902 SPECIAL PHYSIOLOGY. 



the constituents of the food, is traced. The totals under each head 

 in the two Tables correspond, the decimals having been, in some 

 cases, slightly altered in the reduction of grammes into ounces. The 

 upper row of the figures which refer to the urine, represents the ele- 

 ments of the urea; the lower row, those of the non-nitrogenous urinary 

 constituents. Above two-thirds of the hydrogen of the food are con- 

 verted in the body into water, partly by uniting with oxygen already 

 in the food, but chiefly by combination with oxygen from the air. 

 The remainder of the oxygen unites with carbon, to form the carbonic 

 acid of the pulmonary and cutaneous exhalations. Of the entire ex- 

 creta, 32 per cent, pass off by the breath, 17 by the skin, 46.5 by the 

 kidneys, and 4.5 by the alimentary canal. The ultimate products of 

 the chemical metamorphoses of the food within the living body, are 

 regarded essentially as urea, carbonic acid, salts, and water. The 

 small residue consists chiefly of nitrogenous and other matters in the 

 faeces, and of epithelial, and epidermoid losses. 



The intermediate stages of metamorphosis, which occur as the food 

 is assimilated into blood, or solid tissue, and the further exceedingly 

 complex and only imperfectly known changes which these undergo, 

 have been followed, more or less completely, in describing the com- 

 position and use of the different kinds of food (p. 481-5), the modes of 

 their assimilation (p. 621), the office of the several constituents of the 

 blood (p. 707), and the sources of the biliary and urinary pulmonary 

 excretions (pp. 760, 777). A summary or general view of these meta- 

 morphoses may now be given. 



Water appears to undergo no decomposition into oxygen and hydro- 

 gen ; rather it is increased by additional water set free, or actually 

 produced by the union of oxygen and hydrogen in the body itself. It 

 is probably concerned in processes of hydration and dehydration, thus 

 effecting changes in the more complex elements of the body. 



The saline substances of the food pass, for the most part, unchanged 

 through the body, and reappear again in the excretions, especially in 

 the urine ; but the chlorides must undergo temporary decomposition 

 for the formation of the hydrochloric acid of the gastric juice, the 

 chlorine, however, again meeting with appropriate bases. Additional 

 saline matters appear in the excreta besides those in the food, chiefly 

 alkaline sulphates, formed by the oxidation of the sulphur in the albu- 

 minoid compounds of the body, the magnesian phosphates, resulting 

 from the oxidation of the phosphuretted fats of the blood corpuscles 

 and the brain. The ammonia in the breath, in the perspiration, and 

 in the urine, and also the urea, uric acid, and hippuric acid, are saline 

 substances, the products of decomposition of one or more nitrogenous 

 matters in the body. 



The carbhydrates, starch, and sugar, are changed, the first into 

 sugar, and both, probably after transitional mutations, into lactic, 

 oxalic, or other acids. Their elements are ultimately traceable, the 

 carbon, in the carbonic acid of the breath and perspiration, and the 

 hydrogen and oxygen, in water. Sometimes starch or sugar may give 

 rise, apparently by an upward metamorphosis, to biliary or other fatty 

 acids, and thus to fat, which may then be deposited in the tissues as 



