274 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



human urine may show a neutral or even a slightly alkaline reaction, especially 

 after meals. In fact, the reaction of the urine seems to depend directly on the 

 character of the food. Among carnivorous animals the urine is uniformly 

 acid, and among herbivorous animals it is uniformly alkaline, so long as 

 they are using a vegetable diet, but when starving or when living upon the 

 mother's milk that is, whenever they are existing upon a purely animal diet 

 the urine becomes acid. The explanation, as given by Drechsel, is that upon 

 an animal diet more acids are produced (from the sulphur and phosphorus) 

 than the bases present can neutralize, whereas upon a vegetable diet carbonates 

 are formed from the oxidation of the organic acids of the food in quantities 

 sufficient to neutralize the mineral acids. The chemical composition of urine is 

 very complex. Among the constituents constantly present under the conditions 

 of normal life we have, in addition to water and inorganic salts, the following 

 substances : Urea ; uric acid ; xanthin ; creatinin ; hippuric acid ; the urinary 

 pigments (urobilin) ; sulphocyanides in traces ; acetone ; oxalic acid, probably 

 as calcium oxalate ; several ethereal sulphuric acids, such as phenol and cresol 

 sulphuric acids, indoxyl sulphuric acid (indican), and skatoxyl sulphuric acid ; 

 aromatic oxy-acid ; some combinations of glycuronic acid ; some representa- 

 tives of the fatty acids ; and dissolved gases (N and CO 2 ). This list would be 

 very much extended if it attempted to take in all those substances occasion- 

 ally found in the urine. The complexity of the composition and the fact that 

 so many different organic compounds occur or may occur in small quantities 

 is readily understood when we consider the nature of the secretion. Through 

 the kidneys there are eliminated not only what we might call the normal end- 

 products of the metabolism of the tissues, excluding the CO 2 , but also, in 

 large part, the products of decomposition in the alimentary canal, the end- 

 products of many organic substances occurring in our foods and not usually 

 classed as food-stuffs, foreign substances introduced as drugs, etc., all of which 

 are eliminated either in the form in which they are taken or as derivative 

 products of some kind. We shall speak briefly of the most important of the 

 normal constituents, dwelling especially upon their origin in the body and their 

 physiological significance. For details of chemical properties, reactions, meth- 

 ods of preparation, etc. reference must be made to the Chemical section. 



Urea. Urea, which is given the formula CH 4 N 2 O, is usually considered 

 as an amide of carbonic acid, having therefore the structural formula of 



CO<-^jj 2 - It occurs in the urine in relatively large quantities (2 per cent, -f ). 



As the total quantity of urine secreted in twenty-four hours by an adult male 

 may be placed at from 1500 to 1700 cubic centimeters, it follows that from 30 

 to 34 grams of urea are eliminated from the body during this period It is 

 the most important of the nitrogenous excreta of the body, the end-product 

 of the physiological oxidation of the proteids of the body, and also of the 

 albuminoids when they appear in the food. If we know how much urea is 

 secreted in a given period, we know approximately how much proteid has 

 been broken down in the body in the same time. In round numbers, 1 gram 



