ORGANS OF SENSE. 175 



The ciliary vessels which traverse the muscle become 

 intimately amalgamated, so to speak, with its sub- 

 stance, and thus constitute an erectile apparatus 

 (Kouget). The posterior half of the choroid is com- 

 posed of vessels united together by very delicate con- 

 necting tissue, which contains some plasmatic cells. 



Each of the surfaces of the iris is covered by a 

 simple layer of epithelium. That upon its anterior 

 aspect has been already examined (fig. III. 5) ; the 

 epithelium upon its posterior surface (uvea) is com- 

 posed of polygonal pigment cells similar to those of 

 the choroid. In addition to the blood-vessels con- 

 tained in the iris, we find also, in the substance of 

 this membrane, a ring of muscular fibres surrounding 

 the pupil and connected, by its circumference with 

 the converging fibres of the ciliary muscle, and finally 

 a web of connecting tissue full of plasmatic cells. In 

 most eyes, but especially in those of dark color, the 

 majority of these cells contain pigment granules. 



The nerves of the choroid coat and iris (ciliary 

 nerves) are very numerous, and seem intended for 

 the supply of the muscular apparatus belonging to 

 these membranes. 



The retina, which constitutes the third coat of the 

 eye-ball, is coextensive with the choroid coat, beneath 

 which it lies. At the entrance of the optic nerve it 

 is thicker than elsewhere ; in fact a slight prominence 

 is perceptible at this point, which has been designated 

 as the papilla of the retina. At the posterior extre- 

 mity of the antero-posterior axis of the globe, and 

 consequently to the outer side of the pupil, there is, 

 upon the surface of the retina, an elongated depres- 



