THE SCLEROTIC AND CORNEA. 



683 



Arteries and Nerves. The cornea is a non-vascular structure, the capillary 

 vessels? terminating in loops at its circumference. Lymphatic vessels have not 

 as yet been demonstrated in it. The nerves are numerous, twenty or thirty in 

 number: they are derived from the ciliary nerves, and enter the laminated 

 substance of the cornea. They ramify throughout its substance in a delicate 

 network. 



Dissection. In order to separate the sclerotic and cornea, so as to expose the second tunic, 

 the eyeball should be immersed in a small vessel of water. A fold of the sclerotic near its ante- 

 rior part having been pinched up, an operation not easily performed, from the extreme tension 

 of the membrane, it should be divided with a pair of blunt-pointed scissors. As soon as the 

 choroid is exposed, the end of a blowpipe should be introduced into the orifice, and a stream 

 of air forced into it, so as to separate the slight cellular connection between the sclerotic and 

 choroid. The sclerotic should now be divided around its entire circumference, and may be 

 removed in separate portions. The front segment being then drawn forwards, the handle of the 

 scalpel should be pressed gently against it at its connection with the iris, and these being sepa- 

 rated, a quantity of perfectly transparent fluid will escape; this is the aqueous humor. In the 

 course of the dissection, the ciliary nerves may be seen lying in the loose cellular tissue between 

 the choroid and sclerotic, or contained in delicate grooves on the inner surface of the latter 

 membrane. 



Fig. 369. The Choroid and Iris. (Enlarged.) 



Second Tunic. This is formed by the choroid behind; the iris and ciliary 

 processes in front; and by the ciliary ligament, and Ciliary muscle, at the point 

 of junction of the sclerotic and cornea. 



The choroid is the vascular and pigmentary tunic of the eyeball, investing 

 the posterior five-sixths of the globe, and extending as far forwards as the 

 cornea; the ciliary processes being appendages of the choroid developed from 

 its inner surface in front. The iris is the circular muscular septum, which 

 hangs vertically behind the cornea, presenting in its centre a large circular 

 aperture, the pupil. The ciliary ligament and Ciliary muscle form the white 

 ring observed at the point where the choroid and iris join with each other, and 

 with the sclerotic and cornea. 



The Choroid is a thin, highly vascular membrane, of a dark brown or choco- 

 late color, which invests the posterior five-sixths of the central part of the 

 globe. It is pierced behind by the optic nerve, and terminates in front at the 

 ciliary ligament, where it bends inwards, and forms on its inner surface a series 

 of folds or plaitings, the ciliary processes. It is thicker behind than in front. 



