588 HANDBOOK OF PHYSIOLOGY. 



separated irom the retina by a fine elastic membrane, which is either 

 structureless or finely fibrillated. 



The choroid coat ends in front in what are called the ciliary pro- 

 cesses (Fig. 399). 



Structure of the Retina. The retina (Fig. 396) is a delicate mem- 

 brane, concave, with the concavity directed forwards and ending in 

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

 edge the ora serrata. Semitransparent 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 exter- 

 nal white substance, together with nerve cells, it is essentially composed. 



Exactly in the centre of the retina, and at a point thus correspond- 

 ing to the axis of the eye in which the sense of vision is most perfect, is 

 a round yellowish elevated spot, about -^j- of an inch in diameter, having 

 a minute aperture at its summit, and called after its discoverer the yel- 



Fio. 395. Surface view of part of lamella of kitten's cornea, prepared first with caustic potash 

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

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



low spot of Simmering. In its centre is a minute depression called fo- 

 vea centralis. About T V 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 ab- 

 sent. 



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 still considerable uncertainty as 

 to the character (nervous or connective tissue) of some of the layers of 

 the retina. The following ten layers, from within outwards, are usually 

 to be distinguished in a vertical section (Figs. 396, 399). 



1. Membrana limitans internet : a delicate membrane in contact with 

 the vitreous humor. 



2. Fibres of optic nerve. This layer is of very varying thickness in 

 ^different parts of the retina: it consists chiefly of non-medullated fibres 



