366 EXCRETION BY THE SKIN AND KIDNEYS. 



large trunks which form the centres of these stars then pass through the corti- 

 cal substance to the rounded bases of the pyramids, where they form a vaulted, 

 venous plexus corresponding to the arterial plexus already described. The 

 vessels distributed upon 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 pyramids 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 inter- 

 pyramidal substance, enveloped in the same sheath with the arteries. Passing 

 thus to the pelvis of the kidney, the veins converge into three or four great 

 branches, which unite to form the renal, or emulgent vein. A preparation of 

 all the vessels of the kidneys shows that the veins are much more volu- 

 minous than the arteries. 



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 quite 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 upon 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 crys- 

 tallizable, nitrogenized substance, which is discharged by the skin as well as 

 by the kidneys. This has long been recognized as an excrementitious sub- 

 stance ; but the first observations that gave any definite idea of the mechanism 

 of its production were made by Prevost and Dumas, in 1821. At the time these 

 experiments were made, chemists were not able to detect urea in the normal 

 blood ; but Prevost and Dumas extirpated the kidneys from living animals, 

 dogs and cats, and found an abundance of urea in the blood, after certain 

 symptoms of blood-poisoning had been developed. For the first two or 

 three days after the operation there were no symptoms of blood-poisoning ; but 

 finally stupor and other marked evidences of nervous disturbance supervened, 

 when the presence of urea in the blood could be easily determined. These ob- 

 servations were confirmed and extended by Segalas and Vauquelin, in 1822. 

 Since that time, as the processes for the determination of urea in the animal 

 fluids have been improved, this substance has been detected in minute quan- 

 tity in the normal blood. Picard (1856) estimated and compared the propor- 

 tions of urea in the renal artery and the renal vein, and he found that the 

 quantity in the blood was diminished by about one-half in its passage through 

 the kidneys. Still later, urea has been found in the lymph and chyle, in 

 larger quantity, even, than in the blood(Wurtz). 



Bernard and Barreswil (1847) found that animals from which both kid- 

 neys had been removed did not usually present any distinctive symptoms for 

 a day or two after, except that they vomited and passed an unusual quantity 

 of liquid from the intestinal canal. During this time the blood never con- 

 tained an abnormal quantity of urea ; but the contents of the stomach and 



