28 INNERVATION. [CHAP. xvn. 



mor, the deepest of those media. It may be said to commence at 

 the foramen in the sclerotic and choroid by which the optic nerve 

 enters the eye, and to terminate by a finely jagged border (the or a 

 serrata) at the hinder border of the striated part of the ciliary 

 body. It is thicker behind, in the deepest part of the globe, and 

 gradually thinner forwards. Of a pinkish gray tint, and semi-trans- 

 parent when fresh, the images formed upon it may be seen through 

 it from behind, if the sclerotic and choroid coats be first carefully 

 removed. When thrown into accidental folds, it resembles in ap- 

 pearance the vascular gray substance of the cerebral convolutions. 

 The exquisite function performed by this nervous membrane, its 

 expanded form and separability from other structures, have always 

 made it an object of peculiar interest with physiologists, who have 

 not unreasonably expected that important secrets of nervous func- 

 tion might be disclosed by an accurate insight into its structure. 

 Whatever the conclusions to be drawn, it is certain that this 

 structure is elaborate and complex, and worthy of an attentive 

 study. 



The first part of the retina to be described is the fibrous gray 

 layer, which forms the immediate continuation of the optic nerve, 

 and which is seated on its inner surface. This is a layer of fibrous 

 character, radiating from the end of the optic nerve, and apparently 

 consisting of the tubular fibres of that nerve deprived of their white 

 substance; that is, being no longer tubular and white, but solid and 

 gray, and united together more or less into a membrane. This at 

 least seems to us to be certain, that the white substance of Schwann 

 does not exist in the nervous substance of the retina, but ceases as 

 the nerve perforates the sclerotic. It has been particularly de- 

 scribed as existing in the retina of the rabbit; but the fact seems to 

 be, that in this animal the nerve does not end in the retina till some 

 way within the globe, for, after bifurcating and spreading out as 

 a white streak within the choroid, the bundles of nerve-tubes 

 suddenly lose their white lustre, and assume the appearance of the 

 gray fibres of the layer now under consideration. These bundles, 

 both in animals and man, may be seen to anastomose in a close plex- 

 iform manner, especially near the optic nerve, and finally constitute 

 a thin sheet, which becomes thinner and less fibrous as we trace it 

 forwards, until at length it can be no longer discerned. This 

 fibrous gray layer of the retina is united to the hyaloid membrane, 

 containing the vitreous humor, by a layer of nucleated cells almost 

 perfectly transparent, and sometimes very difficult of discovery on 

 that account. It is to be remarked, that the fibrous gray layer is 



