20 CHEMICAL COMPOSITION OF HUMAN BODY. 



rhomboidal scale-like crystals of it are also often found 

 in morbid secretions, as in cysts, the puriform matter of 

 softening and ulcerating tumours, etc. It is soluble in 

 ether and boiling alcohol ; but alkalies do not change 

 it ; it is one of those fatty substances which are not 

 saponifiable. 



The azotized or nitrogenous principles in the human body 

 include what may be called the proper gelatinous and albu- 

 minous substances, besides others of less definite rank and 

 composition, as pepsin and ptyalin, horny matter or keratin, 

 many colouring and extractive matters, etc. 



The gelatinous substances are contained in several of the 

 tissues, especially those which serve a passive mechanical 

 office in the economy; as the cellular or fibro-cellular 

 tissue in all parts of the body, the tendons, ligaments, 

 and other fibrous tissues, the cartilages and bones, the 

 skin and serous membranes. These, when boiled in 

 water, yield a material, the solution of which remains 

 liquid while it is hot, but becomes solid and jelly-like on 

 cooling. 



Two varieties of these substances are described, gelatin 

 and chondrin, the latter being derived from cartilages, 

 the former from all the other tissues enumerated above, 

 and in its purest state, from isinglass, which is the swim- 

 ming bladder of the sturgeon, and which, with the excep- 

 tion of about 7 per cent, of its weight, is wholly reducible 

 into gelatin. The most characteristic property of gelatin 

 is that already mentioned, of its solution being liquid when 

 warm, and solidifying or setting when it cools. The tem- 

 perature at which it becomes solid, the proportion of gela- 

 tin which must be in solution, and the firmness of the 

 jelly when formed, are various, according to the source, 

 the quantity, and the quality of the gelatin ; but, as a 

 general rule, one part of dry gelatin dissolved in 100 of 

 water, will become solid when cooled to 60. The solidi- 

 fied jelly may be again made liquid by heating it ; and the 



