826 



TSE APPAEATV8 OF THE SENSES. 



Fig. 389. 



Fig. 390. 



contimious with the posterior face of the ciliary processes, where it 



afterwards forms the zonida of Zinn. 



(At the papilla conica, all the other elements 

 than the nerve-fibres are entirely absent ; hence this 

 is presumed to be a 

 « blind spot.") 



Blood-vessels. — The 

 retina possesses a par- 

 ticular vascular distri- 

 bution. The arteria 

 centralis retinae, with its 

 vein, enters the optic 

 nerve at a short distance 

 from the globe, and 

 with it passes into the 

 eye; they traverse the 

 papilla, and immediate- 

 ly divide into two 

 branches, one of which 

 is directed upwards, 

 the other downwards. 



Close and fine anastomoses unite the vessels of 

 the retina with the ciliary vessels at the back of the 

 sclerotic. 



CAPILLARIES IN" THE VASCULAK 

 LAYER OF THE RETINA. 



^mli 



DIAGRAM OF THE STRUC- 

 TURE OP THE RETINA. 



p, Pigment cell of the 

 retina connected with a 

 rod ; n, Cone seated on 

 the membrana limitans 

 externa, the inner seg- 

 ment containing a cone 



THE MEDIA OP THE EYE. 



The Crystalline Lens. (Fig. 383, j) 



The lens, as its name implies, is a (solid) trans- 

 ellipsoid, and a needle ; parent body. Sustained at the smaller circumference 

 m, /, Proper fibre con- Qf ^he zone formed by the ciliary processes (behind 

 the pupil, and partially imbedded in the vitreous 

 humour). It is biconvex in shape, and flatter on its 

 anterior than its posterior surface. We have mea- 

 sured the lens of the Horse's eye, and find the follow- 

 ing dimensions : vertical diameter y^ths, and trans- 

 verse diameter ^^ihs, of an inch. The posterior face 

 is evidently more convex than the anterior, for we 

 found the transverse diameter of the last to be ^^ths, 

 and that of the first -j^^ths of an inch. 



Strtjctuke. — The lens is enveloped in a trans- 

 connection of the nucleus parent membrane, the capsule, which contracts no 

 with prolongation of a adhesions with it. Its thickness is uniform in the 

 ganglion cell. Horse, and its tissue is slightly striated transversely ; 



its internal face is lined by a layer of pavement epithelium. 



The proper tissue of the lens is disposed in concentric layers, which the 

 microscope proves to be composed of fibres; the outer layers are almost 

 fluid (gelatinous), but their consistence gradually increases towards the 

 centre. Between the proper tissue and the epithelium of the capsule are 

 two or three layers of round cells, whose dissolution some time after death 

 forms the liquor Morgagni — which is conseq^uently nothing more than the 

 result of a cadaveric phenomenon. 



(The capsule of the lens is composed of tissue exactly similar to the 



necting rod and cone 

 with one of the cells of 

 the membrana fenestra, 

 whose cells are in com- 

 munication with the 

 membrana limitans in- 

 terna, m, /, «, by ncieans 

 of a thick radial fibre 

 with an oval nucleus 

 attached ; g, g, Multi- 

 polar ganglion of nerve 

 elements ; o, p, Optic 

 fibrilla ; g, r, i, Supposed 



