239 



THte vitreous humour, so called from its resemblance to melted glass, 

 is less dense than the crystalline, and more so than the vitreous, and is in 

 considerable quantity in the human eye ; it appears to he secreted by the 

 minute arteries which are distributed to the parietes of the membrane of 

 the vitreous humour ; it is heavier than common water, somewhat albu- 

 minous and saltish*. 



The sclerotica is a fibrous membrane to which the tendons which move 

 the globe of the eye are attached; it supports all the parts of that organ, and 

 these collapse and decay, whenever the continuity of its external covering 

 is destroyed. The use of the choroid, is not so much to avoid a covering 

 to the other parts, as to present a dark surface, destined to absorb the 

 luminous rays, when they have produced on the retina a sufficient im- 

 pression. If it were not for the choroid, the light would be reflected, 

 after having impinged on the nervous membrane, its rays would cross, 

 and produce only indistinct sensations. Mariotte thought that the cho- 

 roid was the immediate seat of vision, and that the retina was only its 

 epidermisf. This hypothesis would never have obtained so much cele- 

 brity, if, besides the objections that analogy might have furnished against 

 it, there had been adduced, in opposition to it, the fact observed in fishes, 

 in which the choroid is separated from the retina, by a glandular body, 

 opaque and incapable of transmitting the luminous rays. The retina 

 loses its form, as soon as it is separated from the vitreous humour, or 

 from the choroid coat, between which it is spread out as a very thin cap- 

 sule, so soft as to be almost fluid. A number of blood-vessels, from the 

 central artery of Zinn, are distributed on the nervous substance of the 

 retina, and give it a slight pink colour. Ought we, with Boerhaave, to 



partly coagulated by heat, and gives a copious precipitate with tannin both before and 

 after the coagulation. Its composition, according to the analyses of Berzelius, is as 

 follows : 



Water 58.0 



Peculiar matter - 35.9 



Muriates, lactates, and animal matter soluble in ajcohdl . 2.4 



Animal matter soluble only in water with some phosphates . 1.3 



Portions of the remaining insoluble cellular membrane . 2.4 



100.0 



The peculiar matter of the lens possesses all the chemical characters of the colouring 

 matter of the blood, except colour. When burnt it leaves a little ash, containing a very 

 small portion of iron. When its solution in water is coagulated by boiling, the liquid in. 

 which the coagulum was formed reddens litmus, containing free lactic acid. [Ann. 

 Phil. 11.385.) Copland 



* The Vitreous Humour possesses the same properties as the aqttews, even its spec, 

 grav. is the same, or only a very little greater. Its constituents, according to the analy- 

 sis of Berzelius, are : 



Water 98.40 



Albumen 0.16 



Muriates and lactates .... 1.42 



Soda, with animal matter soluble only in water . 0.02 



100.00 



Copland. 



f Mr. Jacobs, of Dublin, has discovered a membrane in the eye, situated between 

 the choroids and retina. As no name has yet been given to it, we might without im- 

 propriety call it the tunica tenuissima, as it is the most delicate in the human body. 

 Godman. 



