458 



STKUCTUKE OF THE RETINA. 



[BOOK i. 



Kiihne has made the discovery that the protoplasm of these cells is 

 the seat of remarkable movements, as proved by the different distri- 

 bution of the pigment in them, dependent upon the degree of 

 illumination to which the eye has been subjected. In the pigment 

 cells of a frog which has been kept for several hours in the dark, the 

 pigment is found to be confined to the cell bodies and the roots of the 

 processes coming from these. But if microscopical sections be made 

 of the eyes of frogs which have remained for some time in the sun- 

 light, the pigment will be found to have extended itself much further 

 forward in the cell processes towards the membrana limitans externa, 

 a proportionally smaller quantity remaining in the cell bodies. In the 

 eyes of frogs which have been exposed to light, the retina, when removed, 



Tapetum. 



FIG. 64. KETINAL EPITHELIUM CELLS. (Max Schultze.) 



(a) Cells seen from external surface; (b) and (c) Cells seen in profile, with 

 processes projecting inwards. 



has much epithelium attached to it. Conversely, in the eyes of frogs 

 which have not been exposed to light, the retina can be removed without 

 its epithelial covering. These facts will be again referred to in discussing 

 the functions of the retinal epithelium in regenerating the visual purple. 

 In animals possessing a tapetum, the epithelial layer 

 of the retina is unpigmented in the tapetal area, and the 

 choroid is composed anteriorly of a dense, strongly light-reflecting tissue. 

 In some animals, as the sheep or ox, the tapetum is composed of fine 

 fibrous tissue. In others, as the dog and cat, it is made up of several 

 layers of unpigmented cells which are filled with exceedingly fine 

 crystals (Max Schultze). Some fish, as the bream (Abramis Brama) 

 possess a so-called psuudo-tapetum (Briicke, Kiihne and Sewall 1 ) ; in 

 the bream the retinal epithelium contains, in certain areas, both dark 

 pigment and amorphous strongly light-reflecting Guanin, so that the 

 epithelium seen from before presents a bright or a dark surface ac- 

 cording as the pigment, under the influence of darkness or light, is 

 found in tLe bases or processes of the cells. 



y The preceding description of the structure of the 



in the struc- retina does not apply to every part of its surface, though 

 ture of the we must refer the reader who requires detailed infor- 

 mation on this subject to treatises on histology. Suffice 

 it to say that at the entrance of the optic nerve ( Colli- 

 culus nervi optici) the nerve fibre layer is immensely 



1 Kiihne und Sewall, "Zur Physiologie des Sehepithels. " Verhandl. d. naturhist. 

 Vereins zu Heidelberg, N.S. Vol. n. Heft v. (June, 1880). 



retina in 

 different 

 regions. 



