238 



thickness $ it forms, at the fore-part of the eye, the segment of a smaller 

 sphere : behind it lies the aqueous humour which fills what are called the 

 chambers of the eye; these form spaces divided into anterior and poste- 

 rior, the former, which is the larger of the two, bounded by the cornea 

 at the forepart, and by the iris at the back part; the latter, which is 

 smaller, and separates the crystalline humour from the iris, the posterior 

 part of which, covered by a black pigment, is called the uvea*. The spe- 

 cific gravity of the aqueous humour does not much exceed that of dis- 

 tilled water; some have even thought it less ; it is albuminous, and holds 

 in solution several saline substances!. The chrystalline, enclosed in its 

 membranous and transparent capsule, is a lenticular body rather solid 

 than fluid ; its consistence is particularly great to ward s 4 its -centre; it there 

 forms a kind of nucleus, on which are laid several concentric layers, 

 whose density diminishes as they approach the surface, where the ex- 

 ternal layers, truly fluid, form what Morgagni considered, as a particu- 

 lar liquid on which the lens might be nourished by a kind of imbibition. 

 This body, composed of two segments of unequal convexity, about two 

 lines in thickness, as its centre, consists, of an albuminous substance coa- 

 gulable by heat and alcohol $. Extremely minute arteries given off by 

 the central artery of Zinn, pass through the vitreous humour, and bring 

 to it the materials of its growth and reparation. 



* Some anatomists have doubted the existence of the posterior chamber of the eye ; 

 but to be convinced of its existence, one need but freeze an eye, when there will be 

 found a piece of ice between the chrystalline lens and the uvea. The formation of this 

 icicle is not owing to the admission, through the opening of the pupil, of the aqueous 

 humour which, like all other fluids, expands considerably on freezing 1 , for, the expan- 

 sion of fluids on their freezing 1 , being proportioned to their bulk, the vitreous humour 

 which freezes at the same time as the aqueous, must prevent its retrogade flow through 

 the pupil. Lastly, the uvea or posterior part of the iris is covered with a black point 

 which is easily detached from it ; now, if the anterior part of the chrystalline lens had 

 been in immediate contact with it, it would have been soiled by some of this colouring 

 matter which would have tarnished its natural transparency, indispensable to perfect 

 vision. It is, therefore, undeniable that there does exist a posterior chamber, which is 

 to the anterior in the proportion of two to five, and containing- about two fifths of the 

 aqueous humour, the whole of which is estimated at five grains, and that the iris forms 

 a loose partition between the two portions of the aqueous humour in which the dai'k 

 pigment of the uvea is insoluble. The aqueous humour appears to be the product of 

 arterial exhalation ; it is soon reproduced, as we see after the operation for cataract. 

 .Author's Note. 



j- The Jlqueous Humour. Specific gravity 1.090 at a temperature of 60. From various 

 recent experiments, it appears to be water slightly impregnated with 1st. Albumen ; 

 2d. Gelatin ; 3d. Muriate of soda. Its constituents, according- to the analyses of Ber- 

 zelius, are : 



Water 98.10 



Albumen a trace 



Muriates and lactates .... . . 1.15 



Soda, with animal matter soluble only in water . 75 



100.00 



Copland. 



% In the human eye there are, strictly speaking-, three lenses. Two menisci and one 

 double convex, The anterior meniscus is formed by the cornea and aqueous humour, 

 occupying- all the space anterior to the double convex lens ; the posterior meniscus is 

 formed by the scleorotica, &c. and the vitreous humour, filling the globe of the eye 

 posterior to the double convex lens, or "the lens" proper. Godman. 



The ChrystalUne Lens. Its specific gravity is 1.100. When fresh it has little taste. 

 It putrifics very rapidly. It is almost completely soluble 5n water. The solution is 



