EXCRETION 319 



times as much as a pint in twenty-four hours. The 

 kidneys often discharge as much as 3 pints of water 

 in the same length of time. Dissolved in it is the larger 

 part of the mineral matter requiring to be eliminated. 

 More important than this is the presence in the urine of 

 compounds of nitrogen, sulphur, and phosphorus, the 

 distinctive end-products of protein decomposition. The 

 chief of these bodies is the compound urea which was 

 mentioned in the last chapter. Experience indicates 

 that students must be cautioned against using urea and 

 urine as synonyms. Urea is the leading substance in 

 solution in urine, but the two words are not inter- 

 changeable. 



The waste from the intestine is not easily defined. It 

 is of a miscellaneous character; the bile pigments in a 

 modified form are examples of excretions passing from 

 the body by this route. The water loss from the 

 alimentary canal is normally small. As to the skin, the 

 perspiration is approximately a mineral secretion con- 

 taining little dissolved matter besides "common salt. 

 When it is profuse it may carry in very small amounts 

 organic waste-products like those of the urine. These 

 are somewhat increased when there is kidney disease, but 

 the skin can by no means compensate for the loss of the 

 renal function. 



The Work of the Kidneys. We have now to enlarge 

 upon the work of the kidneys. These are paired organs 

 placed to the right and left of the vertebral column just 

 below the diaphragm. The aorta and the inferior vena 

 cava pass between them. A large but short artery leads 

 from the aorta to either kidney; a corresponding vein 

 connects each kidney with the vena cava. Thus the 

 kidneys are in a position which favors a copious flow of 

 blood through their vessels. A short cut or shunt for the 

 blood is opened through them . Their actual blood-supply 

 in proportion to their mass is exceptionally large. It is 

 said to be exceeded in only one organ, the thyroid gland. 



The microscopic details of the kidney are of such com- 



