484 SPECIAL PHYSIOLOGY. 



sodium and potassium, the phosphates of soda, potash, and of magnesia, 

 perhaps the alkaline sulphates, the phosphate and carbonate of lime, 

 and oxide of iron. Minute traces of manganese and silica are also 

 necessary, the latter being probably combined with fluorine. Such 

 substances as alumina and copper are probably adventitious ingre- 

 dients, and of no essential importance as food. 



7. Water is the most abundant constituent of the animal body, and 

 is a most essential article of food. From the many offices which it 

 performs, dissolving the food, rendering it capable of absorption and 

 entrance into the circulation, facilitating all nutritive, secretive, and 

 excretive processes, and lastly, maintaining the due elasticity and 

 flexibility of the tissues, and their susceptibility of vito-chemical 

 changes, water may be regarded as a common vehicle, in which all 

 other articles of diet are conveyed into, through, and from the animal 

 economy. 



The albuminoid and gelatinoid nutrient substances, resemble each 

 other very closely in composition ; in addition to carbon, hydrogen, 

 oxygen, and sulphur, they contain nitrogen, and have therefore been 

 named, nitrogenous or azotized food ; and, as these substances are 

 especially concerned in the formation of the albuminoid and gelatin- 

 yielding tissues of the body, which indeed cannot be built up without 

 them, they have been designated nutritive or plastic food. Moreover, 

 as they supply the waste which takes place in the muscular and other 

 tissues, they have been likewise called flesh-forming, tissue-forming, or 

 histo-genetic, food. On the other hand, the oleaginous and saccharine 

 substances are composed of carbon, hydrogen, and oxygen only, and 

 are therefore named non-nitrogenous or non-azotized food. The starchy, 

 saccharine, and allied compounds form the carbhydrates ; whilst the 

 fatty substances, still richer in carbon, are named hydrocarbons. As 

 neither of these is ever supposed to be convertible, by the addition of 

 nitrogen, into nitrogenous, plastic, or flesh-forming food, but rather, 

 owing to their richness in carbon and hydrogen, and their poverty in 

 oxygen, to be ultimately used for the purposes of maintaining the ani- 

 mal heat, either being first stored up in the body as fat, or being at 

 once oxygenated through the respiratory process, they have been 

 classed together under the appellation of respiratory, calorific or heat- 

 forming, food. 



These distinctions, which have been chiefly explained and advocated 

 by Liebig, undoubtedly represent a general truth ; but they must be 

 accepted with certain qualifications. In the first place, albuminoid 

 substances may, it would seem, undergo metamorphosis, in the living 

 body, into fatty or even starch-like substances, and so may nourish 

 non-nitrogenous, as well as fleshy or nitrogenous, tissues. Moreover, 

 the nitrogenous tissues of the living body, especially those of the 

 muscles and brain, themselves undergo a most active waste, i. e., a 

 chemical decomposition, of which the essential feature is oxidation ; so 

 that, to a certain extent, they too, in being decomposed, must contrib- 

 ute to the evolution of heat, subserve the respiratory process, and so 

 far act as respiratory food. 



Again, chemical analysis shows, that in the brain especially, but 



