50 SURGICAL APPLIED ANATOMY. [Chap. iv. 



interstitial keratitis, blood-vessels from the arteries at 

 the margin of the cornea penetrate into the substance 

 of the cornea for some distance. As these vessels will 

 be some little way below the surface, and will be covered 

 by the hazy cornea! tissue that is the result of the 

 disease, their scarlet colour is much toned down, and 

 a strand of such vessels is called a " salmon patch." 

 In the condition known as pannus, the cornea 

 appears to be vascularised ; but here, owing to con- 

 tinued irritation, vessels, derived from the neighbour- 

 ing conjunctival arteries, pass over the cornea just 

 beneath its epithelial covering, leaving the cornea 

 proper as bloodless as ever. The term arcus senilis is 

 applied to two narrow white crescents that appear at 

 the periphery of the cornea, just within its margin, in 

 the aged, and in certain morbid conditions. The 

 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 pai't 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 

 anaesthetic. 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 55.) 



The sclerotic, choroid, and iris. The sclerotic 

 is thickest behind, and thinnest about a quarter of 



