436 OltGANS OF THE SENSES. 



1. As a vascular organ (having numerous ciliary or 

 choroid arteries, and groups of veins, forming the vasa vor- 

 ticosa} its purpose is to serve as an organ of calefaction to 

 the nerve membrane beneath it (the retina). We have seen 

 that those organs which contain numerous nerve termina- 

 tions, particularly the organs of the special senses, usually 

 have an abundance of blood vessels, as we see in the papillae 

 of the inner aspect of the fingers, the olfactory membrane, 

 the tongue, etc. 



2. The pigment of the inner surface of the choroid is of 

 great importance to sight ; the retina being transparent, the 

 rays of light fall upon the choroid pigment ; the effect pro- 

 duced is not yet perfectly understood. It may be that this 

 layer absorbs the more irritating rays, and serves as a reflect- 

 ing mirror to the others, which affect the terminal organs of 

 the nerve fibres of the retina ; we shall find, indeed, that the 

 free surface of the sensory elements of the retina is turned 

 towards the choroid, and that these elements are, undoubt- 

 edly, affected only by the rays reflected in this sort of mirror 

 (Ch. Rouget). This pigment layer is not always quite 

 black. There are various shades in different animals ; thus, 

 in the ox, it exhibits metallic reflections exactly resembling 

 the surface of a mirror.. This pigment layer, which is so 

 dark and opaque in other parts, is, perhaps, like the black 

 covering with which the inner surface of a camera obscura 

 is lined, to prevent the irregular reverberation in all direc- 

 tions of the rays of light, and thus to promote the distinct- 

 ness of the sight ; animals which have no choroid pigment 

 (albinos} are scarcely able to endure a strong light (helio- 

 phobia). The choroid pigment is, certainly, a valuable aid to 

 sight, and the weakening of the sight in old age is partly 

 owing to a loss of color of the inner surface of the choroid. 



3. Finally, the muscular elements of the choroid (ciliary 

 muscles} which are developed principally in its anterior part, 

 and joined to erectile prolongations (ciliary process}, are 

 chiefly intended to act upon the crystalline lens, and produce 

 the changes of form which we have studied in regard to 

 accommodation ; great differences of opinion, however, prevail 

 as to the mechanism by which the muscular action affects the 

 lens (Fig. 118). The ciliary muscle is composed of longitu- 

 dinal and circular fibres. The former act by drawing for- 

 ward, from a fixed point at the junction of the sclerotic coat 

 and the cornea (near the canal of Schlemm, sinus circularis 

 iridis}, the whole choroid membrane, and, consequently, the 



