48 SPECIAL SENSES. 



etc., points that we do not propose to consider. It is well 

 known that the surface of the cornea is exquisitely sensitive. 



Choroid Coat. Calling the sclerotic and the cornea the 

 first coat of the eyeball, the second is the choroid, with the 

 ciliary processes, the ciliary muscle, and the iris. This was 

 called by the older anatomists the uvea, a name which was 

 later applied, sometimes to the entire iris, and sometimes to 

 its posterior, or pigmentary layer. We shall describe, how- 

 ever, the choroid and ciliary processes together as the second 

 coat, and then take up the ciliary muscle and the iris. 



The choroid is distinguished from the other coats of the 

 eye by its dark color and its great vascularity. It occupies 

 that portion of the eyeball corresponding to the sclerotic. It 

 is perforated posteriorly by the optic nerve, and is connected 

 in front with the iris. It is very delicate in its structure, and 

 is composed of two or three distinct layers. Its thickness is 

 from Y^J- to ^ of an inch. 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, nerves (the long ciliary arteries and the 

 ciliary nerves), and very loose connective tissue. This is 

 sometimes called the membrana fusca, though it can hardly 

 be called a distinct layer. It contains, in addition to the 

 vessels, nerves, and fibrous tissue, a few irregularly-shaped 

 pigment-cells. 



The rest of the choroid is composed of two distinct lay- 

 ers ; an external, vascular, and an internal, pigmentary layer. 

 The vascular layer consists of numerous arteries, veins, and 

 capillaries, arranged in a peculiar manner. The layer of ca- 

 pillary vessels, which is internal, is sometimes called the middle 

 layer of the choroid, or 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 

 pigmentary layer. The plexus of capillaries is closest .at the 



