428 TEXT-BOOK OF PHYSIOLOGY. 



The nerves of the kidney are derived from the renal plexus and 

 follow the course of the blood-vessels to their termination. 



The Renal Duct. The excretory duct of the kidney, the ureter, 

 is a musculo-membranous tube about 5 mm. in diameter when dis- 

 tended, 30 cm. in length, and extends from the hilum to the base of 

 the bladder. The upper extremity is expanded and within the renal 

 sinus becomes irregularly branched, giving rise to a number of short 

 tubes, called calyces, each of which embraces the apex of a Malpighian 

 pyramid. The interior of the expanded portion of the ureter is 

 known as the pelvis. The wall of the ureter consists of a mucous 

 membrane, a muscle coat, and an external fibrous investment. 



MECHANISM OF URINE SECRETION. 



The secretion of urine is a complex process and susceptible of 

 several interpretations. It was originally inferred by Bowman that, 

 as the kidney presents anatomically an^prjajratu^Jor^mtration, the 

 capsule with its enclosed glomerulus, and an" apparafusTof "secretion, 

 the epithelium of the urinary tubules, the elimination of the urinary 

 constituents from the blood is accomplished by the two processes of 

 filtration and secretion; that the water and highly diffusible inorganic 

 salts simply pass by diffusion, under pressure, through the walls of 

 the glomerular capillaries, while the organic constituents are removed 

 by the epithelium lining the tubules. 



Influenced largely by the facts of blood-pressure Ludwig advanced 

 the view that the factors concerned in the secretion of urine were 

 purely physical; that in consequence of the high pressure in the 

 vessels of the glomeruli, due to the resistance offered by the smaller 

 efferent vessel, all the urinary constituents were filtered off in a 

 state of extreme dilution. In order to account for the higher per- 

 centage of the organic constituents in the urine, it was assumed that 

 as the dilute urine passed through the tubules the water was partly 

 reabsorbed, passing by diffusion into the lymph and blood until the 

 urine acquired its normal characteristics. In support of this view, 

 a large number of facts relating to the influence of an increase and a 

 decrease of pressure in the blood-vessels of the glomeruli, the velocity 

 of the blood-stream, etc., in determining the rate of urinary flow 

 were adduced, all of which apparently indicated that the former 

 stood to the latter in the relation of cause and effect, and that the 

 formation of urine was accomplished entirely by physical forces. 



The progress of physiologic investigation, however, has thrown 

 some doubt on the validity of this physical interpretation, and has 

 rather served to support the view of Bowman that the organic con- 

 stituents at least are removed from the blood by a process of selection 

 on the part of the epithelium of the convoluted part of the urinary 



