VARIATIONS IN THE COMPOSITION OF THE URINE 353 



that women usually perform less mental and physical work than men 

 and that their digestive system is not so active. 



Variations at Different Seasons and at Different Periods of the Day. 

 The changes in the quantity and composition of the urine that may 

 be referred directly to conditions of digestion, temperature, sleep, exer- 

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

 so to separate these influences that the true modifying value of each 

 can be fully appreciated. For example, there is nothing which pro- 

 duces such marked variations in the composition of the urine as diges- 

 tion. Under strictly physiological conditions, the modifying influence 

 of digestion must always complicate observations on the effects of 

 exercise, sleep, season, period of the day etc. ; and the urine is con- 

 tinually varying in health, with the physiological modifications in the 

 different processes and conditions of life. 



At different seasons of the year and in different climates, the urine 

 presents certain variations in its quantity and composition. It seems 

 necessary that a tolerably definite quantity of water should be dis- 

 charged from the body at all times ; and when the temperature or the 

 hygrometric condition of the atmosphere is favorable to the action of 

 the skin, as in a warm, dry climate, the quantity of water in the urine 

 is diminished and its proportion of solid matters is correspondingly 

 increased. On the other hand, the reverse obtains when the action 

 of the skin is diminished from any cause. 



At different times of the day the urine presents certain important 

 variations. It is evident that the specific gravity must vary with the 

 relative proportions of water and solid constituents. The urine first 

 discharged in the morning is dense and highly colored ; that passed 

 during the forenoon is pale and of a low specific gravity ; and in the 

 afternoon and evening it is again deeply colored and its specific gravity 

 is increased. Its acidity is also subject to certain variations, which have 

 already been mentioned. 



Influence of Mental Exertion. Although the influence of mental 

 exertion on the composition of the urine has not been very closely 

 studied, the results of the investigations that have been made are in 

 many regards quite satisfactory. It is a matter of common remark that 

 the secretion of urine often is modified to a considerable extent through 

 the nervous system. Fear, anger and various violent emotions some- 

 times produce a sudden and copious secretion of urine containing a 

 large proportion of water, and this is often observed in cases of hysteria. 

 Intense mental exertion will occasionally produce the same result. 



Internal Secretion. It is possible that the kidneys are the seat of 

 an internal secretion analogous to the internal secretion of the supra- 



