936 



THE SENSORY APPARATUSES. 



Fiff. 509. 



Layer of ganglion-cells. 4. Molecular layer. 5. Inner granular layer. 6. In- 

 termediate layer. 7. Outer granular layer. 8. Outer limitary membrane. 9. 

 Layer of rods and cones. 10. Pigmentary layer of the choroid, if this be attached 

 to the retina, as Schultze proposes. 



It is to be remarked, that at the om serrafa all the nerve-elements of the 

 retina disappear. 



Pigment Layer. — This is represented by a row of cells spread on the posterior 

 face of the rod-and-cone stratum. These cells have 

 many small prolongations, which are insinuated be- 

 tween the rods and cones, and the elongated or 

 rounded pigmentary granules of a more or less dark 

 colour. 



(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 particular 

 vascular distribution. The arteria centralis retinm, 

 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 immediately 

 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. 



The distribution of the vessels in the different strata of the retina is variable. 

 They are only found in the layer of nerve-fibres, in the retina of the Horse, Rabbit, 

 and Guinea-pig ; but they are seen in all the layers — that of the rods excepted — 

 of the retina of the other animals. 



CAPILLARIES IN THE VASCULAR 

 LAYER OF THE RETINA. 



THE MEDIA OP THE EYE. 



1. The Crystalline Lens (Fig. 502,/). 



The lens, as its name imphes, is a (solid) transparent body, sustained at the 

 smaller circumference of the zone formed by the ciliary processes (behind the 

 pupil, and partially embedded in the vitreous humour). It is biconvex in shape, 

 and flatter on its anterior than its posterior surface. "We have measured the lens 

 of the Horse's eye, and find the following dimensions : vertical diameter -j^ 

 and transverse diameter ^^ of an inch. The posterior face is evidently more 

 convex than the anterior, for we found the transverse diameter of the last to be 

 r%, and that of the first y\ of an inch. 



Structure. — The lens is enveloped in a transparent membrane — the capsule 

 — which is not adherent to it. Its thickness is uniform in the Horse, and its 

 tissue is slightly striated transversely ; its internal face is lined by a layer of 

 pavement epithelium. (It is a homogeneous elastic membrane, partly of cuticular 

 formation by epithelial cells, and partly an altered product of the embryonic 

 connective tissue.) 



The proper tissue (substantia propria) of the lens is disposed in concentric 

 layers, the outer of which are almost fluid (gelatinous), but their consistence 

 gradually increases towards the centre. These layers are composed of hexagonal 



