THE INTERNAL COAT THE. NERVOUS TUNIC 



647 



a single layer of columnar epithelial cells whose bases, of generally hex- 

 agonal outline, rest upon and are firmly adherent i.. I lie inner surface 

 of the choroid coat, and from whose free borders irregular processes 

 extend inward between the ele- 

 ments of the rod and cone layer. 

 These epithelial cells have a finely 

 granular cytoplasm. Their nucle- 

 us 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 (fuscin) 

 by which the cytoplasm of the cell 

 is more or less completely filled. 



The disposition of the pigment 

 within the epithelial cell appar- 



ently corersponds to, and is de- 

 pendent upon, the effect of light 

 upon the retina. In an eye ex- 



FIG. 546. THE RETINA OP A CHILD'S 

 EYE; MERIDIONAL SECTION. 



a, nerve fiber layer, the broad bases of 

 Miiller's fiber cells show distinctly; 6, 



posed to the action of light at the blood-vessel; c, large ganglion cell layer; 



, inner reticular layer; e inner nuclear 



layer;; /, outer reticular layer, with a 

 prominent fiber layer of Henle; g, outer 

 nuclear layer; h, layer of rods and cones; 

 \ la ^ r of pig^ented epithelium; k, 

 choroid coat. Hematem and eosm. 

 photo. X 225. 



instant of death, the pigment 



granules accumulate in the irreg- 



ular processes of the cells which 



surround the rods and cones, the 



outer or basal portion of the cell 



being relatively free from pig- 



ment. In an eye which is shaded from the light, or in one removed in 



comparative darkness, the pigment has apparently retracted until it lies 



entirely with the body of the cell. Even under 

 these conditions the extreme base of the cell fre- 

 quently presents a narrow zone which is rela- 

 tively free from pigment. Similar changes in 

 the disposition of the pigment undoubtedly occur 

 in the living eye under the influence of exposure 

 to varying degrees of light. 



The function of this pigment and of the pe- 

 culiar changes in its disposition is still somewhat 

 speculative, but it may, without doubt, be safely 



asserted that these phenomena are concerned with the renewal of 



the visual purple of the outer segments of the rods after the same has 



FlG. 547. PlGMENTED 



EPITHELIUM OP THE 

 RETINA, VIEWED IN 

 TRANSECTION. 



X 500. (After Fuchs.) 



