390 EXCRETION BY THE SKIN AND KIDNEYS. 



tion of the organism, but it contains matters evidently derived from the 

 food. Its constitution is varying Avith every different condition of nutrition, 

 with exercise, bodily and mental, with sleep, age, sex, diet, respiratory 

 activity, the quantity of cutaneous exhalation, and, indeed, Avith every con- 

 dition that affects any part of the system. There is no fluid in the body 

 that presents such a variety of constituents as a constant condition, but in 

 which the proportion of these constituents is so variable. It is for this 

 reason that in the table of the composition of the urine, the ordinary limits 

 of variation of its different constituents have been given ; and it has been 

 found necessary, in treating of the individual excrementitious products, to 

 refer to some of the variations in their proportion in the urine. 



Variations iviih Age and Sex. There are decided differences in the 

 composition of the urine at different periods of life and in the sexes. These 

 undoubtedly depend in part upon the different conditions of nutrition and 

 exercise and in part upon differences in the food. Although the quantities 

 of excrementitious matters present great variations, their relations to the 

 organism are not materially modified, except, perhaps, at an early age ; and 

 the influence of sex and age operates merely as these conditions affect the 

 diet and the general habits of life. 



It has been stated that urea does not exist in the urine of the foetus : but 

 in a specimen of urine taken from a still-born child deliA - ered with forceps, 

 examined by Elliot and Isaacs, the presence of urea Avas determined. Beale 

 found urea in a specimen taken at the seventh month. Observations upon 

 children between the ages of three and seven have shown that at this period 

 of life, the urea excreted in proportion to the weight of the body is about 

 double the quantity excreted in the adult. The chlorine in the urine of chil- 

 dren is about three times the quantity in the adult ; and the quantities of 

 other solid matters are also greater. The quantity of water excreted by the 

 kidneys in children, in proportion to the weight of the body, is very much 

 greater than in the adult, being more than double. Between the ages of eight 

 and eighteen years, the urinary excretion gradually approximates the stand- 

 ard in the adult. It has been observed that crystals of calcium oxalate are 

 much more frequent in the urine of children between four and fourteen 

 years of age than in the adult. 



There are not many definite obsen*ations on record upon the composition 

 of the urine in the later periods of life. It has been shown, however, that 

 there is a decided diminution, at this time, in the excretion of urea, and that 

 the absolute quantity of urine is someAvhat less. 



The absolute quantity of the urinary excretion in women is less than in 

 men, and the same is true of the quantity in proportion to the Aveight of the 

 body ; still, the differences are not very marked, and the proportion of the 

 urinary constituents being subject to modifications from the same caus< 

 in men, the small deficiency, in the feAv direct observations on 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. 



