504 THE SENSE OF SIGHT. 



nected, by means of delicate fibres issuing from them, with the 

 nerve-vesicles of the retina, and even to become continuous 

 with the radiating processes which some of these vesicles pre- 

 sent. Concerning the use of these bodies, the discovery of 

 their connection with the sensitive part of the retina supports 

 the opinion entertained by Kolliker and H. Miiller, that their 

 special office is to receive and transmit impressions of light. 



The structures of which the granular layer is composed are 

 indicated in the figure. 



The nervous layer is composed of nerve-corpuscles and nerve- 

 fibres. The nerve-corpuscles are the outermost, and are most 

 numerous over the yellow spot, and absent altogether from the 

 point of entrance of the optic nerve. They are imbedded in 

 fine molecular matter, which also forms a layer outside them. 

 The nerve-fibres radiate as a fine membranous network from 

 the point of entrance of the optic nerve, of whose fibres they 

 are the continuation. They end probably in the nerve-corpus- 

 cles. The fibres are absent from the yellow spot. 



Two of the fibres of Mutter are, for the sake of illustration, 

 arranged in the figure separately on each side of the layer 

 which they perforate. About the connection of the fibres of 

 Miiller there is some uncertainty. They are supposed to be 

 connected by their outer ends with the rods and cones; and 

 by their inner, which are thought to be modifications of con- 

 nective tissue, they rest on the membrana limitans. Between 

 these points they are supposed to have connections also with 

 some of the other structures through which they pass, espe- 

 cially with the inner layer of nuclei. 



The retinal bloodvessels ramify chiefly in the nervous 

 layer. 



The structures which have been just described are modified 

 in their distribution over the yellow spot in the following man- 

 ner: Of the columnar layer, or membrani Jacobi, the cones 

 greatly predominate; of the nervous layers the cells are nu- 

 merous, while the nerve-fibres are absent. There are capilla- 

 ries here, but none of the larger branches of the retinal ar- 

 teries. Opposite the fovea centralis, there are, moreover, neither 

 the granular, nor the fine molecular layer, nor the fibres of 

 Miiller. 



By means of the retina and the other parts just described, 

 a provision is afforded for enabling the terminal fibres of the 

 optic nerve to receive the impression of rays of light, and to 

 communicate them to the brain, in which they excite the sen- 

 sation of vision. But that light should produce in the retina 

 images of the objects from which it comes, it is necessary that, 



