ORGANS OF THE SENSES. 519 



That these gelatinous fibres constitute the principal portion 

 of the fibrous layer of the retina, and that no tubular nerve 

 fibres exist in the retina itself, are points upon which not the 

 smallest doubt can be entertained. 



Of the reality of the transformation of the tubular into 

 the gelatinous nerve filament, that is, the conversion of a 

 tubular, unbranched, and unnucleated structure into a 

 branched, and nuclear tissue, great misgivings might be well 

 entertained ; an attentive study of the structure of the fibrous 

 gray layer of the retina renders it very difficult, however, to 

 deny the reality of such a structural transition. 



The vascular lamina is the last of the layers of the retina : 

 it would appear to be entirely distributed upon the inner 

 surface of the fibrous layer ; for if we take a perfectly fresh 

 eye and spread the retina out with its inner surface upwards, 

 we can readily see the larger blood-vessels filled with blood 

 corpuscles, and having the fibrous layer situated immediately 

 behind them. (See Plate LXVII. fig. 2.) 



The optic nerves consist of several bundles of nerve tubules : 

 these are very slender and brittle, and interspersed with deli- 

 cate globular cells ; in these last two particulars these nerves 

 correspond with the white fibrous portions of the brain. 



The transparent media of the eye are the vitreous body and 

 the crystalline lens with its capsule. 



Vitreous Body. 



The vitreous humour is enclosed in a perfectly structureless 

 and exceedingly delicate membrane called the hyaloid mem- 

 brane : this does not enclose the whole of the vitreous 

 humour, but is deficient behind the crystalline lens, it being 

 inserted into the side of the capsule of that body. 



From all points of the inner surface of this membrane 

 fibres proceed; these interlace with each other in such a 

 manner as to form a cellated structure. 



The size and structure of these cells may be readily seen 

 with an inch or half-inch object-glass ; and the best view of 



