646 MANUAL OF HISTOLOGY. 



nerve fibres. At the edge of this zone the most exquisite recurrent 

 capillary loops may be seen. In birds, amphibia, and fishes the .retina 

 is even entirely devoid of vessels, but the membrana hyaloidea is fed by 

 a network which probably also supplies the retina (ffyrtl, H. Miiller). 



The question now arises at the conclusion of this long description of 

 the retina, What is the arrangement and connection of its nervous 

 elements ? and here we have only hypotheses to offer. That the rods and 

 cones may be regarded as the terminal perceptive elements of the mem- 

 brane is to our mind a point upon which there can be now but very little 

 doubt. Bacillary cells, besides, we know have recently been recognised 

 as the terminal structures of other nerves of special sense. Another ques- 

 tion arises here also from which we cannot refrain, although a physio- 

 logical one, namely, What are the relative purposes of "the rods and cones 1 



We hav* learned from the foregoing description, that the point at 

 which the power of perception is most intense, the fovea centralis, pre,- 

 sents only cones in the human eye. Mammals of more. nocturnal habits 

 ( 316) have, on the other hand, rods alone throughout the whole retina. 

 It is to be remembered, also, that towards the external border of the retina 

 the nervous fibrous mass undergoes considerable and progressive diminu- 

 tion. 



The probability has been pointed out by Schuttze, that the latter 

 structures are endowed with the powers of perceiving quantitative differ- 

 ences in light and in distance, while the cones, besides possessing both 

 these, are sensible to colours also, that is, to qualitative differences in 

 light. The fibres, then, of both these retinal structures, which traverse 

 the external granular layer, may be regarded as nervous elements together 

 with the corpuscles. The efforts, however, of earlier investigators to 

 follow up these nervous fibres in a direct perpendicular course through 

 the internal layers, down to the stratum of the ganglion cells, are to be 

 looked upon as fruitless. Since the provisional ending of both rod and 

 cone fibres in the inter-granular layer, consequently in the very middle 

 of the retina, has been pointed out by Schultze, and also the apparent 

 origin of another set of the finest fibrillae pursuing an altered course 

 inwards, it seems vain to hope to demonstrate, with our present modes 

 of investigation, and through such a complicated system of fibres, the 

 connection between the cones and ganglion cells and optic fibres. In 

 this respect the grey matter of the nervous centres and retina resemble 

 each other. 



There has been no lack, however, of other views regarding the retina 

 in recent times. 



Thus Henle contrasts the external half of the membrane, as far as the 

 inter-granular layer, under the name of "mosaic, stratum" (musivische 

 Schicht), with the internal half or "true nervous layer." We fail to per- 

 ceive any great advantage in this. 



An effort has recently been made also by Krause to prove that the 

 retina, down to the inter-granular layer, his membrana fenestrata, is by 

 no means of nervous nature. Apart from anatomical considerations, he 

 bases his views upon the fact, that some weeks after section of the optic 

 nerve, the nerve fibres and ganglion cells of the retina may be observed 

 to have undergone fatty degeneration, but the whole rod and cone apparatus 

 to remain unaltered. 



As to the composition of the retina we know but little. Some investi- 

 gations by C. Schmidt have brought into notice a substance contained in 



