CHAPTER X 



THE URINE 



The urine is sometimes spoken of as a secretion, but this is not 

 strictly correct ; speaking broadly, it may be said that a secretion 

 is something which is formed in a part for the purpose of being 

 eventually utilised by the system. This does not apply to the 

 urine, the chief constituents of which are not even prepared in 

 the kidneys, but only separated by them ; moreover, the urine 

 having once been formed is of no further use to the body, and is 

 excreted. An excretion, therefore, is something removed from 

 the system as being no longer required, and the retention of 

 which would be harmful. Such a removal is effected by the 

 kidneys, which may in a sense be regarded as the filters of the 

 body, regulating the composition of the blood by removing from 

 it waste and poisonous products, and maintaining, as will be 

 later explained, its proper degree of chemical neutrality. 



The method adopted by the kidney for the secretion of urine 

 has been for many years one of the chief fields for physiological 

 dispute. A pair of very vascular glands are capable of removing 

 from the blood a fluid which is essentially different in composi- 

 tion to the blood itself. The blood is neutral in reaction, the 

 urine acid or alkaline, depending on the class of animal ; the 

 blood is a proteic fluid, the urine in a state of health is free from 

 protein ; the blood contains sugar, the urine contains none ; the 

 blood has one colouring matter, the urine another ; the blood 

 contains urea and salts in small quantities, the urine contains 

 them in relatively large amounts ; the blood maintains the whole 

 of its inorganic material in solution or packed away in such a 

 form as to be readily soluble, the urine may be of such concen- 

 tration or reaction as to be unable to retain its substances in 

 solution. Nevertheless, the kidney only takes from the blood 

 what is brought to it, for, with the single exception of hippuric 

 acid, none of the other urinary constituents are formed in the 

 gland. There is no other body-secretion which exhibits these 

 striking differences, and, further, there is no other gland which 

 resembles the peculiar histological structure of the kidney. 



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