THE EYE. 



739 



company the vessels, dividing repeatedly, and ^vith frequent anasto- 

 moses, especially in the annuJus minor, to the pupilhiry margin, where 

 their mode of termination is at present unkno\Yn. The elements of all 

 these nerves are, in the trunks, of the medium and fine kinds, from 

 0-002-0-004, and in the iris of not more than 0-001-0-002 of a line. 

 I have never noticed ganglion-cells in them, nor in the ciliary muscle, 

 ■where Bochdalek describes them as existing. 



Some authors, and among them, recently, Bochdalek, believe that 

 they have seen scattered nerves in the choroid — a circumstance that I 

 am unable to confirm from my own observations. Quite recently, 

 Rainey (" Philos. Magaz.," Ma}^ 1851, p. 420) describes a transversely 

 striped choroideal muscle, occupying the posterior part of the choroid 

 and extending through its entire thickness in the form of variously de- 

 cussating layers, which he says may be most easily demonstrated in the 

 eye of the Sheep. I agree with Henle in thinking that these state- 

 ments rest upon deceptive grounds ; at all events, in the situation in 

 question, either in animals or in Man, I can discover nothing like mus- 

 cular fibres. 



§ 227. Nervous membrane [retina). — The retina is the innermost of 

 the five tunics of the eyeball, and is in close apposition with the choroid, 

 though not coextensive with it, ending at the ora serrata in an undula- 

 ted margin [margo undulato-dentatus s. ora serrata retince), which is 

 very intimately connected, on the one side with the cJioroid, and on the 

 other with the hijaloid membrane. A continuation of the retina on the 

 ciliary portion of the hyaloid membrane, which is described by many 

 anatomists, does not exist. 



The retina is a delicate mem- 

 brane ; when recent, almost per- 

 fectly transparent and clear, and 

 after death whitish and opaque. 

 It commences at the point of en- 

 trance of the optic nerve, with which 

 it is, in part, continuous. Its thick- 

 ness at first is 0-1 of a line, but 

 as it extends anteriorly it soon 

 diminishes to 0-06, until ultimately, 

 close to the anterior border of the 



Fig. 502, 



iliflllSflPiiff!^'---^ 



-A- 



 ^ 



Fig. 302. — Vertical transverse section of the retina of Man, from the posterior portion of 

 the membrane, magnified 250 diameters: a, hyaloid membrane with decumbent nuclei: b, 

 mcmbrana limit ans ; c, clear globules (epithelium?) ; d, expansion of the optic nerve; c, layer 

 of gray nerve-substance;/, internal granular layer; g, fine-granular layer, in which the 

 radiating fibres are more distinct than elsewhere; A, external granular layer; i, internal divi- 

 sion of the bacillar layer, with the "cones;" k, external division, with the prolongations of 

 the " cones" and the true " rods." 



