114 ANIMAL MECHANISM. 



Explanation of Figure 1. 



This figure represents 

 the mode, and, in fact, 

 the place into which the 

 cotiching needle is in- 

 troduced, in the opera- 

 tion of couching. 



A The pupil is seen 

 through the transpa- 

 rent cornea. 



B Thezm. 



C The needle, with 



the handle elevated so as to depress the point. 



D The lens and point of the needle in outline: this precisely 

 represents the position of the lens after couching. To complete 

 the operation, it must be carried a little back before withdrawing 

 the needle. 



VITREOUS HUMOR. 



Beyond the two humors we have been describing, is 

 the third, differing essentially from either of them. In 

 the first place, in volume it far exceeds the others, qp- 

 cupying more than two thirds of the whole interior and 

 posterior of the ball. Its consistence is that of the white 

 of an egg, but kept in its place by its own appropriate 

 capsule ; it presents many interesting phenomena. When 

 the sack is punctured with a pin, it flows out slowly, in 

 consequence of its adhesiveness. Like the preceding 

 humors, it is transparent, allowing the free passage of 

 light through its substance, and also possesses the addi- 

 tional quality of allowing the rays to separate again, as 

 they leave the point at which they were converged, just 

 back of the lens. Observation proves that the vitreous 

 humor is kept in place by being lodged in cells. Per- 

 haps a piece of sponge might give a tolerable idea of the 

 cellular structure, admitting it to be as transparent as the 

 water which it absorbs. On its fore part, it has a depres- 

 sion, in which the posterior convexity of the lens is 

 lodged, as represented in this diagram. Concave, 

 therefore, in front, and convex behind, gives another kind 

 of optical glass, known as the meniscus, the crescent, 

 faintly resembling the first quarter of the new moon. If 



