200 PHYSIOLOGY. 



have been compared to the glasses of a telescope, and the 

 coats to the tube which keeps them in their places. They are 

 three in number, and though all are perfectly transparent, 

 they differ in density, and in the space which they respec- 

 tively occuptl 



18. Aqueous Humour.-^-The aqueous humour is as clear aa 

 the purest water, from whence its name, although its specific 

 gravity is greater ; containing, as it does, a little albumen, 

 gelatine, and muriate of soda. It fills the entire space be- 

 tween the cornea and cyrstalline lens. The iris floats in it, 

 suspended like a curtain. The space between the iris and 

 the cornea is called the anterior chamber ; and that between 

 the iris and the crystalline lens, the posterior chamber of the 

 eye. This fluid is secreted by a very thin, transparent mem- 

 brane which encloses it, though some say it exhales from the 

 vessels of the iris, and its chief office is to distend the cor- 

 nea, and preserve the iris in a moist condition, fit for the dis- 

 charge of its appropriate functions. This fluid is also in a 

 constant state of secretion and absorption ; for when 

 entirely discharged, in extracting the lens, for cataract, we 

 find that in a few hours it will be restored again. In old 

 age, its quantity is lessened, and the eye becomes flatter, 

 needing the aid of convex glasses. Fishes have no aqueous 

 humour at all, as the water, in which they swim, answers the 

 same purpose, as this was designed for, in land animals. 



19. Besides this, the aqueous humour probably aids in 

 adjusting the eye to different distances. As the lens lies im- 

 mediately behind it, it is evident that, when the quantity of 

 fluid is increased, it will press the lens back nearer the re- 

 tina, while, at the same time, it renders the cornea more 

 convex, and the reverse happens when the quantity is dimi- 

 nished. When the distemsion is very great, the pressure 

 causes a cloudiness or opacity of the cornea, thereby pro- 

 ducing blindness. This not unfrequently happens with 

 horses that have been fed during the winter on dry food, 

 either placed on a level with the head or above it. In the 



