222 EXCRETION. 



There are not many definite observations on record upon 

 the composition of the urine in the latter periods of life. It 

 has been shown, however, that there is a decided diminution, 

 at this period, in the excretion of urea, and that the absolute 

 quantity of the urine is somewhat smaller. 



The absolute quantity of the urinary excretion in women 

 is less than in men, and the same is true of the proportionate 

 amount of these principles to the weight of the body ; still, 

 the differences in the proportionate excretion are not very 

 marked, and the amount of all these principles being subject 

 to modifications from the same causes as in men, the small 

 deficiency, in the few direct observations upon record, may 

 be in part, if not entirely, explained by the fact that women 

 usually perform less mental and physical work than men, 

 and that their digestive system is generally not so active. 



Variations at Different Seasons, and at Different Peri- 

 ods of the Day. The changes in the quantity and com- 

 position of the urine which may be directly referred to 

 the conditions of digestion, temperature, sleep, exercise, 

 etc., have long been recognized by physiologists ; but it is 

 difficult, if not impossible, so to separate these influences, 

 that the true modifying value of each can be fully appre- 

 ciated. For example, there is nothing which produces such 

 marked variations in the composition of the urine as the 

 digestion of food. So marked, indeed, is its influence, that 

 some writers of authority incline to the belief that the 

 greatest part of what have been regarded as the most im- 

 portant excrementitious matters is derived from the food, 

 and not from physiological disintegration of the tissues. 

 Under strictly physiological conditions, the modifying in- 

 fluence of digestion must always complicate observations 

 upon the effects of exercise, sleep, season, period of the 

 day, etc. ; and the urine is continually varying in health, 

 with the physiological modifications in the other processes 

 and conditions of life. It will be sufficient for our purpose 



