MALPIGHIAN BODIES OF THE KIDNEY; 



311 



Fig. 173. 



-MALPIGHIAK BODIES 

 THE KIDNEY. 



the urinary tubes, as may keep the pressure within the vessels 

 very nearly at a uniform standard ; anl a distinct and very 

 curious provision exists for its separation. The extremity of 

 many of the uriniferous tubes is made to include little knots 

 or bunches of capillary vessels, 

 which have extremely thin 

 walls (fig. 173) ; and a vast 

 number of such knots, which are 

 termed " Malpighian bodies," 

 after the name of their dis- 

 coverer, are scattered through 

 the cortical portion ( 368) of 

 the kidney. To these the blood 

 brought to the organ by the 

 renal artery is first conveyed; 

 and the membranes that sepa- 

 rate the interior of the capil- 

 lary vessels from the cavity of 

 the uriniferous tube, being of 

 extreme thinness, water is 

 readily able to traverse them; and will do so in larger or 

 smaller quantity, according as the pressure upon the walls 

 of the capillaries is greater or less. The blood which has 

 passed through these is next conducted to another set of 

 capillaries, which form a network upon the part of the tube 

 that is lined by the secreting cells ; and it is there subservient 

 to the elaboration of the solid part of the secretion. Hence 

 the quantity of water in the urinary secretion depends in part 

 upon the amount exhaled from the skin, being greatest 

 when this is least, and vice versa, and in part upon th& 

 quantity which has been absorbed by the vessels. The quan- 

 tity of solid matter in the secretion has but little to do with 

 this ; for it depends upon the amount of waste of the muscular 

 and other tissues that has been occasioned by their activity 

 (367); and also upon the quantity of surplus aliment which 

 has to be discharged through this channel, there being no 

 other vent for it ( 348). 



370. Next to the excretions formed by the liver and the 

 kidneys, that of the Skin probably ranks in importance. A 

 large quantity of watery vapour is constantly passing-off from 

 the whole surface of Man and other sofVskin&ed animals; 



