ANATOMY OF THE EYEBALL. 677 



tion to blood-vessels, nerves and fibrous tissue, a few irregularly shaped pig- 

 ment-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 lluyschiana. The arteries, which 

 are derived from the posterior short ciliary arteries and are connected with 

 the capillary plexus, lie just beneath the pigmentary 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 very peculiar appearance, and they have been called the vasa vorti- 

 cosa. The pigmentary portion is composed, over the greatest part of the 

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

 measuring ^J^ to y^nr of an inch (12 to 16 p.) in diameter. These cells are 

 filled with pigmentary 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 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 occu- 

 pied 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 downward 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 mm.) in length. They 

 are sixty to eighty in number. The larger folds are of nearly uniform size 

 and are regularly arranged around the margin of the crystalline lens. Be- 

 tween 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 membrane, 

 continuous anteriorly with the hyaloid membrane of the vitreous humor, 

 called the zone of Zinn. 



The ciliary processes present blood-vessels, which are somewhat larger 

 than those of the rest of the choroid. The pigmentary cells are smaller and 

 are arranged in several layers. The anterior border of the processes is free 

 and contains little or no pigment. 



Ciliary Muscle. This muscle, formerly known as the ciliary ligament 

 and now sometimes called the tensor of the choroid, is the agent for the 

 accommodation of the eye to vision at different distances. Under this view, 

 the ciliary muscle is an organ of great importance, and it is essential, in the 

 study of accommodation, to have an exact idea of its relations to the coats 

 of the eye and to the crystalline lens. 



The form and situation of the ciliary muscle are as follows : It surrounds 



