VARIATIONS IN THE COMPOSITION OF THE URINE. 437 



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 Periods of the Day. The changes 

 in the quantity and composition of the urine which may be directly referred to the con- 

 ditions 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 appreciated. 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 important 

 excrementitious matters is derived from the food and not from physiological disintegration 

 of the tissues. Under strictly physiological conditions, the modifying influence of diges- 

 tion 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 physio- 

 logical modifications in the other processes and conditions of life. It will be sufficient 

 for our purpose to note the most important of these variations and to endeavor to appre- 

 ciate the conditions which combine to produce them, assigning to each one its proper 

 value. 



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 discharged from the body at all times ; and, when the tem- 

 perature 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. This fact is a matter 

 of common remark as well as of scientific observation. 



At different periods of the day, the urine presents constant and important variations. 

 It is evident that the specific gravity must be constantly varying with the proportion of 

 water and 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 tolerably definite diurnal 

 variations, which have already been noted. 



Variations produced by Food. An immense number of observations have been made 

 upon the influence of ordinary food and upon diet restricted to particular articles. These 

 facts have necessarily been considered more or less fully in connection with the origin 

 of the urinary constituents ; but it is important, in studying the influence of muscular 

 exercise, mental effort, etc., to constantly bear in mind the variations occurring under 

 the influence of the ingesta. 



Water and liquids generally increase the proportion of water in the urine and dimin- 

 ish the specific gravity. This is so marked after the ingestion of large quantities of 

 liquids, that the urine passed under these conditions is sometimes spoken of by phys- 

 iologists as the urina potus. This must be borne in mind in clinical examinations of 

 the urine. It is a curious fact, however, that, when an excess of water has been taken 

 for purposes of experiment, the diet being carefully regulated, the absolute amount of 

 solid matters excreted is considerably increased. This is particularly marked in the 

 urea, but it is noticeable in the sulphates and phosphates, though not to any great 



