to enlarge 

 the opening. Briefly, the pupil is contracted by the circular fibers, 

 and dilated by the straight or radiating fibers, thus the amount of 

 light admitted within the eye is regulated. 



The retina is the innermost coat, of many layers, within the 

 choroid. This is a very delicate structure in which are the be- 

 ginnings of the optic nerve fibers. It is the coat which is essential 

 to vision no retina, no vision. The outermost layer of the retina 

 is the one which contains the rods and cones, or the visual cells. 

 Like the sclera and choroid, the retina is incomplete in front. 

 When first exposed to the air (in the dissection of an eye) it is 

 clear and shining in appearance, presenting an opalescent play 

 of color with a general violet tinge, due to the "visual purple" 

 contained in delicate pigment cells. 



From the cells in the retina delicate fibers are prolonged and 

 gathered together to form the optic nerve, which pierces the 

 choroid and the sclerotic, passes through the optic foramen of 

 the orbit, and thence back to the brain. The optic disc is the spot 

 where the optic nerve leaves the retina; it is situated a little to the 

 nasal side of the center of the retina (Figs. 214, 216). Of course the 

 optic disc is not a portion of the retina proper, and no sense of 

 vision is stimulated here. It is rather an area where the nerves 

 and vessels are transmitted through the other coats of the eyeball. 



The macula lutea is a spot in the center of the retina opposite the mid- 

 point of the normal pupil. In the center of this spot is a depression called 

 ihefovea centralis which is the center of vision; only the cone-shaped visual 

 cells are here present. 



The vitreous body is glass-like, as its name signifies, both in 

 appearance and transparency. It consists of a jelly-like substance 

 contained in a hyaloid membrane within the three coats. It trans- 

 mits and directs the rays of light to the retina; also it aids in pre- 

 serving the shape of the eyeball (Fig. 214). 



The crystalline lens is situated immediately in front of the 

 vitreous body, in a shallow depression like a cup on the anterior 

 surface. It is a double convex lens with a capsule, both perfectly 

 transparent so that light may pass through, and it is able to converge 

 the rays of light so that they will fall correctly upon the retina. 



The lens is behind the iris, the margin of the pupil resting 



22 



