OF SECRETION AND EXCRETION. 



Fig. 167. 



vides itself into several minute branches, terminating in convoluted capillaries, 

 which are collected in the form of a ball (m, m) ; and from the interior of the 

 ball, the solitary efferent vessel, e /, arises, which passes out of the capsule by 

 the side of the single afferent vessel. This ball seems to lie loose and bare in 

 the capsule, being attached to it only by its afferent and efferent vessels (Fig. 

 166, m). The efferent vessels, on leaving the Malpighian bodies, separately 

 enter the plexus of capillaries, p, surrounding the tubuli uriniferi, st, and supply 

 that plexus with blood ; from this plexus the renal vein arises. Thus there is 

 a striking analogy between the mode in which the tubuli uriniferi are supplied 

 with blood, for the purpose of elaborating their secretion, and the plan on which 

 the hepatic circulation is carried on. For as the secretion of the Liver is formed 

 from blood conveyed to it by one large vessel, the vena portae, which has col- 

 lected it from the venous capillaries of the chylopoietic viscera, and which sub- 

 divides again to distribute it through the liver, so the secretion of the Kidney 

 is elaborated from blood which has already passed through one set of capillary 

 vessels, those of the Malpighian tufts; this blood is 

 collected and conveyed to the proper secreting surface, 

 however, not by one large trunk (which would have 

 been a very inconvenient arrangement), but by a mul- 

 titude of small ones, the efferent vessels of the Mal- 

 pighian bodies, which may be regarded as collectively 

 representing the vena portse, since they convey the blood 

 from the systemic to the secreting capillaries. Hence 

 the Kidney may be said to have a portal system within 

 itself. This ingenious view of Mr. Bowman's finds 

 support from the fact that in Reptiles the efferent ves- 

 sels of the Malpighian bodies (which receive their blood, 

 as elsewhere, from the renal artery) unite with the 

 branches of the vena portaa, to form the secreting plexus 

 around the tubuli uriniferi. Here, therefore, the 

 blood of the secreting plexus has a double source, the 

 vessels which supply it receiving their blood in part 

 from the capillaries of the organ itself, and in part 

 from those of viscera external to it; just as, in the 

 Liver, the secreting plexus is supplied in part by the 

 nutritive capillaries of the organ itself, which receive 

 their blood from the hepatic artery, and in part by the 

 blood conveyed from the chylopoietic viscera through 

 the vena portae. 



636. These admirable researches of Mr. Bowman on the structure of the Mal- 

 pighian bodies, and on the vascular apparatus of the Kidney, have thrown great 

 light upon the mode in which the Urinary secretion is elaborated. One of the 

 most remarkable circumstances attending this excretion, in the Mammalia par- 

 ticularly, is the large but variable quantity of water which is thus got rid of 

 the amount of which bears no constant proportion to that of the solid matter 

 dissolved in it. The Kidneys, in fact, seem to form a kind of regulating valve, 

 by which the quantity of water in the system is kept to its proper amount. 

 The amount of exhalation from the Skin, which, with that from the Lungs, is 

 the other principal means of removing superfluous liquid from the blood, is liable 

 to be greatly affected by the temperature and degree of humidity of the air 

 around ( 647) : hence, if there were not some other means of adjusting the 

 quantity of fluid in the bloodvessels, it would be subject to continual and very 

 injurious variation. This important function is performed by the Kidneys; 

 which allow such a quantity of water to pass into the urinary tubes, as may keep 

 the pressure within the vessels at a nearly uniform standard. The quantity of 



Distribution of the Renal ves- 

 sels ; from Kidney of Horse : a, 

 branch of Renal artery : a/, af- 

 ferent vessel ; m, m, Malpighian 

 tufts ; ef, ef, efferent vessels ; p, 

 vascular plexus surrounding 

 the tubes ; st, straight tube ; ct, 

 convoluted tube. 



