240 NUTRITION 



(but not respectively). The salts and the water ingested are mostly 

 excreted in unchanged condition. In our knowledge of the katabolism 

 of proteids, fats, and carbohydrates there are as yet large gaps, and hence 

 our description of this must be at present partly conjectural; indeed, in 

 some of its aspects largely so. We shall do best, perhaps, by keeping 

 out attention mainly on the nitrogen, the carbon-dioxide, and the water 

 of katabolism. For practical convenience, however, we must describe 

 this general katabolic excretion under the heads of the respective ex- 

 creting organs : the kidneys excreting urine, the lungs carbon dioxide and 

 water, the rectum feces, the liver bile, and the skin also water and 

 carbon dioxide. Aside from the value of metabolic theory, these excretory 

 menstrua have very large practical importance, and hence their respective 

 compositions and modes of excretion from the body must be thor- 

 oughly understood. 



The Urine. The urine excretes about 94 per cent, of the nitrogen 

 involved in proteid katabolism, 3 or 4 per cent, of the katabolic carbon, 

 about one-half of the excreted water, and a large part of the inorganic 

 salts used in the body. The nitrogen comes partly from the wasting 

 tissues, but in varying proportions also from the circulating proteid. 

 The carbon in the urine comes to a slight extent from the carbo- 

 hydrates and fats of the tissues and the "food" still circulating in the 

 blood, but mostly from the degenerating proteid molecules of the cells. 

 The water is larely ingested as such, but about one-fourth part of it 

 appears to be liberated or even compounded from the fats and car- 

 bohydrates katabolized in the body. 



First, as to the nitrogen. As we saw above, the proteids absorbed 

 from the intestine pass into the latter's epithelium as proteoses or pep- 

 tones, or else (Bayliss and Starling) this epithelium constructs the 

 peptones from the amido-acid products of tryptic zymolysis. These 

 cells or the endothelium of the capillaries, or both, probably dehydrate 

 the proteoses or peptones and change them over into the serum-albumin, 

 serum-globulin, etc., of the blood. As such native proteids, then, the 

 protein material from the gut passes through the portal vein on its way 

 to the liver, first soaking through the spleen. There is no evidence 

 that any of the circulating or food-pro teid is stopped by the liver except 

 when fat and carbohydrate are entirely lacking in the food. In this case, 

 owing to the energetic demand of the body for these two proximate 

 principles to furnish heat and power, some of the food-proteid from the 

 intestine may be retained and changed into these substances along 

 with urea. This urea-part of the suggested anabolism, however, would 

 represent an unlikely waste of precious nitrogenous material, unless, 

 indeed, the tissue-pro teids and the circulating food-pro teids are so closely 

 allied that their separation is impossible. For this possible formation 

 of fat, glycogen, and urea from protein in the liver, Dubois suggests the 

 following equation, not as just what actually takes place, but as something 

 similar perhaps to the reaction. (The proteid-formula is Lieberkiihn's 

 and also Loew's conjecture for albumin.) 



