THE IRIS, RETINA, AND HUMORS. 



385 



on which are seen, c, c, the vasa vorticosa ; <#, d, ciliary nerves ; 0, ciliary 

 ligaments ; /*, anterior face of iris ; <7, pupil. 



Fig. 199. Fig. 200. 



The veins of the choroid. The arteries of the choroid. 



Fig. 200: a, a, exterior surface of the choroid and the iris, showing the 

 arterial network of these two membranes, supplied by the ciliary arteries, 

 which, after having traversed the sclerotic, divide into, 5, , posterior cili- 

 aries for the choroid, and <?, c, anterior ciliaries for the iris. 



The iris, though arising, as has been said, from the choroid, is con- 

 structed in a different way, its tissue mainly consisting of un- 

 striped muscular fibre, except in the case of birds, which present 

 the striped variety. It is extremely vascular, its arteries being derived 

 from the ciliary. The color of the eye depends on the color of the front 

 of the iris ; the posterior portion is covered with black pigment. 



The ciliary muscle, for such it has been proved to be by Dr. Wallace, 

 of New York, is of the unstriped kind ; its action is to move the lens. 

 In birds it is of the striped variety. 



The retina intervenes between the vitreous humor and the choroid 

 coat ; it arises from the tubules of the optic nerve, which have _ 



-, . . . -, . , i , The retina. 



cast off their covering investitures on their passage through the 

 sclerotic. It extends forward to the ciliary body ; is perfectly trans- 

 parent during life, though it soon becomes semi-transparent. Its struc- 

 ture, a knowledge of which is of the utmost importance in the theory of 

 vision, will be presently described. 



201. In the optical axis of the eye there is upon the ret- 



ina a spot of about the twentieth of an inch in diam- 

 eter, called the yellow spot of Soemmering. Its posi- 

 tion is shown in Fig. 201. The entrance of the optic 

 nerve is at the blind spot which is marked at some dis- 

 tance on one side. The vitreous humor is contained in 

 spot of soem- a delicate mesh of transparent tissue, which The vitreous 



causes it, when removed, to present the aspect humor - 

 of a jelly. In front it receives the crystalline lens, which is contained 

 in a closed capsule. Eound the lens there is a passage, known as the 

 canal of Petit, which enables the ciliary muscle to move the lens. The 



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