THE SOURCES OF ENERGY 



73 



urine increases, and vice versa. Thus the secretory activity of 

 the kidneys can be regulated. Similar regulatory mechanisms 

 exist everywhere over the animal body, and the nerves mainly 

 responsible belong to what is called the sympathetic nervous 

 system. 



Now look at the minute structure of the kidney. Here we 

 represent, very simply, and on a much bigger scale than the 

 natural one, one of the tubules that excrete the water, urea, etc. 

 First of all the blood in the renal artery becomes distributed 

 through a multitude of branches or twigs of this bloodvessel, and 

 we may regard each twig as going to a peculiar structure called 

 a glomerulus. 



Capillary 

 knot x 



Outer (Jill 

 of glomerulus 



FIG. 20. THE GLOMERULUS OF A URTNIFEROUS TUBULE, WITH ITS 

 BLOODVESSELS. HIGHLY MAGNIFIED. 



This glomerulus is like a bulb at the end of a urinif erous tubule. 

 )ne part of the bulb is pushed in, and a twig of the renal artery 

 enters and breaks up into a little complex knot of capillaries, 

 from which a twig of the renal vein gathers up the blood and 

 takes it away. This little twig of the renal vein again breaks up 

 into capillaries, which surround the rest of the urinif erous tubule, 

 as shown in Fig. 19. Thus we see a rich blood-supply carried 

 by capillaries surrounding the parts of the gland that secrete. 

 AVater is secreted from the blood in the capillaries of the glomeru- 

 lus, and this water oozes through the inner wall of the bulb, 

 and so goes into the uriniferous tubule. Then the cells of the 

 latter take urea, etc., from the blood in the capillaries that sur- 



