242 NUTRITION 



presence of caustic alkali; the latter absorbs the carbonic dioxide and the 

 nitrogen is collected in a graduated tube. From the amount of this 

 nitrogen the quantity of the urea decomposed is calculated, every gram 

 of urea giving 37.1 cubic centimeters of nitrogen by this method. The 

 nitrate and oxalate of urea have importance in examinations for urea. 



The non-nitrogenous portion of the product of the katabolic process 

 in proteid is ultimately oxidized, like other such substances, to carbon 

 dioxide and water, perhaps by way of glycogen or dextrose. The sul- 

 phur probably goes into the sulphates of the urine and of the feces. 

 Such are the hypotheses which at present seem rather more probable 

 than others to many biochemists and physiologists. 



The water of the urine varies in amount largely at different times, 

 but on the average perhaps runs within 200 c.c. of 1| liters daily. It is 

 derived from several sources. Part (perhaps two-fifths) is ingested 

 directly as cold or warm drinks ; part, about two-fifths, is ingested mixed 

 chemically or mechanically with the food; the other fifth is produced 

 anew by the body-katabolism. (The water excreted by the kidneys is 

 only about half that excreted by the body, most of the remainder going 

 out through the skin, and half a liter or less through the respiratory 

 tubes.) The ingested water so far as known is not altered, unless it be 

 mechanically, for there is no evidence that any of it is broken up, nor 

 that it combines chemically with any element of protoplasm. If it did 

 so, however, we could not know it, and the possibilities of its chemical 

 reactions in the metabolism are very many. The water actually pro- 

 duced in the body is made by the oxidation of hydrogen. The fats 

 especially are productive water-formers, for they contain much more 

 hydrogen than is necessary to satisfy their oxygen, and are therefore 

 fuel of the best type. The empirical formula of stearin, for example, is 

 C 57 H 110 O 6 , which shows at a glance the large amount of hydrogen with 

 a maximum combustion-value available for oxidation, the oxygen for 

 which respiration supplies. The carbohydrates and the proteids like- 

 wise contribute to the water-making, the former much more largely than 

 the latter. The water is produced largely in the muscles from the 

 combustion of glycogen and dextrose, but also wherever carbohydrate and 

 fatty food or tissue is katabolized with the absorption of oxygen. The 

 oxidative process probably occurs to some extent everywhere in the site 

 of the former molecules. The katabolism precedes the oxidation rather 

 than vice versa, for oxygen has no power to break down the protoplasmic 

 molecule ; it has however, great chemical affinity for simple combustibles 

 which are free to combine with it. 



THE COMPOSITION OF THE URINE is a matter of much importance 

 theoretically and practically, for it is this liquid which offers the best 

 chance to learn what goes on chemically at different times under a multi- 

 tude of various conditions, dietetic, metabolic, and pathologic, within 

 the hidden tissues. The urine is able still to teach the physiologist 

 much more even than it has taught him about metabolism, and the 

 physician very much about the condition of his patient. 



