432 THE SOLIDS. 



ramifies in the interstices left between the terminal loops of 

 the tubes (see Plate LIX. fig. 2.) : the second origin is in 

 the plexus of capillaries surrounding the tubes formed by 

 the junction of the capillaries of both artery and vein ; from 

 these two origins and plexuses branches proceed; these, 

 uniting with each other, form larger vessels, which also pass 

 through the medullary part of the kidney in sets. (See Plate 

 LVIII.^. 4, 5.) 



Portal Vein. Each Malpighian tuft or plexus of vessels 

 is formed, like other plexuses, of capillaries derived in part 

 from an artery and in part from a vein. As a single arterial 

 twig proceeds to each Malpighian dilatation the afferent 

 vessel, so a single venous twig departs from it the efferent 

 vessel ; this vessel is usually of much smaller size than the 

 artery, and terminates in the capillaries which encircle the 

 uriniferous tubes. (See Plate LX. Jig. 2.) 



The efferent vessel of the Malpighian tuft resembles in its 

 origin and distribution the portal vessel of the liver, being 

 in connection with capillaries, at both its origin and its 

 termination : in consequence of this resemblance it has been 

 termed by Mr. Bowman the portal vein of the kidney ; and 

 as each Malpighian plexus has a separate efferent vessel, 

 to the aggregate of these that gentleman applies the term 

 " portal system " of the kidney. 



The above description of the vascular distribution belong- 

 ing to the kidney differs, in some important respects, from 

 that given by Mr. Bowman, as we shall now proceed to 

 make apparent. 



In the first place, Mr. Bowman describes the afferent vessel 

 of the Malpighian tuft as piercing the membrane of the en- 

 larged extremity of the tube, and as forming within this, 

 by its numerous subdivisions, the Malpighian plexus : the 

 efferent vessel, in like manner, perforating the membrane of 

 the dilatation, or proper capsule. 



This view of the position of the Malpighian plexus is un- 

 doubtedly incorrect ; this plexus, like every other plexus 

 belonging to glands formed on the tubular or follicular types, 

 is situated on the external surface of the basement membrane, 

 that is, embracing the globular head of the tubes, and lying 



