SOURCES OF THE UREA. 777 



of carbon. One ounce, taken as the ordinary daily excretion, con- 

 tains about 220 grains of nitrogen. 



The sources of the urea are evidently nitrogenous organic com- 

 pounds, which have undergone decomposition by partial oxidation. 

 It constitutes the highest product of oxidation of the albuminoid and 

 gelatinoid substances in the body. It is derived, partly from the tis- 

 sues, but partly from the food, merely assimilated into blood ; not, as 

 was at one time supposed, from the tissues only. This is proved by 

 many facts. Thus, the urea is always increased after meals, espe- 

 cially about three or four hours after the food is taken. In animals 

 fed on too little nitrogenous food to counterbalance the waste of the 

 albuminoid tissues, more urea is given off than the nitrogen in the 

 food would form; when the waste is just compensated for, then the 

 urea is equal to the nitrogen in the food ; lastly, when an excess of 

 nitrogenous food is given, the weight of the animal increases, and, 

 after a time, an excess of urea is eliminated. Urea is still excreted, 

 even in starving animals, though in smaller quantity than usual ; it is 

 increased by feeding them on a vegetable diet containing nitrogen, 

 especially on bread and beans; its quantity is still greater, on a mixed 

 vegetable and animal diet, but it is greatest of all, on an exclusively 

 animal diet. In a dog weighing 30 kilogrammes, the daily excretion 

 of urea, with a pure animal diet, varied from 150 to 180 grammes 

 that is, it equalled sJ^th or y^^th of the weight of the body. In* 

 Man, with an exclusively animal diet, the daily quantity excreted was 

 found by Lehmann to be about 820 grains, with a mixed diet 500 

 grains, with a vegetable diet 347 grains, and with a completely non- 

 nitrogenous diet 237 grains. The researches of Dr. E. Smith confirm 

 these results, and further show that an animal diet increases the ex- 

 cretion of carbonic acid from the lungs. In other experiments, the 

 quantity excreted daily, on a superabundant animal diet, was found 

 to be nearly 3 oz. ; on a moderately animal diet continued for ten 

 days, from 1} to 2 oz.; and after a diet of sugar, prolonged for four 

 days, the daily quantity of urea was rather less than J oz. Not only, 

 then, is urea formed largely from the food, but chiefly so, the quan- 

 tity derived from the tissues, as above shown, when a non-nitrogenous 

 saccharine diet was taken, being less than half the ordinary daily 

 amount. Sometimes even more is eliminated during a total abstinence 

 from food, as if, in the latter condition, an animal maintained its tem- 

 perature by waste of its nitrogenous tissues. 



In the female, from her smaller frame, her less active nutrient 

 metamorphoses, and the smaller quantity of food consumed, the daily 

 quantity excreted is about f- of an oz. Proportionally to the weight 

 of the body, it is less abundantly formed in women ; but children up 

 to seven years old excrete about twice as much urea, proportionally, 

 as adults, and infants more than children; in old age, the relative 

 amount is diminished. The effect of age depends upon the diminished 

 activity of the nutritive functions, and the smaller quantity of the food. 

 Exercise was formerly believed to increase the quantity of urea, and 

 rest to have the apposite effect ; but recen>t observations show that 

 the immediate effect of exercise is to diminish the excretion of urea,. 



