250 



REN. 



ation of blood in the tuft than occurs pro- 

 bably in any other part of the vascular system ; 

 a delay that must be increased by the tor- 

 tuosity of the channels to be traversed. 



The other system of capillaries, or that 

 surrounding the uriniferous tubes, corre- 

 sponds, in every important respect, with that 

 investing the secreting canals of other glands. 

 Its vessels anastomose with the utmost free- 

 dom on every side, and lie on the deep sur- 

 face of the membrane that furnishes the 

 secretion. 



Mr. Bowman has applied the term "portal 

 system of the kidney " to the series of vessels 

 connecting these two, on account of the close 

 analogy it seems to bear to the vena porta, 

 intervening, like it, between two capillary net- 

 works, the first of which answers to that in 

 which the vena porta originates, and the 

 second to that in which the vena porta ter- 

 minates. The capillary plexus surrounding 

 the tubes differs, therefore, from that of other 

 glands, and agrees with that of the liver, in 

 its receiving blood that has previously tra- 

 versed another system of capillary vessels. 



The correctness of the analogy which Mr. 

 Bowman has drawn between the circulation 

 of the kidney and that of the liver is very 

 beautifully shown by his observations on the 

 kidney of the boa-constrictor, an animal 

 which may be regarded as the type of those 

 in which, besides the renal artery, the kidney 

 receives a portal vein derived from the hinder 

 part of the body.* Mr. Bowman thus de- 

 scribes the organ in question : " The kidney 

 of the boa, being composed of isolated lobes 

 of a compressed reniform shape, displays all 

 the points of its structure in peculiar sim- 

 plicity and beauty. At what may be termed 

 the hilum of each lobe, the branches of the 

 vena porta and duct separate from those of 

 the renal artery and emulgent vein ; the two 

 former spreading side by side, in a fan-like 

 form, over the opposite surfaces of the lobe, 

 while the two latter enter its substance and 

 radiate together in a plane midway between 

 these surfaces. The lobe is made up of the 

 ramifications of these four sets of vessels, in the 

 following mode (fg. 161.). Each duct, as it 

 runs over the surface, sends down a series of 

 branches which penetrate in a pretty direct 

 manner towards the central plane. Arrived 

 there, they curl back, and take a more or less 

 retrograde course towards the surface, and, 

 finally, becoming more convoluted, terminate 

 in the Malpighian bodies, which are all 

 situated in a layer at some distance within the 

 lobe, parallel to the central plane, and nearer 

 to it than to the surface. The ducts never 

 anastomose. The artery subdivides into ex- 

 tremely minute twigs, no larger than capil- 

 laries, which diverge on either hand and 

 enter the Malpighian bodies. The efferent 

 vessels are of the same size as the afferent, 

 and, on emerging, take a direct course to the 

 surface of the lobe, and join the branches 

 of the vena porta there spread out. The 



* Vide ante, p. 232-3. 



branches of the portal vein on the surface 

 send inwards a very numerous series of twigs 



Fig. 161. 



fUT 



U7 



a 



Plan of the arrangement of the elements of the 

 kidney, in the boa constrictor, by Mr. Bowman. 



a, arterial branch in the centre of the lobule, 

 sending afferent twigs to the Malpighian bodies on 

 each side. The efferent vessels are seen running to 

 the branches of the portal vein, p v, p v, on the sur- 

 faces of the lobule. The plexus surrounding the 

 tubes is seen at p, running from the portal vein to 

 the emulgent vein, ev, which lies in company with 

 the artery in the centre of the lobiile. The urinife- 

 rous tube, t, is seen commencing in the M. body, and 

 passing to the branch of the ureter, u r, u r, at the 

 surface of the lobule where it accompanies the portal 

 vein. The M. bodies are seen diminishing in size, 

 as the tubes become shorter towards the thin edge 

 of the lobule b. 



of nearly uniform capacity, and only a little 

 larger than the vessels of the capillary plc.vus, 

 in which they almost immediately terminate. 

 This is the plexus surrounding the uriniferous 

 tubes. It extends from the surface to the 

 central plane of the lobe, and there ends in 

 the branches of the emu/gent r>ciu. n 



" Thus the efferent vessels of the Malpighian 

 bodies are radicles of the portal vein, and, 

 through the portal vein, empty themselves, as 

 in the higher tribes, into the plexus surround- 

 ing the uriniferous tubes. The only real 

 difference between this form of kidney and 

 that of Mammalia is that there is here a 

 vessel bringing blood that has already passed 

 through the capillaries of distant parts, to be 

 added to that coming from the Malpighian 

 bodies, and to circulate with it through the 

 plexus surrounding the tubes. The efferent 

 vessels of the Malpighian bodies run up to 

 the surface, in order to throw their blood 

 through the whole extent of the capillary 



