420 NERVOUS SYSTEM. 



They run from before backwards, and the branches which con- 

 cur to form them, being adjacent with, and parallel to each other, 

 for the most part, form large curves, the convexity of which is 

 forwards; they, moreover, anastomose freely, and thereby pro- 

 duce a vascular sort of net-work, filling up^the concavity of some 

 of the curves. These veins, called the Vasa Vorticosa, are 

 nearer the external surface of the choroides than the arteries, 

 and are assembled into twelve or fourteen trunks, which, en- 

 gaging in the sclerotica, near its middle, run for some distance 

 in its substance, and. then, by their junction, are reduced to four 

 or five in number. The latter, disengaging themselves from 

 the eye, join, subsequently, the ophthalmic vein. 



In addition to the veins mentioned, the long ciliary arteries 

 have their venae comites, which take a course parallel to and 

 adjoining them. These veins do not observe the vortical ar- 

 rangement of the others ; they bring back the blood of the iris, 

 and terminate in the larger trunks of the others. 



This structure has been most cautiously explored by the ce- 

 lebrated Soemmering, and his observations have tended very 

 much to determine the opinions of anatomists concerning many 

 parts of the eye. A curious remark of his is, that " the human 

 eye may be distinguished from that of animals by a form of this 

 vascular net work, entirely peculiar; for example, in the eye of 

 the ape, its vascular tissue differs not only from that of the hu- 

 man subject, but also from that of the dog, and still more evi- 

 dently from that of the calf. From which cause, it would be 

 as easy to distinguish with a microscope, the choroides, well 

 injected, of different animals, even a piece of only the forty- 

 eighth part of an inch in extent, as it is easy to distinguish a 

 poplar stripped of its leaves from an oak, a pear tree, an apple 

 tree, or any other tree, by the arrangement of its trunk and 

 branches. 



The choroides, on its internal face, is not smooth, but velvety, 

 which becomes still more conspicuous when the eye is finely 

 injected and examined with a microscope. Meckel considers 

 the appearance to depend upon its very fine tissue of vessels. 

 This surface is called Tapetum. In the bullock, and some 

 other animals, at a particular part, it presents a shining, silvery 

 appearance, and may be torn off from the external surface. 



