THE MALPIGHIAN CORPUSCLES. 



215 



Section 



grouped into cones, the base of each cone being 

 outward, and the point toward the pelvis of the 

 kidney. The cortical substance, however, envel- 

 ops the cones nearly to their points. It is of a 

 red color, and is the seat of the secreting action. 

 The urine, as it arises, passes along the fine con- 

 vergent vessels, the uriniferous tubes, and these, 

 coalescing as they approach the points of the cones, 

 give origin to what are termed the ducts of Bellini. 

 From these the secretion passes into the calices, 

 thence into the pelvis, and so along the ureter into 

 the bladder. In the cortical substance there are 

 large numbers of dark points, the Malpighian bod- 

 kidney. i es . Their diameter is about -j-Lj- of an inch. 

 Mr. Bowman has demonstrated that the minute structure of the cortical 

 portion is as follows : The uriniferous tubes, as they approach it, under- 

 go bifurcation in such a way that the branches continually arising have, 

 for the most part, a diameter of about -^-$ of an inch. As they enter it 

 they are contorted, and at their ends present small capsules or flask- 

 shaped sacs. Each of the capsules is entered by a twig of Str * uctureofthe 

 the renal artery, which at once divides into loop-like branch- Malpighian 

 es constituting a tuft, and which delivers the blood to a cor P us 

 vein originating in the interior of each tuft. These structures are known 

 as the Malpighian corpuscles. The vein and artery pass out of the cor- 

 puscles usually at the same point ; the vein, however, instead of deliv- 

 ering its blood at once to the renal vein, forms a plexus on the sides of 

 a uriniferous tube, in this simulating the mechanism of the portal vein, 

 which begins in a capillary system and ends in one. It is supposed that 

 the exudation of the water of the urine takes place in the Malpighian 

 body, and the secretion of the solid portions from the cells which cover 

 the uriniferous tubes. 



The chief feature of this structure is, therefore, that in a sac formed 

 upon a uriniferous tube, a tuft of capillaries, the walls of which are of ex- 

 treme tenuity, permits water to escape from the blood supplied by the 

 emulgent artery. The blood, thus concentrated by loss of its water, 

 passes into the veinlets which originate in the interior of the tuft ; these, 

 converging into a little trunk, less in diameter than the twig Circulation of 

 of the emulgent artery, escape along with that vessel from thebioodinthe 

 the capsule ; but, instead of discharging its contents into the kidne ^- 

 renal vein, it ramifies in a plexus on the walls of a uriniferous tube, thus 

 affording a miniature representation of the portal vein, beginning in a 

 capillary system and ending in one. From the plexus the commencing 

 capillaries of the renal veins arise. 



