THE RYE. 181 



old observers imagined it resembled molten glass ; which at 

 first sight it does appear to be somewhat like. When the 

 sclerotic, the choroid, and the retina are cat through, the 

 vitreous humour does not flow out like water, which we 

 have asserted it to be ; but hangs as a thick clear jelly from 

 the incision. And why is this '? Because the cavity of the 

 vitreous humour is lined with a fine, clear, and perfectly 

 transparent membrane, which is formed into cells {Fig 19). 

 Each of these cells, which is rather minute, constitutes a 

 closed sac ; and in this little bag, without an opening, the 

 water is contained. The reader can now understand why 

 the vitreous humour would appear dense, without actually in 

 this particular exceeding the aqueous humour. 



The use of the humours is to distend the globe of the 

 eye ; or to keep the sac formed by the sclerotica and the 

 cornea continually on the stretch, or always of a globular 

 form. 



The aqueous humour fills a cavity, bounded anteriorly 

 by the cornea, and posteriorly by the crystalline lens (see 

 Fig 19). This limited space is divided into two chambers. 

 The anterior chamber is by far the largest, and consists of 

 all that space between the posterior of the cornea and the 

 anterior of the iris. There is, however, a small dark space 

 between the back of the iris and the front of the crystalline 

 lens, which is termed the posterior chamber. When a 

 puncture is made through the cornea so as to let out the 

 aqueous humour, the cornea is corrugated, or thrown into 

 various minute and irregular folds. Yet this cornea, the 

 shape of which appears to depend upon the pressure of the 

 aqueous humour from behind, is a very compound sub- 

 stance. If it be cut out from its situation, and taken 

 between the points of the finger and thumb, it w^ili be dis- 

 covered to be laminated ; because one part will be felt to 

 slide upon another. It consists of several minute layers, 

 as many as fifteen ; though it is not thicker than a sixpence. 

 It is also composed of two distinct parts. The most for- 

 ward of which is fibrous, and the most posterior elastic. 

 This last part is so elastic, that if freed it will maintain its 

 original form, and assume no other. 



The crystalline lens (Fig 19 . 2), or humour, is retained 



