580 



THE EYE 



itself (Lewis*), but is formed from the overlying area of the 

 ectoderm. 



THE LAYERS OF THE RETINA. The retina may be said 

 to consist of ten layers, which from without inward are : 



1. The pigment epithelium. 



2. The layer of rods and 

 cones. 



3. The external limiting 

 membrane. 



4. The outer nuclear layer. 



5. The fibre layer of Henle. 



6. The outer reticular layer. 



7. The inner nuclear layer. 



8. The inner reticular layer. 



9. The ganglion cell layer. 



10. The nerve fibre layer. 

 To these several layers an 



additional one, the internal 

 limiting membrane, is fre- 

 quently added. The first five 

 of these layers are contained 

 within the neuro- epithelial 

 portion of the retina, the last 

 five form its cerebral or neural 

 portion. 



1. The pigment epithelium (layer of pigment cells) consists of a 

 single layer of columnar epithelial cells whose bases rest upon and 

 are firmly adherent to the inner surface of the 

 choroid coat, and from whose free borders ir- 

 regular processes extend inward between the 

 elements of the rod and cone layer. These 

 epithelial cells have a finely granular cytoplasm. 

 Their nucleus is oval, somewhat flattened, and 

 placed near the base of the cell ; it is, however, 

 obscured or even entirely hidden by the mass 

 of dark pigment granules by which the cyto- 

 plasm of the cell is more or less completely filled. 



The disposition of the pigment within the epithelial cell appar- 

 ently corresponds to, and is dependent upon, the effect of light 

 upon the retina. In an eye exposed to the action of light at the 

 * Am. J. of Anat., 1904. 



FIG. 425. THE RETINA OF A CHILD'S EYE ; 

 MERIDIONAL SECTION. 



a, nerve fibre layer, the broad bases of 

 Muller's fibre cells show distinctly ; 6, blood 

 vessel ; c, large ganglion cell layer ; d, inner 

 reticular layer; e, inner nuclear layer; /, 

 outer reticular layer, with a prominent fibre 

 layer of Henle ; </, outer nuclear layer ; i, 

 layer of pigmented epithelium ; fc, choroid 

 coat. Hematein and eosin. Photo, x 225- 



FIG. 426. PIGMENTED 



EPITHELIUM OF THE 



KETINA VIEWED IN 



TRANSECTION. 



x 500. (After Fuchs.) 



