STRUCTURE OF THE KIDNEY. 409 



separation of the superfluous jftm'd of the system. When a section of the 

 Kidney is slightly magnified (Fig. 128, B), the cut surface is seen to be 

 studded by a number of little dark points ; each one of which, when exa- 

 mined under a higher magnifying power, is found to consist of a knot of 

 minute blood-vessels, formed by the convolutions of thin-walled capilla- 

 ries (Fig. 129, m). It has been shown by Mr. Bowman, that each one 

 of these knots is included in a flask-like capsular dilatation, connected 

 with one of the tubuli uriniferi ; several such capsules, it appears, being 

 usually developed from the sides of each tubulus, like currants upon a 

 stalk. Each of these vascular tufts (called Malpighian bodies, after 

 their discoverer) is directly supplied by a branch of the renal artery 

 (Fig. 129, af) ; which, upon piercing the capsule, subdivides into a 

 group of capillaries ; and these, after forming the convoluted tuft, coa- 

 lesce into a single efferent trunk (ef\ which may be considered as re- 

 presenting (in a small way) the vena portse. For the efferent trunks 

 of the Malpighian bodies discharge their blood into the capillary plexus, 



Fig. 129. 



Distribution of the Renal vessels, from Kidney of Horse : a, branch of Renal artery ; af, afferent vessel : 

 m, m, Malpighian tufts; ef, ef, efferent vessels; p, vascular plexus surrounding the tubes; st, straight tube; 

 ct, convoluted tube. Magnified about 30 diameters. 



which surrounds the tubuli uriniferi, and from which the solid matter 

 of the urinary secretion is elaborated ; just as the vena portse supplies 

 the capillary plexus, from which the biliary secretion is elaborated in 

 the liver. In Reptiles (in which, as in Fishes, the kidney is partly 

 supplied by the hepatic portal system), the efferent vessels of the Mal- 

 pighian bodies unite with branches of the portal vein to form the secre- 

 ting plexus around the tubuli uriniferi ; and even in Birds this arrange- 

 ment still seems to prevail to a certain extent. Thus all the blood 

 which the secreting plexus receives, has already passed, in each case, 

 through a set of capillaries within or without the organ ; those, namely, 

 of the Malpighian bodies, or those of the parts supplying the hepatic 

 portal system. The special purpose of the Malpighian bodies appears 

 to be, to allow of the transudation of the water of the blood, which is 

 filtered off (so to speak) through the thin walls of their capillaries, and 

 thus passes into the tubuli uriniferi. It is well known that the fluid 

 and solid constituents of the urinary secretion bear no constant relation 

 to each other ; the amount of fluid depending mainly upon the degree 



