THE RETINA. 19 



retina, where the sense of sight is especially developed, no nerve-termina- 

 tions, looped or otherwise, have yet heen found, (except by Krause,) and 

 that, therefore, the opinion that each sensitive point of the retina cor- 

 responds to the extremity of a separate nerve-fibre is not founded in 

 fact. 



The observations of all who have recently examined the minute struc- 

 ture of the retina, concur in describing the existence of numerous cells 

 and globules surrounding the fibrous expansion of this membrane, and 

 situated chiefly along its internal surface and within the meshes formed by 

 the interlacing of the individual nerve-fibres. These cellular bodies 

 appear to be of different kinds, although, as Henle observes, it is probable 

 that the several varieties met with, are only the same cells in different stages 

 of development. The larger and more perfectly developed cells imme- 

 diately surround the fibrous layer. By Valentin,* who first accurately 

 described them, they were considered as identical with the ganglion-cor- 

 puscles of nervous substance. According to his account, which has since 

 been generally confirmed, these cells when viewed separately are seen to 

 consist of an external transparent membrane, granular contents, and a 

 clear vesicular nucleus, containing a single particle in its centre. These 

 cells lie closely packed together, and by this compression frequently lose 

 their original round form. Valentin states that they are situated only on 

 the internal surface of the fibrous expansion of the optic nerve, and within 

 the meshes of this layer: Henle f makes a similar remark. But Han- 

 nover,} Krause, Pappenheim,|| and others have observed them on the ex- 

 ternal as well as the internal surface of this layer, which appears, therefore, 

 to be imbedded in cells. The observations of Dr. Todd and Mr. BowmanH 

 also, would seem to shew that the cells may occur on both sides. Pap- 

 penheim is of opinion that the fibrous expansion consists of two distinct 

 laminae : and Huschke, adopting this view, as well as the opinion that the 

 cells are situated on both surfaces of the fibrous expansion, thinks that the 

 external stratum of the cells corresponds to one lamina of the fibrous layer, 

 the internal stratum to the other. Henle** discusses at some length, the 

 question as to whether these vesicles should be regarded as analogous to 

 ganglion-corpuscles, and is inclined to doubt the analogy. For, as he 

 observes, beyond their general cellular character, they bear no other re- 

 semblance to ganglion-corpuscles ; and he is of opinion that they probably 

 constitute a kind of transparent epithelium, which serves to invest the 

 delicate nerves composing the fibrous layer. Very shortly after death 

 these cells break up, and the place which they occupied becomes a con- 

 fused granular mass, in which are scattered, often in a linear direction, 



* Repertorium, 1837, p. 25. t Allg. Anat. p. 663. 



J MUller's Archiv. 1840, p. 340. Valentin's Repertorium, 1842, p. 169. 



|| Huschke, Eingeweiden und Sinnes-org. des Menschl. Korpers, p. 719. 



II Op. cit.p. 29. ** Op. cit. p. 665. 



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