54 CHEMICAL COMPONENTS OP THE HUMAN BODY. 



19. All the Protein-compounds are very liable to decomposition; this change 

 taking place in them spontaneously, when they are exposed to the air at ordinary 

 temperatures ; and being very readily induced by oxidizing agents, alkalies, &c., 

 the effect of which is promoted by heat. As yet, however, no satisfactory clue 

 has been obtained to the very complex composition of these substances ; since 

 they cannot be resolved by analysis (like complex inorganic bodies) into two or 

 more compounds whose synthesis reproduces the original. Still, it is very inte- 

 resting to observe that, whilst the ultimate decomposition of the protein-com- 

 pounds resolves them (with oxygen taken from the atmosphere) into water, 

 carbonic acid, and ammonia, various organic compounds may be generated by 

 a less complete separation of their components. Thus by the action of oxidiz- 

 ing substances, the formic, acetic, butyric, caproic, and other organic acids of 

 the same group, which occur naturally in the animal body, may be obtained ; 

 and there is a strong probability that the ordinary fatty acids may be generated 

 by a similar change ( 40). Again, by the prolonged action of caustic alkalies 

 upon the protein-compounds, a crystalline substance is obtained, which is termed 

 Leucine; this forms colorless scales, destitute of taste and odor; it is soluble 

 in water, and sublimes unchanged. It consists of 12C, 12H, IN, 40. There 

 is not at present any evidence that this substance is ever produced in the liv- 

 ing body; but a strong interest attaches to it, from the fact that it may be 

 procured from .Gelatin, as well as from the Protein-compounds. Another 

 compound obtained by the same reaction is called Tyro&ine; it crystallizes in 

 brilliant needles ; and its formula is 16C, 9H, IN, 50. The tendency to 

 decomposition which exists in this class of substances, not merely occasions the 

 re-arrangement of their own elements in new compounds of a different character, 

 but also tends to produce similar changes in other substances ; and it is probable 

 that this kind of agency takes place to a great extent in the living body. Thus 

 a protein-compound, in a certain stage of decomposition, will convert starch into 

 sugar; in a more advanced state of change, it will convert sugar into lactic acid, 

 mannite, and vegetable mucus, or into alcohol and carbonic acid ; and by the 

 same agency, lactic acid may be resolved into butyric acid, hydrogen, and car- 

 bonic acid. 1 This property of exciting change in other substances, whilst 

 themselves passing into decay, makes it very important that the history of the 

 protein-compounds in the living body should be fully made out ; since it is 

 obvious that they are not merely required as histogenetic materials, but that 

 they also take an important part in the transformation of other substances by 

 their action as ferments. 



20. Of the whole series of protein-compounds, Albumen is obviously the one 

 which may be considered as the proper pabulum of the Animal tissues gene- 

 rally; since we have evidence that from it, in combination with fatty matter and 

 mineral ingredients, all the tissues of the body may be generated. The store 

 of nutriment laid up within the egg, from which in due time the chick is de- 

 veloped, with its bones, muscles, nerves, tendons, ligaments, membranes, skin, 

 horny bill, feathers, &c., is composed of nothing else ; and the albumen of the 

 blood of the adult animal is continually being withdrawn from it, to be applied 

 in like manner to the maintenance of these various tissues. 3 We shall find, 

 moreover, that the other histogenetic substances, when employed as food, must 

 be reduced to the state of albumen, before they can be appropriated by the 

 living system. The properties of Albumen may be studied in the white of the 

 egg, or in the serum of the blood, from either of which situations it may be 



1 See Prof. Liebig's "Familiar Letters on Chemistry," 3d ed. p. 207. 



2 Even if it should be proved that the Gelatigenous tissues are ever formed from Gelatin 

 taken in as food, which the Author believes to be highly improbable (see \ 35), yet there 

 can be no question that they are formed from the albuminous part of the blood, when (as 

 in all Herbivorous animals) the aliment contains no gelatinous component. 



