THE EYE 



153 



ness of its blood-supply; it seems to be specially con- 

 cerned with nutrition. The innermost coat, reminding 

 one of the plate or film in the camera, is the retina. 

 It consists in part of nerve fibers which run from every 

 region of it to the point of departure of the optic nerve. 

 This nerve, in leaving the eye, necessarily perforates the 

 middle and the outer coats. 



The middle coat adheres to the outer everywhere but 



FIG. 39. A vertical section of the right eye and its lids, (c) is the 

 cornea, (I) the crystalline lens, its margins shielded by the iris, (s.r.) 

 is the superior rectus muscle, (i.r.) the inferior rectus. The optic nerve 

 (o.n.) is not cut by the section but is to be thought of as lying back of 

 its plane, that is, toward the nose. 



in front. Within the circle of the cornea it falls back and 

 forms the iris, pierced by a round opening, the pupil. 

 The iris is the part which gives the distinctive color to 

 the eye. It is provided with muscle fibers of the smooth 

 variety adapted to narrow or to widen the pupil. The 

 iris serves the same purpose as a diaphragm in a camera; 

 it limits the admission of light when the illumination is 

 strong and it has another use which will be pointed 

 out presently. The circular form would seem the natural 



