MALPIGHIAN BODIES OF THE KIDNEY, 311 
the urinary tubes, as may keep the pressure within the vessels 
very nearly at a uniform standard ; an’ 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- 
vate the interior of the capil- 
lary vessels from the cavity of 
the uriniferous tube, being of Fiz- 113 ares ae or 
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 uyon 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 versdé,—and in part upon the 
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 soft-skinned animals; 
