THE IRIS. THE RETINA. 



687 



point of entrance of the optic nerve; the arteria centralis retinae piercing its 

 centre. This is the only part of the surface of the retina from which the 

 power of vision is absent. 



Fig. 372. The Arteria Centralis Retinae, Yellow Spot, etc., the Anterior Half of the Eyeball 



being removed. (Enlarged.) 



Structure. The retina is composed of three layers, together with blood- 

 vessels : 



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'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 perpendicularly 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 sol : d, like 

 the columnar rods, but consist of an external membrane with fluid contents. 

 Bv their deep ends, both kinds are joined to the fibres of Miiller. 



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

 the retina. It consists of two laminae 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 thicker, 

 and its constituent particles are globular. The innermost layer is the thinner; 

 its component particles are flattened, looking like pieces of money seen edge- 

 ways; hence it has been -called, by Bowman, ihenummular layer. 



The internal or nervous layer is a thin semitransparent 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 course towards the anterior margin of the retina, the bundles interlace, 

 forming a delicate net, with flattened elongated meshes. The nerve-fibres which 

 form this layer differ from the fibres of the optic nerve in this respect ; they 

 lose their dark outline, arid their tendency to become varicose ; and consist 



