72 



THE MECHANISM OF LIFE 



Surface e of hidn^^, 



carried to the liver, where they are converted into this latter 

 substance. The blood circulating in the body therefore contains 

 a certain small proportion of urea. This is the waste product 

 of nitrogenous metabolism ; it has no useful function, and it must 

 be removed from the system. 



We consider the kidneys in some detail, because they may be 

 taken as types of glands, structures which we have not, so far, 

 described. 



Each of them, then, is an organ provided with (1) an artery 

 carrying blood into it from the aorta ; (2) a vein carrying blood 



away from it into the 

 posterior vena cava ; 



(3) a duct, the ureter, 

 which carries away the 

 water and other sub- 

 stances taken from 

 the blood that flows 

 into the gland ; and 



(4) nerves. When in 

 action, blood contain- 

 ing urea and other 

 waste substances, as 

 well as an excess of 

 water, is continually 

 flowing into the kid- 

 neys through the renal 

 arteries, while the same 

 blood deprived of these 

 waste substances and 

 of a certain quantity 

 of water is continually 

 flowing out through 



the renal veins. Water containing the urea, etc., gathers in 

 the cavities of the kidneys and slowly trickles down the 

 ureters into the urinary bladder, from which it is periodically 

 expelled. The nerves that enter the kidney go to the arteries, 

 and they act by exciting the muscles in the walls of these 

 vessels to expand or contract, thus altering their caHbre, 

 and so increasing or diminishing the quantity of circulating 

 blood. If the calibre of the renal veins remains the same, the 

 blood-pressure of the kidneys increases, and so the secretion of 



Caviiy of 

 The kidney 



Fig. 19. — One of the Secretoey Units (or 

 Urinieerous Tubules) of the Kidney 

 represented in a very Diagrammatic 

 Way. Magnified. 



