110 ANIMAL MECHANISM. 



and closing the pupil. An opinion is current, founded 

 undoubtedly in truth, that the aqueous humor is never 

 suffered to remain long at a time, but, on the contrary, is 

 constantly poured in and again drawn off by an infinite 

 number of invisible ducts. By being stationary, it would 

 become speedily turbid, and finally lose its transparency. 

 A knowledge of the rapidity of the secretion, has been 

 the means of encouraging occulists to undertake novel 

 methods of extracting cataracts, a kind of dark mote, 

 through the cornea, as the most certain mode of restoring 

 sight. Twentyfour hours after drawing off the aqueous 

 humor, by a puncture, the anterior chamber will be full 

 again. 



Old age, characterized by a gradual decay in the vigor 

 of all the individual organs, shows also its insidious ap- 

 proach in the eye. Vessels that have toiled with untiring 

 diligence to the meridian of life, begin to show a loss 

 of energy. Those which have carried the new, pure 

 liquid, forward a less quantity in a given time than for- 

 merly, while those whose task it was to convey away 

 the old stock, are dilatory in the performance of their 

 work. Hence, from being kept too long in the reservoir, 

 in consequence of a tendency to become more turbid, 

 does not allow the light to pass with former facility to the 

 nerve : elderly persons, therefore, have indistinct vision 

 from this cause, similar to looking through a smoky at- 

 mosphere. The writer has a favorite Newfoundland dog, 

 whose eyesight is impaired in this way. Fishes have no 

 aqueous humor at all, as it could be of no service in the 

 element in which they swim : the water surrounding 

 them is the aqueous humor to their organs. Kept, as the 

 humor is, in its own capsule, gives other advantages to 

 the apparatus of vision : it is a concavo-convex glass, ab- 

 solutely and indispensably requisite in an instrument that 

 will produce an image by the same laws that govern the 

 human eye. A sensible diminution in the quantity of 

 this fluid, is very apparent in people advanced in years : 

 the cornea becomes flatter ; the segment of the transpa- 

 rent cornea is so altered, that rays of light are no longer 

 converged as in younger days. This, together with cor- 

 responding derangements within the globe, constitutes 



