390 STEUCTUEE OF THE EETINA. 



described as an expansion of the optic nerve, intervenes between the vit- 

 reous humor and the choroid coat. 



Regarding it as composed of distinct layers, the innermost of which, 

 Construction in contact with the hyaloid membrane, is called the fibrous 

 f nd Jacobs S ra 7 ^ a 7 er ' ar i ges fr m tne tubules of the optic nerve, which 

 membrane, have cast off the white substance of Schwann ; and in pass- 

 ing, we may dwell emphatically upon the point that at that spot, where 

 it exists alone, that is to say, where the optic nerve is entering the eye, 

 vision can not be performed. Beneath, or outside this fibrous layer, 

 comes the gray vesicular layer: it is analogous to the vesicular matter 

 of the brain. The two layers thus far described are served with capillary 

 blood-vessels of extreme minuteness. Outside of the gray vesicular lay- 

 er is the granular layer, which, as its name imports, consists of a conge- 

 ries of granules, which are puobably the origin of the vesicles, new ones 

 arising from this layer continually. Yet again, outside of the granular 

 layer, comes a delicate sheet, known as the membrane of Jacob, but which 

 is formed, in reality, from the juxtaposition of a set of rod-shaped and 

 conical bodies, the thicker ends of the rods being outward, the thinner 

 inward. 



ig. 202. Fig. 202 shows the partial detachment of 



the membrane of Jacob from the exterior of the 

 retina. The membrane appears as delicate 

 shreds, and may be advantageously demon- 

 strated after the removal of the choroid, the 

 specimen being placed under water. 



In the preceding description I have followed 

 the course usually taken by former anatomists, 

 who describe the retina as consisting of suc- 

 cessive layers or strata, but much more philo- 

 Membrane of Jacob. sophical views are obtained by considering it in 



Per endicuiar ^e manner introduced by H. Miiller, that is to say, in its 

 examination radial section. From this it appears that the four strata 

 of the retina. ^ QyQ ^^0,^^ y^ \ t Jacob's layer of rods and cones ; 2. 

 The granular layer ; 3. The vesicular layer ; 4. The fibres of the optic 

 nerve, are, in reality, all connected in such a way that, passing in a radial 

 direction as respects the globe of the eye, all these different elements are 

 successively combined, constituting what is termed the radiated fibre 

 system. Thus from each of the proper fibres of the optic nerve a thread- 

 Eadial fibre like body passes radially through the thickness of the retina, 

 system. including in its outward passage a vesicle, and again, beyond 

 that, a granule, and, still farther, a cone, and terminating in a rod ; so 

 that from the extremity of the rod there is a continuous communication 

 through the thickness of the retina to the fibres of the optic nerve ; the 



