466 OF NUTRITION. 



Cells. Very frequently, however, a deposit is formed within these cells, 

 through a secreting process effected by themselves ( 651) ; which may have 

 an entirely different character. Thus the cells of Adipose tissue elaborate 

 Fatty matter, the cells of Epidermis appear to draw off Horny matter, and the 

 cells of the Epithelium fill themselves with products of various kinds, which 

 were either pre-existing in the Blood, or are generated by a simple transforma- 

 tion of its elements ( 649). These last might be regarded, equally with the 

 contents of the more permanent Tissue-cells, as products of Nutritive action ; 

 but it will be more convenient to consider them with a view to their destination, 

 which is altogether different. 



615. The composition of the greater part of the Fibrous tissues, however, 

 is very different ; for they all yield to boiling water the substance called Gela- 

 tin, which does not seem capable of the same degree of organization with the 

 Protein compounds. This may be obtained by boiling portions of Skin, Areo- 

 lar tissue, Serous membrane, Tendon, Bone, &c., in water, for some time ; after 

 which the decoction is allowed to cool, when it solidifies into a jelly of greater 

 or less thickness. Some tissues dissolve readily in this manner, and little resi- 

 dual (or fibrinous) substance is left ; this is especially the case with areolar 

 tissue, serous membranes, and (in a less degree) with skin. Others require a 

 long boiling for the extraction of any Gelatin, and even then it is obtained in 

 but small quantity ; of this kind are the elastic fibrous tissue and some forms 

 of cartilage. A peculiar modification of this principle exists in most of the 

 permanent cartilages ; and has received the name of Chondrin. Gelatin is 

 not found in the blood, nor in any of the healthy fluids ; and most Chemists 

 are of opinion that it is rather a product of the operation practised to separate it, 

 than a real constituent of the living solids. This idea seems inconsistent, 

 however, with the fact that the gelatinous tissues will exhibit, without any 

 preparation, the best marked of the chemical properties which are regarded 

 as characteristic of Gelatin, that, namely, of forming a peculiar insoluble 

 compound with Tannin ; and the Tanno-Gelatin which may be obtained by 

 precipitating Gelatin from a solution, and that which results from the action 

 of Tannin on Animal membrane, appear to be precisely analogous in every 

 respect, save the presence of structure in the latter, and its absence in the 

 former. Gelatin is very sparingly soluble in cold water ; by contact with 

 which, however, it is caused to swell up and soften. It is readily dissolved 

 by hot water ; and forms so strong a jelly on cooling, that 1 part in 100 of 

 water becomes a consistent solid. Its reaction with Tannic acid is so distinct 

 that 1 part in 5000 of water is at once detected by infusion of Galls. The fol- 

 lowing are the results of four analyses of Gelatin, by Scherer and Mulder. 



X 



SCHERER, MULDER. 



Carbon . . . 50-557 50-774 50-048 50-048 



Hydrogen . . 6-903 7-152 6.477 6-643 



Nitrogen . . 18-790 18-320 18-350 18-388 



Oxygen . . . 23-750 23-754 25-125 24-921 



The formula adopted by the former is 48 C, 41 H, 7k N, 18 O ; that of the 

 latter is 54 C, 42 H, 9 N, 20 O. Neither of these can be yet regarded as 

 satisfactorily determined ; and it is therefore useless to speculate upon the 

 mode in which Gelatin is produced by a metamorphosis of Protein-compounds. 

 That it cannot be converted, in the living body, into Albumen or Fibrin, 

 would appear from the considerations already stated (554). A kind of sugar, 

 termed Glycicoll, may be obtained from Gelatin, by the action of Sulphuric 

 acid, or by boiling it in caustic Alkali : this substance crystallizes in large 

 prisms, which are colourless, taste sweet, and feel gritty' between the teeth ; 



