ANATOMY OF THE EYEBALL. 49 



posterior portion of the membrane. The veins are external 

 to the other vessels. They are very numerous and are dis- 

 posed in curves converging to four trunks. This arrange- 

 ment 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 20 ^ 00 to Y^V<r ^ an 

 inch in diameter. These cells are filled with pigmentary 

 granulations of uniform size, and give to the membrane its 

 characteristic dark-brown or chocolate color. The pigmen- 

 tary granules in the cells are less numerous near their centre, 

 where a clear nucleus can readily be observed. In the an- 

 terior 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. 



Ciliary Processes. The anterior portion of the choroid 

 is arranged in the form of folds, or plaits projecting inter- 

 nally, called the ciliary processes. The largest of these folds 

 are about -$ of an inch in length. They are from sixty to 

 eighty in number. The larger folds are of nearly uniform 

 size, and are regularly arranged around the margin of the 

 crystalline lens. Between these folds, which constitute about 

 two-thirds of the entire number, are smaller folds, lying, 

 without any regular alternation, between the larger. Within 

 the folds, are received corresponding folds of the thick mem- 

 brane, continuous anteriorly with the hyaloid membrane of 

 the vitreous humor, called the zone of Zinn. 



The ciliary processes present blood-vessels, which are some- 

 what larger than those of the rest of the choroid. The pig- 

 mentary cells are smaller and are arranged in several layers. 

 The anterior border of the processes is free, and contains 

 little or no pigment. 



