CHAP. VII 



EETINAL EXCITATION 



331 



iS 



* '& / 





I. On examining vertical sections of the human retina and that 

 of the higher animals under the microscope, eight distinct layers 

 at least can be distinguished. 

 Taken from without, inwards, these 

 are : 



1. The layer of pigment epi- 

 thelium-cells; 



The layer of rods and cones 

 (Jacob's mosaic membrane) ; 



3. The outer nuclear or granu- 

 lar layer : 



4. The outer molecular or reti- 

 cular layer : 



f>. The inner nuclear or granu- 

 lar layer ; 



6. The inner molecular or reti- 

 cular layer ; 



7. The layer of ganglion 

 cells ; 



8. The layer of optic nerve - 

 fibres. 



In addition to these, two delicate 

 membranes have been described- 

 the mernbrana limitans externa 

 between the second and third layers, 

 and the membrana limitans iuterna 

 between the eighth layer and the 

 hyaloid membrane of the vitreous 

 body. 



Fig. 154 shows under a high 

 magnification the appearance of 

 the strata in the retina of a human 

 eye, enucleated from a living man, 

 and immediately placed in a fixing 

 solution. 



It is very difficult from this 

 simple figure to form a clear idea 

 of the histological elements of 

 which the retina is built up and 

 of their reciprocal relations. But they have been fully studied 

 by Golgi's silver method and by Ehrlich's vital methylene blue 

 method. 



The most external layer (formerly described with the choroid 

 coat) consists of a single stratum of hexagonal epithelial cells, 

 which as seen from the surface form a regular mosaic, and from 

 the side present an outer, nucleated, non-pigmented part, and an 

 inner, pigim-nted portion prolonged into filiform processes or 



:.. 



l.i 



Fie.. I"i4. Vertical section of human retina. 

 lixi'il immediately after enucleation of Bye- 

 ball in 'IVlhesiiiesxky fluid. Magnified 510 

 diameters. (1>. Cliiaiini.) 1. layer of ],i-- 

 menled epithelial cells; 2, layer of rods 

 and rimes; :;. outer nuclear (granular) 

 layer; 4, outer molecular layer; 5, inner 

 nuclear (granular) layer : (!, inner molecular 

 layer; 7, layer nf uanjilinn cells; S, layer 

 of iiptie nerve-iibres ; /.'., ineuiliiana limi- 

 tans e\terna ; l.i., inemluana limitans 

 interna. 



