EXCRETION 199 



tubule present the appearance of a secreting mechanism. The 

 large soft, cloudy cells which line them are eminently fitted to 

 take from the blood, or rather from the lymph which fills the 

 tissue-spaces which intervene between the walls of the capillary 

 bloodvessels and tubules, the various substances which they 

 excrete. (3) The loop of Henle is a remarkable piece of 

 apparatus, the purpose of which has been a subject of much 

 controversy. Looking at it from the point of view of hydro- 

 statics, it seems safe to conclude, from its extremely narrow 

 bore, that it raises the pressure of the fluid in the glomerulus and 

 first contorted portion ; but it may have other functions also. 

 A consideration of the arrangement of the bloodvessels of 

 the kidney bears out the conclusion that the secreting apparatus 

 is divisible into at least two separate portions, possibly into 

 three. The glomeruli are supplied by short and relatively 

 wide arterioles. Each arteriole breaks up, as soon as it enters 

 the capsule, into a bunch of capillary vessels, which, in the 

 same abrupt manner, reunite to form a venule. On leaving 

 the capsule, this little vein behaves in a fashion for which the 

 only parallel is to be found in the portal system of the liver. 

 Instead of uniting with a larger vein, it again breaks up into 

 capillary vessels, which supply the contorted tubules and 

 loops of Henle. The medulla of the kidney is supplied by long 

 arterial capillaries of the usual type. The short arterioles of 

 the glomeruli are controlled by nerves which, constricting 

 them, or allowing them to dilate possibly by actively causing 

 them to dilate rapidly diminish or increase the amount of 

 blood passing through their tufts of capillary vessels. Here, 

 therefore, is a mechanism by which the glomeruli can be sud- 

 denly flushed with blood a condition favourable to exudation 

 into the urinary tubules. The interposition of a second set 

 of capillaries prevents this sudden flushing from unduly disturb- 

 ing the pressure in the vascular system as a whole. In the 

 renal-portal capillaries of the kidney the blood-pressure is 

 fairly constant and, presumably, low. The use of the term 

 " renal-portal " is justifiable, not only on the ground that 

 the vessels of the kidney behave like those of the portal system 

 of the liver, but also owing to the very significant fact that in 

 fishes and amphibia the kidney actually has a double blood- 

 supply. In such an animal as the frog the glomeruli are 



