ANATOMY OF THE BALL 313 



little to the nasal side of its center, the pupil. It is attached 

 to the corneo-sclerotic line. It contains circular and radi- 

 ating fibers. The iris divides the space between the cornea 

 and lens into two chambers, anterior and posterior the lat- 

 ter of which is very small. The "color of the eyes" depends 

 on the color of the anterior surface of the iris ; its posterior 

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

 pil 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 pos- 

 terior hemisphere. Just external to the point of entrance of 

 the nerve is the macula lutea, a small yellow area in the cen- 

 ter of which is the fovea centralis; this last is exactly in the 

 axis of distinct vision. Nine layers of cells are usually de- 

 scribed 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 important 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 abundant 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 pre- 

 sents 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 conveyed from them 

 to the brain by the optic nerve. The fibers of the second 

 nerve, composing one layer, are pale and transparent. The 

 blood supply of the retina is from the arteria centralis 



