462 TEXT-BOOK OF PHYSIOLOGY 



of the spinal cord and pass forward in the small splanchnic nerves. Experi- 

 ment has shown that these nerve-fibers have both vaso-constrictor and 

 vaso-dilatator functions. The presence of specific secretor fibers for the 

 epithelium has not been satisfactorily demonstrated. 



The Renal Duct. The renal duct, the ureter, is a musculo-membranous 

 tube about 5 mm. in diameter when distended, 30 cm. in length, and extends 

 from the hilum to the base of the bladder into which it empties. 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 physiologic mechanism concerned in the secretion of the urine, re- 

 duced to its simplest terms, consists of (i) the afferent vessel, the glomeru- 

 lus, the efferent vessel, the capillaries and the veins; (2) the glomerular cap- 

 sule and the epithelium of the tubule throughout its entire extent. Fig. 208. 



The secretion of urine by this mechanism is a complex process and 

 susceptible of several interpretations. It was originally inferred by Bowman, 

 an inference based on the histologic structure of the blood-vessels and tubules 

 and their relation one to the other, that there is here present (i) an ap- 

 paratus for filtration, the capsule with its enclosed glomerulus, and (2) an 

 apparatus for secretion, the tubule with its epithelium, and therefore 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 filtration, under pressure, through the walls of 

 the glomerular capillaries, while the organic constituents are removed by the 

 epithelium lining the tubules; that the water, descending the tubules from the 

 capsules, plays the part of a solvent for the crystallizable compounds secreted 

 by the epithelium, in consequence of which they are the more readily carried 

 by the moving fluid into the pelvis of the kidney. 



Ludwig, influenced largely by the facts of blood-pressure, advanced the 

 view a few years later that the factors concerned in the secretion of urine 

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

 the glomeruli, due to the high pressure in the renal artery on the one hand, 

 and to the resistance offered by the smaller efferent vessel on the other hand, 

 all the urinary constituents are filtered off in a state of extreme dilution. 

 In order to account for the higher percentage of the organic constituents in 

 the urine than in the blood, it was assumed that as the dilute urine passed 

 through the tubules the water and possibly other substances as well, that were 

 necessary for the nutrition of the body, were partly reabsorbed, passing by 

 diffusion into the lymph and blood until the urine acquired its normal 

 characteristics and degree of concentration. 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 



