386 A MANUAL OF PHYSIOLOGY 



which are of the same general type as those of the mucous 

 and serous salivary glands. The lachrymal glands are 

 serous like the parotid ; and the tears secreted by them 

 contain some albumin and salts, but little or no mucin. The 

 sexual secretions and milk will be best considered under 

 reproduction (Chap. XIV.), so that there remain only the 

 urine and the secretions of the skin to be treated here. 



I. Excretion by the Kidneys. 



The Chemistry of the Urine. Normal urine is a clear 

 yellow liquid of acid reaction, and with an average specific 

 gravity of about 1020, the usual limits being 1015 and 1025, 

 although when water is taken in large quantities, or long 

 withheld, the specific gravity may fall to 1005, or even less, 

 or rise to 1035, or even more. The quantity passed in 

 twenty-four hours is very variable, and is especially 

 dependent on the activity of the sweat-glands, being, as a 

 rule, smaller in summer when the skin sweats much, than. 

 in winter when it sweats little. The average quantity for 

 an adult male is 1200 to 1600 c.c. (say, 40 to 50 oz.).* 



Composition of Urine. It is essentially a solution of ureai 

 and inorganic salts, the proportion of the latter being 

 about 1*5 per cent., or double the usual amount in physio- 

 logical liquids. Besides urea, there are other nitrogenous. 

 bodies in much smaller quantity, such as uric acid and the.- 

 allied xanthin bases, hippuric acid, and kreatinin ; some of 

 these at least are products of the metabolism of proteids 

 within the tissues. And besides the inorganic salts there are 

 certain organic bodies indol, phenol, pyrokatechin, skatols 

 united with sulphuric acid, which are primarily derived 

 from the products of the putrefaction of proteids within the 

 digestive tube. In tabular form the composition of urine,. 



* The average quantity of urine varies not only with the season, but 

 also with the habits of the person, especially as regards the amount of 

 liquid taken. The average for seventeen healthy (American) students, 

 whose urine was collected for six to eight successive days in winter, was 

 1166 c.c. The highest average in any one individual for the observation 

 period was 1487 c.c. (for seven days), and the lowest 743 c.c. (for eight 

 days). The greatest quantity passed in anyone period of twenty-four 

 hours was 2286 c.c. (by the individual whose average was the highest). 

 The smallest quantity passed in twenty-four hours was 650 c.c. (by the.- 

 individual whose average was the lowest). 



