r VARIATIONS IN THE COMPOSITION OF THE URINE. 391 

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

 The changes in the quantity and composition 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 so to sepa- 

 rate these influences that the true modifying value of each can be fully 

 appreciated. For example, there is nothing which produces such marked 

 variations in the composition of the urine as the digestion of food. Under 

 strictly physiological conditions, the modifying influence 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 various processes and conditions of 

 life. 



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

 sents certain variations in its quantity and composition. It seems necessary 

 that a tolerably definite quantity of water should be discharged from the 

 body at all times ; and when the temperature or the hygrometric condition of 

 the atmosphere is favorable to the action or the skin, as in a warm, dry cli- 

 mate, 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 varia- 

 tions. It is evident that the specific gravity must be constantly varying 

 with the relative proportions of water and of solid constituents. According 

 to Dalton, 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. The acidity is also subject to certain variations, which 

 have already been mentioned. 



Influence of Mental Exertion. Although the influence of mental exer- 

 tion upon the composition of the urine has not been very closely studied, the 

 results of the investigations which have been made upon this subject are in 

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

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

 nervous system. Fear, anger, and various violent emotions, sometimes pro- 

 duce a sudden and copious secretion of urine containing a large proportion of 

 water, and this is often observed in cases of hysteria. Intense mental exer- 

 tion will occasionally produce the same result. In studying the influence 

 of cerebral activity upon the composition of the urine, Byasson found that 

 by mental exertion the quantity of urine was increased ; the urea was also 

 increased ; the phosphoric acid was increased about one-third ; the sulphuric 

 acid was more than doubled ; and the chlorine was nearly doubled. 



The products of spontaneous decomposition of the urine have a certain, 

 chemical interest but are of no physiological importance. 



