618 ORGANS OF THE SENSES. 



The CHOROID is a thin, highly vascular membrane, of a dark brown or chocolate 

 color, which invests the posterior five-sixths of the central part of the globe. It 

 is pierced behind by the optic nerve, and terminates in front at the ciliary ligament, 

 where it bends inwards, and forms on its inner surface a series of folds or plaitings, 

 the ciliary processes. It is thicker behind than in front. Externally, it is con- 

 nected by a fine cellular web (membrana fused) with the inner surface of the 

 sclerotic. Its inner surface is smooth, and lies in contact with the retina. The 

 choroid is composed of three layers, external, middle, and internal. 



Fig. 305. The Veins of the Choroid. (Enlarged.) 



The external layer consists of the larger branches of the short ciliary arteries, 

 which run forwards between the veins before they bend downwards to terminate 

 on the inner surface. This coat consists, however, principally of veins, which 

 are disposed in curves ; hence their name, vense vorticosas. They converge to four 

 or five equidistant trunks, which pierce the sclerotic midway between the margin 

 of the cornea and the entrance of the optic nerve. Interspersed between the 

 vessels, are lodged dark star-shaped pigment cells, the fibrous offsets from which, 

 communicating with similar branches from neighboring cells, form a delicate 

 network, which, towards the inner surface of the choroid, loses its pigmentary 

 character. 



The middle layer consists of an exceedingly fine capillary plexus, formed by the 

 short ciliary vessels, and is known as the tunica Ruyschiana. The network is 

 close, and finer at the hinder part of the choroid than in front. About half an 

 inch behind the cornea, its meshes become larger, and are continuous with those 

 of the ciliary processes. 



The internal or pigmentary layer is a delicate membrane, consisting of a 

 single layer of hexagonal nucleated cells, loaded with pigment granules, and 

 applied to each other, so as to resemble a tessellated pavement. Each cell 

 contains a nucleus, and is filled with grains of pigment, which are in greater 

 abundance at the circumference of the cell. In perfect albinos this epithelium 

 contains no pigment, and none is present in the star-shaped cells found in the 

 other layers of the choroid. 



The ciliary processes should be next examined ; they may be exposed, either by detaching the 

 iris from its connection with the ciliary ligament, or by making a transverse section of the globe, 

 and examining them from behind. 



The Ciliary processes are formed by the plaiting or folding inwards of the 

 middle and internal layers of the choroid, at its anterior margin, and are received 



