CORNEA. 413 



parency of the cornea, by equably distending the cells. This 

 may be proved by a simple experiment, viz. by forcing the 

 fluid by pressure from a part of a recent cornea, when the 

 portion pressed becomes opaque, but has its transparency 

 restored by the return of the fluid on the removal of the 

 pressure. In old persons the cornea becomes denser in its 

 structure, and usually presents an opaque ring at its circumfer- 

 ence, called Arcus Senilis. 



The cornea is covered by the tunica conjunctiva, which 

 adheres firmly to it, but may be separated after maceration. It 

 can also be separated from the sclerotica after maceration, and 

 a slight degree of putrefaction, especially if the parts, when in 

 this situation, are suspended a short time in boiling water. 



The cornea is lined internally by a fine serous membrane 

 (membrane of Descemet,) called the Capsule of the aqueous hu- 

 mor; which will be evident if the coats of the eye are boiled. 

 In this case the cornea hardens, and the capsule of the aqueous 

 humor appears detached from it, like the cuticle raised by 

 vesication. 



In a sound state, the sensibility of the cornea varies consi- 

 derably.* The vessels in its structure do not carry red blood ; 

 it is, however, much changed in its structure by inflammation ; 

 and it is said that blood has been found between its lamina, in 

 consequence of violent strokes upon the eye. 



It is the segment of a smaller sphere than the sclerotica : and 

 therefore is more convex. The degree of convexity is very 

 different in different persons ; those in whom it is very great are 

 necessarily short-sighted. It is not perfectly circular, but rather 

 oval : the transverse diameter being the longest. 



The cornea and sclerotica are connected to each other by 

 sloping surfaces. The edge of the sclerotica projects over the 

 internal lamen of the cornea, and the edge of the cornea passes 

 under the external surface of the sclerotica. 



The separation above mentioned proves the cornea and scle- 



* It is the opinion of Dr. Physick, whose numerous and successful operations 

 on the eye have afforded him many opportunities of judging, that the incision 

 of the cornea always occasions some pain, which is very different, as to its in- 

 tensity, in different persons. 



35* 



