KIDNEYS, SKIN, AND GENERAL EXCRETION. 395 



in the body, and acting as a poison deposited usually in the 

 joints, give rise to the familiar symptoms of gout and kin- 

 dred diseases. 



The aromatic bodies, such as hippuric acid, kresol, in- 

 dol and skatol are found in very small quantities. Hippuric 

 acid, however, occurs in relatively very large amounts in 

 the renal secretion of herbivorous animals. The kresol, in- 

 dol and skatol are substances which have resulted from the 

 disintegration of albumens in the intestine, but having 

 been absorbed by the blood are eliminated again through 

 the kidneys. 



The characteristic color of the renal secretion is due to 

 a number of pigments. The best known of these is the 

 pigment known as uro-bilin. This uro-bilin is probably de- 

 rived from the same source as the bilirubin of the bile, that 

 is, from the disintegration of red corpuscles, or more ex- 

 actly, from the haemoglobin of red corpuscles. 



The mineral salts include a number of mineral sub- 

 stances which are the products of tissue disintegration, but 

 in addition excesses of salts which have been directly elim- 

 inated from the blood. Experiments show that when cer- 

 tain mineral salts in increased quantities reach the blood 

 they are at once eliminated in this manner. On this account 

 the eating of even excessive amounts of salt has no directly 

 injurious effect whatever upon the body. 



It is sometimes of the utmost importance to the physi- 

 cian to be able to determine the composition of urine, espe- 

 cially when the presence of sugar or albumen is suspected. 



Quite a large additional number of chemical substances 

 occur in more or less minute quantities in the secretion, but 

 the nature of these substances from a chemical standpoint 

 is so complicated, and in the real physiology of the body 

 they play such an unimportant role that in this discussion 

 they are omitted altogether. 



By way of summary it may be stated then that the real 

 significance of the physiology of the kidney lies in its 

 power to eliminate superfluous or injurious substances, 



