THE SENSES. 199 



between the cells anastomosing with horizontal branches, and forming a 

 deep intra-epithelial plexus, from which fibres ascend, till near the surface 

 they form a superficial intra-epithelial network. 



Structure of Choroid (tunica vasculosa). This coat of the eye- 

 ball is formed by a very rich network of capillaries (chorio-capillaris) out- 

 side which again are connective-tissue layers of stellate pigmented cells 

 (Fig. 25) with numerous arteries and veins. 



The choroid coat ends in front in what are called the ciliary processes 

 (Fig. 365). 



Structure of Retina. The retina (Fig. 370) is a delicate mem- 

 brane, concave, with the concavity directed forward and ending in front, 

 near the outer part of the ciliary processes in a finely notched edge, the 

 ora serrata. Semi-transparent when fresh, it soon becomes clouded and 

 opaque, with a pinkish tint from the blood in its minute vessels. ' It results 

 from the sudden spreading out or expansion of the optic nerve, of whose 

 terminal fibres, apparently deprived of their external white substance, 

 together with nerve cells, it is essentially composed. 



Fia. 369. Surface view of part of lamella of kitten's cornea, prepared first with caustic potash 

 and then with nitrate of silver. (By this method the branched cornea-corpuscles with their granular 

 protoplasm and large oval nuclei are brought out.) x 450. (Klein and Noble Smith.) 



Exactly in the centre of the retina, and at a point thus corresponding to 

 the axis of the eye in which the sense of vision is most perfect, is a round 

 yellowish elevated spot, about -fa of an inch in diameter, having a minute 

 aperture at its summit, and called after its discoverer the yelloio spot of 

 Scemmering. In its centre is a minute depression called fovea centralis. 

 About -^ of an inch to the inner side of the yellow spot, and consequently 

 of the axis of the eye, is the point at which the optic nerve begins to 

 spread out its fibres to form the retina. This is the only point of the 

 surface of the retina from which the power of vision is absent. . 



The retina consists of certain nervous elements arranged in several 

 layers, and supported by a very delicate connective tissue. 



From the nature of the case there is considerable uncertainty as to the 

 character (nervous or connective tissue) of some of the layers of the retina. 

 The following ten layers, from within outward, are usually to be distin- 

 guished in a vertical section (Figs. 370, 373). 



