CHEMICAL COMPOSITION OF THE BODY. 81 



such substances as Leucin, Glycin, and Taurin ; the Biliary and Urinary 

 acids, Kreatin, Kreatinin, and Urea; and the azotized pigmentary sub- 

 stances. The latter or nonazotized group are divisible into those which 

 contain less oxygen than the albuminous compounds, as the fats; or more 

 oxygen, as the carbo-hydrates, glycogen, milk, sugar, and the like. The 

 ultimate results of the further oxidation of both classes of compounds are 

 carbonic acid and water. 1 The fats and carbo-hydrates ingested into the 

 stomach, after some preparatory processes, are absorbed into the blood, and 

 undergo the same changes as the corresponding compounds resulting from 

 the breaking up of albumen and its allies. The tissues therefore, as Dr. 

 Marcet has recently pointed out, 2 may be regarded as formed of three dif- 

 ferent classes of substances, namely, those which constitute the mature or 

 functionally active tissue, and are characterized by being insoluble in water ; 

 next, those on their way to form the living tissue which are soluble in 

 water but of colloidal nature, i.e., incapable of diffusion ; and, finally, those 

 which are effete, having performed their function, and are on the road to be 

 eliminated. These are soluble in water and crystalloid. It will thus be 

 seen that Plants and Animals are directly opposed to each other in their 

 chemical relations : Plants forming organic compounds, decomposing car- 

 bonic acid, fixing the carbon and setting free the oxygen, and, so to speak, 

 absorbing force ; whilst Animals effect the decomposition of the organic 

 compounds, taking up oxygen, and eliminating carbonic acid, water, and 

 urea, and giving off force in the form of heat, motion, etc. In considering 

 the proximate principles entering into the composition of the body, we shall 

 first describe the albuminous compounds and their derivatives; secondly, 

 the hydro-carbonaceous, and thirdly, the mineral compounds, following 

 as far as possible the order of descent from the higher to the lower planes 

 of complexity of constitution which experiment and observation render 

 probable. 



53. ALBUMINOUS COMPOUNDS. S As already stated, Plants alone are 

 capable of producing albuminous substances or " Proteids" from the direct 

 union of their elements; and from this source are derived all the supplies 

 required for the maintenance of their bodies by the Herbivora, which again 

 yield them up to the Carnivora. The chief facts deserving of attention in 

 the chemical history of the Proteids are, that they are all composed of Car- 

 bon, Hydrogen, Nitrogen, Oxygen, and Sulphur, and so complex is the 

 grouping of their atoms that it is almost impossible to represent them by 

 any rational formula. No less than sixteen different forms of albumen have 

 been described by chemists, viz., Serum-albumen, Ov-albumen, Paralbumen, 

 Paraglobuliu, Fibrinogeu, Serum-Casein, Alkali- (Sodium- or Potassium-) 

 Albumen, Casein, Myosiu, Syntouiu, Coagulated Albumen, Fibrin, Vitellin, 

 Ichthin, Amyloid and Prot-acid. The differences between these are, however, 

 comparatively slight,*and are chiefly referable to their greater or less solubility 

 in saline or acid solutions, or in the temperature at which they coagulate by 

 heat. A knowledge of their percentage composition is more easily attained, 



1 The successive steps may be traced, perhaps, most distinctly in the case of guanin 

 (C 5 H 5 N 5 O) which is u constituent of the pancreas, through hypoxanthin (C 5 H 4 N 4 O) 

 and xanthin (C 5 H 4 N 4 O 2 ) to uric acid (C 5 H 4 N 4 O 3 ) ; uric acid again, when acted on 

 by oxidizing agents, yielding urea (CH 4 N 2 O), allantoin (C 4 H 6 N 4 O 3 ), oxalic acid 

 (C 2 O. 2 OH 2 ), and carbonic add. 



2 An Experimental Inquiry into the Nutrition of the Animal Tissue, 1874. 



3 See Eichwald, Beitrage zur Chernie der Gewebebildenden Substanzen und ihre 

 Abkommlinge. Berlin, 1873. Nasse Studien uber die Eiweisskorpen, Pfliiger's 

 Archiv, Band vi, 187^, p. 589, and Band vii, 139. 



