772 SPECIAL SENSES. 



The choroid is distinguished from the other coats of the eye by its dark color and its 

 great vascularity. It occupies that portion of the eyeball corresponding to the sclerotic. 

 It is perforated posteriorly by the optic nerve and is connected in front with the iris. 

 It is very delicate in its structure and is composed of two or three distinct layers. Its 

 thickness is from -^ to ^V f an mcn - I ts thinnest portion is at about the middle of the 

 eye. Posteriorly it is a little thicker. Its thickest portion is at its anterior border. 



The external surface of the choroid is connected with the sclerotic by vessels, nerves 

 (the long ciliary arteries and the ciliary nerves), and very loose connective tissue. This 

 is sometimes called the membrana fusca, although it can hardly be called a distinct layer. 

 It contains, in addition to the vessels, nerves, and fibrous tissue, a few irregularly-shaped 

 pigment-cells. 



FIG. 242. Choroid coat of the eye. (Sappey.) 



1, optic nerve ; 2, 2, 2, 2, 3, 3, 3, 4, sclerotic coat, divided and turned back to show the choroid ; 5, 5, 5, 5, the cornea, 

 divided into four portions and turned back ; 6, 6, canal of Schlemm ; 7, external surface of the choroid, traversed 

 by the ciliary nerves and one of the long ciliary arteries ; 8, central vessel into which open the vasa vorticosa ; 9, 

 9, 10, 10, choroid zone ; 11, 11, ciliary nerves; 12, long ciliary artery ; 13, 13, 13, 13, anterior ciliary arteries ; 14, iris ; 

 15, 15, vascular circle of the iris ; 16, pupil. 



The rest of the choroid is composed of two distinct layers ; viz., an external, vascular, 

 and an internal, pigmentary layer. The vascular layer consists of numerous arteries, veins, 

 and capillaries, arranged in a peculiar manner. The layer of capillary vessels, which is 

 internal, is sometimes called the middle layer of the choroid, or the tunica Euyschiana. 

 The arteries, which are derived from the posterior short ciliary arteries and are connected 

 with the capillary plexus, lie just beneath the pigmentary layer. The plexus of capilla- 

 ries is closest at the posterior portion of the membrane. The veins are external to the 

 other vessels. They are very numerous and are disposed in curves converging to four 

 trunks. This arrangement gives the veins a very peculiar appearance, and they have 

 been called the vasa vorticosa. The pigmentary portion is composed, over the greatest 

 part of the choroid, of a single layer of regularly polygonal cells, somewhat flattened, 

 measuring from ^^ to ^fa of an inch in diameter. These cells are filled with pig- 

 mentary granulations of uniform size, and they give to the membrane its characteristic 

 dark-brown or chocolate color. The pigmentary granules in the cells are less numerous 

 near their centre, where a clear nucleus can readily be observed. In the anterior por- 

 tion of the membrane, in front of the anterior limit of the retina, the cells are smaller, 

 more rounded, more completely filled with pigment, and present several layers. Beneath 

 the layer of hexagonal pigment-cells, the intervascular spaces of the choroid are occupied 

 by stellate pigment-cells. 



