416 NERVOUS SYSTEM. 



anatomists to consider these membranes as one and the same, 

 notwithstanding their obvious difference of structure and of pro- 

 perties. Their adhesion yields to protracted maceration. The 

 cornea is covered in front by a continuation of the tunica con- 

 junctiva, which unites the eyeball to the eyelids. This may be 

 proved by dissection, by maceration, and by its sloughing off 

 entirely along with the epidermis of animals that are subject to 

 this process, as the locust, snakes, and others. On its posterior 

 face, it is covered by the membrane of the aqueous humour, 

 which may be rendered evident by steeping it in spirits of wine; 

 whereby the latter membrane is made more hard, and may be 

 torn off. 



The cornea, in a natural state, is perfectly transparent, and 

 readily transmits the rays of light. It consists of an indefinite 

 number of laminae, which are placed one against the other like 

 the leaves of a book, and are united by a delicate transparent 

 cellular substance which permits the laminae to slide upon each 

 other. These laminae are kept moist and pellucid by an inter- 

 stitial secretion of a fluid equally pellucid with themselves, the 

 abundance of which in health gives to the eye its brilliancy, and 

 the deficiency of it in illness and in death causes the eye to look 

 dim and somewhat opaque. Its evaporation, which no doubt is 

 continually occurring, is as constantly supplied by a fresh and 

 abundant secretion. The motion of the eyelids sweeps the resi- 

 duum, after the evaporation of its watery particles, from the sur- 

 face of the cornea: without this process, the residuum appears as 

 a thin layer or film of albuminous matter spread over the cornea, 

 when the eye is kept open without winking for a considerable 

 time. 



The cornea has not the fibrous structure of the sclerotica, yet 

 the application of mechanical force to the eyeball shows that it 

 is stronger. Neither has it vessels, conveying red blood natu- 

 rally, yet, in a state of inflammation, its capillaries dilate so as 

 to admit red blood, and deposite coagulating lymph between its 

 layers. It is common for anatomists to attribute a want of sensi- 

 bility to it in a natural state: as a general rule, this is fallacious; 

 for many persons, where the eyes are not inflamed, suffer ex- 

 tremely from its being cut in cataract, and sometimes faint from 

 the pain, while others are truly unconscious of the incisions 

 made through it. Of this opinion, I have a full assurance from 



