118 ANIMAL MECHANISM. 



down from the forehead, to join the cheek. To compre- 

 hend, clearly, the manner in which the eye is fastened, 

 before, observe how the skin turns over the edge 

 of the lid, going about three quarters of an inch back, 

 striking the ball to which it is made fast, then folded back 

 upon itself, adhering to the whole anterior surface of the 

 cornea, dipping down and finally mounting over the 

 margin of the lower lid, and ultimately losing itself on 

 the face. As we cannot recognise this, on a living eye, 

 it will at once lead one to suppose it is as clear as glass, 

 which is the case. Streaks of blood, when the eye is 

 inflamed, lie covered over by the tunica conjunctiva. 

 Now if particles of sand, or other irritating substances 

 get under either eye-lid, they cannot possibly enter but 

 little way, before reaching the duplication of this trans- 

 parent skin ; there is no danger, therefore ; the offending 

 matter cannot get so far between the socket and ball, 

 backward, as to abridge the free motion of the organ, or 

 do a permanent injury to the parts. This partition, or 

 doubling over of the conjunctiva, is a curious provision, 

 as we are thereby enabled to reach the source of irrita- 

 tion. The principle of introducing eye-stones, to extract 

 foreign matter, is this, and not owing, as vulgarly sup- 

 posed, to the crawling about of a smooth piece of sulphate 

 of lime, on some forty or fifty feet. The stone is so 

 much larger than the extraneous body, already there, 

 that it excites a proportionably larger quantity of tears, to 

 wash it away : in essence, therefore, we submit to a 

 greater temporary evil, to get rid of a lesser one. Pre- 

 cisely on this principle, a person chewing tobacco, is 

 constantly spitting : the vile weed is so offensive to the 

 nerves about the region of the throat and tongue, a 

 stimulant so unnatural and uncongenial to the constitu- 

 tion of the body, that the saliva is poured out, with in- 

 creasing copiousness, to wash it from the mouth. 



Serpents annually shed their skins, which, unaccount- 

 able as it at first appears, are whole over the holes where 

 the eyes were. That thin sheet, so very clear and fine 

 in texture, is the conjunctiva, showing its origin, hence 

 a similar origin may safely be inferred over other eyes. 



