RETINA. G21 



yellow spot, of Summering ; having a central depression at its summit, the fovea 

 centralis. The retina in the situation of the fovea centralis is exceedingly thin, so' 

 much so, that the dark color of the choroid is distinctly seen through it ; so that 

 it presents more the appearance of a foramen, and hence the name " foramen of 

 Sommering" at first given to it. It exists only in man, the quadrumana, and some 

 saurian reptiles. Its use is unknown. About T V of an inch to the inner side of 

 the yellow spot is the point of entrance of the optic nerve ; the arteria centralis 

 retinse piercing its centre. This is the only part of the surface of the retina from 

 which the power of vision is absent. 



Fig. 307. The Arteria Centralis Retinse, Yellow Spot, etc., the Anterior Half 

 of the Eyeball being removed. (Enlarged.) 



Sclerotic 

 Chore iff, 

 Eetina, 



Structure. The retina is composed of three layers, together with bloodvessels : 



External or columnar layer (Jacob's membrane). 

 Middle or granular layer. 

 Internal or nervous layer. 



The bloodvessels do not form a distinct layer ; they ramify in the substance of 

 the internal layer. 



The external or Jacob" 1 s 'membrane is exceedingly thin, and can be detached from 

 the external surface of the retina by the handle of the scalpel, in the form of a 

 flocculent film. It is thicker behind than in front, and consists of rod-like bodies 

 of two kinds : 1. Columnar rods, solid, nearly of uniform size, and arranged per- 

 pendicularly to the surface. 2. Bulbous particles or cones, which are interspersed 

 at regular intervals among the former ; these are conical or flask-shaped, their 

 broad ends resting upon the granular layer, the narrow-pointed extremity being 

 turned towards the choroid ; they are not solid, like the columnar rods, but consist 

 of an external membrane with fluid contents. By their deep ends, both kinds are 

 joined to the fibres of Muller. 



The middle or granular layer forms about one-third of the entire thickness of 

 the retina. It consists of two laminas of rounded or oval nuclear particles, separated 

 from each other by an intermediate layer, which is transparent, finely fibrillated, 

 and contains no bloodvessels. The outermost layer is the thickest, and its con- 

 stituent particles are globular. The innermost layer is the thinnest ; its component 

 particles are flattened, looking like pieces of money seen edgeways ; hence it has 

 been called by Bowman, the nummular layer. 



The internal or nervous layer is a thin semi-transparent membrane, consisting 

 of an expansion of the terminal fibres of the optic nerve and nerve cells. The 

 nerve fibres are collected into bundles, which radiate from the point at which the 

 trunk of the optic nerve terminates. As they proceed in a tolerably straight 



