330 EXCRETION 



Bowman called the branches which connect together the vessels of the 

 Malpighian tuft and the capillary plexus surrounding the tubes, the 

 portal system of the kidney. These intermediate vessels form a coarse 

 plexus surrounding the prolongations of the pyramids of Ferrein into 

 the cortical substance. 



The renal, or emulgent vein takes its origin in part from the capil- 

 lary plexus surrounding the convoluted tubes and in part from vessels 

 distributed in the pyramidal substance. A few branches come from 

 vessels in the envelopes of the kidney, but these are comparatively 

 unimportant. The plexus surrounding the convoluted tubes empties 

 into venous radicles which pass to the surface of the kidney, and these 

 present a number of little radiating groups, each converging toward a 

 central vessel. This arrangement gives to the vessels under the fibrous 

 envelope of the kidney a peculiar stellate appearance, forming what are 

 called the stars of Verheyn. The large trunks which form the centres 

 of these stars then pass through the cortical substance to the rounded 

 bases of the pyramids, where they form a vaulted venous plexus cor- 

 responding to the arterial plexus already described. The vessels dis- 

 tributed on the straight tubes of the pyramidal substance form a loose 

 plexus around these tubes, except at the papillae, where the network is 

 much closer. They then pass into the plexus at the bases of the pyra- 

 mids to join with the veins from the cortical substance. From this 

 plexus a number of larger trunks arise and pass toward the hilum, in 

 the axis of the interpyramidal substance, enveloped in a sheath with 

 the arteries. Passing to the pelvis of the kidney, the veins converge 

 into three or four branches, which unite to form the renal vein. A 

 preparation of all the vessels of the kidneys shows that the veins are 

 much more voluminous than the arteries (see Frontispiece). 



The capsule of the kidney has a lymphatic plexus connected with 

 lymph-spaces below ; and lymph-spaces, in the form of large slits, exist 

 between and around the convoluted tubes. 



The nerves are abundant and are derived from the solar plexus, their 

 filaments following the renal artery in its distribution in the interior of 

 the organ and ramifying on the walls of the vessels. 



MECHANISM OF THE PRODUCTION AND DISCHARGE OF URINE 



The most important constituent of the urine is urea CO(NH 2 ) 2 

 a crystallizable nitrogenous substance, which* is discharged by the skin 

 as well as by the kidneys. This has long been recognized as an excre- 

 mentitious substance ; but the first observations that gave a definite idea 

 of the mechanism of its production were made by Prevost and Dumas 



