ORGANS OP THE SENSES. 517 



surface of the choroidal epithelium, and the tails disposed in 

 an opposite direction. Although these cells adhere together 

 with sufficient firmness to constitute a distinct membrane, it 

 would appear that they possess a certain power of movement 

 upon each other, for it is only on such a supposition that we 

 can explain satisfactorily the fibrous appearance which this 

 membrane frequently presents when viewed in extenso. (See 

 Plate LXYII. fig. 9.) 



The tunica Jacobi, although certainly not a nervous struc- 

 ture, is yet properly enumerated as one of the layers of the 

 retina, since it never adheres on the removal of the latter to 

 the choroidal epithelium, but always to the second or granular 

 layer of the retina itself : on account of its extreme frailty 

 and delicacy, this membrane is only to be satisfactorily 

 studied in extremely fresh eyes. A few hours after death 

 the cells separate from each other, and the heads of the cells 

 become disjointed from the tails, so that in the course of a 

 short time not a vestige of the membrane remains. 



Each cell of the stratum bacillosum bears not an inexact 

 resemblance to a human spermatozoon, than which it is how- 

 ever less considerable in size. 



The granular layer consists not of granules merely, as the 

 name implies, but of numerous nuclei imbedded in granular 

 matter, and each of which contains several dark spots which 

 reflect the light strongly. This layer is of considerable thick- 

 ness, and is described in the "Physiological Anatomy" as being 

 divided into two, of which the inner is much the narrower, 

 by a pale stratum which can only be seen by very careful 

 manipulation. The nuclei of which it is composed bear much 

 resemblance to those which occur in the convolutions of the 

 brain, and are most probably of the same nature. (See Plate 

 LXVII. figs. 5, 6.) 



The next is the ganglionary layer: this appears to have 

 been hitherto altogether overlooked ; its discovery supplies a 

 desideratum in the anatomy of the eye, and clearly shows 

 the really nervous character of the granular and vesicular 

 strata of the retina, which many persons have been much 

 disposed to doubt. 



