NITROGENOUS WASTE-PRODUCTS 541 



would in cold-blooded animals. It is to this that the exhausting effects 

 of the Tropical Climate are to be referred. A temperature of 86 F. 

 in an atmosphere saturated with moisture is almost unbearable, and 

 physical work is, for Europeans at least, an impossibility, while a 

 temperature of 110 F. in a perfectly dry atmosphere can be endured, 

 and even a considerable amount of physical work performed, without 

 any exceptional discomfort, by persons in normal health whose tem- 

 perature-regulating mechanisms are in good order. 



Among the remaining carbonaceous waste-products under normal 

 physiological conditions may be enumerated Methane which is derived 

 from bacterial fermentations in the intestine but is exhaled mainly 

 through the lungs. The quantity of methane produced by carnivora 

 and animals which subsist upon a mixed diet, such as ourselves, is 

 normally a very small proportion of the total carbon output, but in 

 herbivora it may become very appreciable. Oxalic Acid is regularly 

 found in normal urine in very small amounts, the normal excretion 

 being about 0.02 grams in twenty-four hours. Its origin is unknown. 

 As it is a frequent product of bacterial fermentations it may have an 

 alimentary origin, and, again, the administration of sodium oxalate 

 leads to the appearance of the unchanged oxalic acid in the urine, and 

 a number of foodstuffs, particularly fruits and vegetables, contain 

 oxalates which would therefore appear in the urine. On the other hand, 

 the output of oxalic acid continues on a pure protein diet and, on a 

 normal diet, is stated to be enhanced by the administration of con- 

 siderable quantities of gelatin, so that we may conjecture that the 

 urinary oxalic acid is in part produced by the metabolism of the tissues. 

 The output of oxalic acid is also stated to be increased in diabetes. 



Lactic Acid is only found in the urine in partial asphyxia, or after the 

 most extreme muscular exertion; its appearance in the urine indicates 

 imperfect oxidation of carbohydrates or else extraordinarily excessive 

 production by the muscular tissues. 



THE NITROGENOUS WASTE-PRODUCTS. 



Of the various nitrogenous waste-products, Urea: 



/NH 2 



co 



is quantitatively the most important. The daily output of this sub- 

 stance varies with the quantity of protein which is ingested, but for 

 the adult man subsisting upon a mixed diet the daily excretion is about 

 thirty grams, for a woman somewhat less. This corresponds to from 

 84 to 90 per cent, of the total nitrogenous output. 



The quantity of urea which is excreted varies directly with the 

 quantity of protein ingested. We have seen in preceding chapters that 

 animal tissues do not store up proteins and that their storage-capacity 



