42 SURGICAL APPLIED ANATOMY. [Chap. iv. 



crescents are placed at the upper and lower margins, 

 and their points meet midway on either side of the 

 cornea. They are due to fatty degeneration of the 

 corneal tissue, and the change is most marked in the 

 layers of the cornea just beneath the anterior elastic 

 lamina, i.e., in the part most influenced by the marginal 

 blood-vessels. In. spite of its lack of a direct blood- 

 supply, wounds of the cornea heal kindly. The cornea 

 is very lavishly supplied with nerves, estimated to 

 be from forty to forty-five in number. They are 

 derived from the ciliary nerves, enter the cornea 

 through the fore part of the sclerotic, and are dis- 

 tributed to every part of the tunic. In glaucoma, 

 a disease the phenomena of which depend upon greatly 

 increased intraocular pressure, the cornea becomes 

 ansesthetic. This depends upon the pressure to which 

 the ciliary nerves are exposed before their branches 

 reach the cornea. (See also Nerve supply of the eye- 

 ball, page 47.) 



The sclerotic, choroid, and iris. The sclerotic 

 is thickest behind, and thinnest about a quarter of 

 an inch from the cornea. When the globe is ruptured 

 by violence it is the sclerotic that most commonly 

 yields, the rent being most usually a little way from 

 the cornea, i.e., in or about the thinnest part of the 

 tunic. A rupture of the cornea alone from violence 

 is not common. The sclerotic may be ruptured while 

 the lax conjunctiva over it remains untorn. In such 

 a case the lens may escape through the rent in the 

 sclerotic, and be found under the conjunctiva. It is 

 mainly to the denseness and unyielding character of the 

 sclerotic that must be ascribed the severe pain (due to 

 pressure on nerves) experienced in those eye affections 

 associated with increased intraocular tension (glaucoma, 

 etc.). The choroid is the vascular tunic of the globe, and 

 carries its main blood-vessels. Between the choroid 

 and sclerotic are two thin membranes, the lamina 



