312 THE SENSES 



the latter of which is very small. The " color of the eyes" 

 depends on the color of the anterior surface of the iris; its pos- 

 terior surface has a constant dark purple hue. The size of the 

 pupil is subject to variations to be noted later. 



Inside the choroid is the retina, which is that part of the eye 

 capable of receiving impressions of sight. Anteriorly it reaches 

 nearly to the ciliary processes. Externally it is in contact with 

 the choroid, and internally with the hyaloid membrane of the 

 vitreous humor. It is penetrated by the optic nerve a little within 

 and below the center of the posterior hemisphere. Just external 

 to the point of entrance of the nerve is the macula lutea, a small 

 yellow area in the center of which is the fovea centralis ; this 

 last is exactly in the axis of distinct vision. Nine layers of cells 

 are usually described as composing the retina. From without 

 inward they are (i) the pigment layer, (2) rods and cones, (3-6) the 

 four granular layers, (7) nerve cells, (8) expansion of fibers of the 

 optic nerve, (9) the limitary membrane. Of these, the most im- 

 portant is the layer of rods and cones. The rods, or cylinders, 

 extend through the thickness of the membrane and have between 

 them, at intervals, flask- shaped bodies, the cones. At the macula 

 lutea only the cones exist. Elsewhere the rods are more abun- 

 dant than the cones. The length of the cones is about half that 

 of the rods, and they occupy the inner aspect of the membrane. 

 The layer of nerve cells presents cells communicating on the 

 one hand with the rods and cones and on the other with fibers 

 of the optic nerve. The rods and cones are the only parts of 

 the retina possessing special sensibility, impressions being con- 

 veyed from them to the brain by the optic nerve. The fibers 

 of the second nerve, composing one layer, are pale and transpar- 

 ent. The blood supply of the retina is from the arteria cen- 

 tralis retinae, which enters the optic nerve just before it expands, 

 and, running in its substance, is distributed as far as the ciliary 

 processes anteriorly. 



The Crystalline Lens is a biconvex transparent body situ- 



