262 UNCONSCIOUS NERVOUS OPERATIONS 



dioxide. To the kidneys, then, belongs the important 

 function of removing from the body the waste product 

 of nitrogenous food in all the living tissues. 



382. Urea. The final result of the changes which 

 nitrogenous foods undergo is urea, and its chief ultimate 

 source is the most abundant tissue muscular tissue. 

 There is, however, no urea in the muscles. Other steps 

 in its production lie between the muscles and the kidneys. 

 The spleen, lymphatic glands, and other glands have to do 

 with its formation, but the final and most distinctive changes 

 appear to occur in the liver. If the urea is not removed 

 from the system, it destroys life. The quantity daily 

 secreted by the human organism is about five hundred 

 grains, which is normally 2 per cent of the total excre- 

 tion from the kidneys. 



383. Relation between the Kidneys and the Skin. The 

 relation between the skin and the kidneys is such that if 

 one channel of excretion becomes clogged, extra labor 

 appears to be thrown upon the other. If perspiration is 

 checked, there is, along with diminished activity of the 

 sweat glands, constriction of the blood vessels of the sur- 

 face, followed by dilation of those of the viscera, which 

 permits an increased flow of blood in the internal organs, 

 including the kidneys. The secretion of the kidneys, there- 

 fore, becomes more abundant. When by warmth or exer- 

 cise the sweat glands are stimulated, the reverse is the case : 

 the vessels of the skin are dilated, while those of the abdo- 

 men are constricted, and the renal secretion becomes scanty. 



384. The Kidneys and the Alimentary Canal. Still more 

 important seems to be the connection between the amount 

 of the kidney secretion and the water absorbed by the 

 walls of the alimentary canal. When a large quantity of 

 water is drunk, it passes directly into the circulation, and, 



