652 SPECIAL SENSES 



connected in front with the iris. It is delicate in structure and com- 

 posed of two or three distinct layers. Its thickness is g 1 ^ to ^5 of an 

 inch (0.3 to i millimeter). Its 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 and nerves (the long ciliary arteries and the ciliary nerves) 

 and very loose connective tissue. This is sometimes called the mem- 

 brana fusca, although it can hardly be regarded as a distinct layer. It 

 contains, in addition to bloodvessels, nerves and fibrous tissue, a few 

 irregularly-shaped pigment-cells. 



The vascular layer of the choroid consists of arteries, veins and 

 capillaries, arranged in a peculiar manner. The layer of capillary 

 vessels, which is internal, is sometimes called the tunica Ruyschiana. 

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

 and are connected with the capillary plexus, lie just beneath the pig- 

 mentary layer of the retina. The plexus of capillaries is closest at the 

 posterior portion of the membrane. The veins are external to the 

 other vessels. They are very abundant and are disposed in curves 

 converging to four trunks. This arrangement gives the veins a peculiar 

 appearance, and they have been called 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 2~oVo 

 to ygVff of an inch (12 to 16 ft) in diameter. These cells are filled with 

 pigmentary granules of uniform size and give to the membrane its 

 characteristic dark-brown or chocolate color. The pigmentary granules 

 in the cells are less abundant near their centre, where a clear nucleus 

 can readily be observed. In the anterior portion 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. The 

 cells next the layer of rods and cones are regarded as constituting the 

 outer, or pigmentary layer of the retina. These cells send little hair- 

 like processes inward between the rods and cones. 



Ciliary Processes. The anterior portion of the choroid is arranged 

 in the form of folds or plaits projecting internally, called the ciliary 

 processes. The largest of these folds are about ^ of an inch (2.5 milli- 

 meters) in length. They are sixty to eighty in number. The larger 

 folds are of nearly uniform size and are arranged regularly around the 

 margin of the crystalline lens. Between these folds, which constitute 

 about two-thirds of the entire number, are smaller folds, lying, without 



