456 STRUCTURE OF THE RETINA. [BOOK I. 



posed of multipolar nerve-cells which unquestionably communicate 

 by certain of their processes with the fibres of the second layer, 

 and by other more delicate processes with the delicate reticulum 

 which constitutes a great part of the fourth layer. 



The fourth layer (4, Fig. 62), termed the inner molecular layer, is 

 composed in part of fibres belonging to the connective-tissue frame- 

 work, which afford support to a delicate reticulum which doubtless 

 is the medium of communication between the layers which lie internal 

 and external to it. 



The fifth layer (5, Fig. 62), or internal granular layer (also inner 

 nuclear layer), is conspicuous for the presence of the so-called granules, 

 viz. small transparent nucleated spherical bodies with two poles, of 

 which one points towards the inner the other towards the outer 

 molecular layer, and which are doubtless connected with the networks 

 of those layers. These granules are considered by all to belong to 

 the nervous elements of the retina. 



In addition we observe, however, in the fifth layer certain granules 

 which are probably connective-tissue cells, and radiating fibres (fibres 

 of Muller) which belong to the connective-tissue framework, which is 

 specially well developed in the granular layers. 



The sixth layer (6, Fig. 62), or outer molecular layer, possesses a 

 structure similar to that of the fourth or inner molecular layer, consisting 

 of fibres of which some doubtless belong to the connective-tissue frame- 

 work and merely afford support for a truly nervous reticulum ; this 

 outer molecular layer is much less deep than the inner molecular layer. 



The seventh layer (7, Fig. 62), or external granular layer (also outer 

 nuclear layer), presents many strata of bodies resembling in the main 

 those characteristic of the inner granular layer, and like them 

 presenting nuclei and two poles of which the inner pass to the 

 reticulum of the outer molecular layer ; the granules offer however 

 peculiarities : Istly their external poles very clearly are connected 

 with either the rods or cones of the ninth layer : 2ndly the granules 

 which are connected with the cones (cone- granules) are larger, are 

 situated in the more external strata of the layer, and are directly 

 joined to the cones without the intermediation of fibres, whilst the 

 rod-granules are smaller, are joined to the rods by fibres and present 

 two transverse stripes. 



The eighth layer (8 3 Fig. 62) is composed of the external limiting 

 membrane and, like the first layer, is composed of a fibrillated 

 membrane which forms the external boundary of the connective- 

 tissue framework of the retina; within this boundary the retina 

 is vascular, outside it is absolutely free from blood-vessels. The 

 external limiting membrane is perforated by the communications 

 between the rods and cones and the outer granular layer. 



The ninth layer (9, Fig. 62), bacillary layer or layer of rods and 

 cones, is composed of the bodies which are by common consent and for 

 undeniable reasons considered to be the end-organs which are directly 

 excited by luminous undulations, which initiate the state of activity, 



