THE URINARY ORGANS. 



335 



and medullated nerve-fibers. The former accompany the arteries 

 and may be traced along these to the Malpighian corpuscles. 

 From the plexuses surrounding the vessels small branches are 

 given off, which end on the muscle-cells of the media. According to 

 Berkley, small nerve-fibrils may be traced to the uriniferous tubules, 

 which pierce the membrana propria and end on the epithelial cells. 

 Smirnow has also traced nerve-fibers to the epithelial cells of the 

 uriniferous tubules and the Malpighian corpuscles. Dogiel has 

 shown that medullary (sensory) nerve-fibers terminate in the 

 adventitia of the arteries of the capsule. 



The secretory processes of the kidney can be considered only 

 briefly in this connection. The theories concerning uriniferous 

 secretion may be grouped under two heads : namely, the theory 

 of C. Ludwig, who believed that all the constituents of the urine 



A 



Fig. 270. A, Direct anastomosis between an artery and vein in a column of Berlin 

 of child ; B, bipolar rete mirabile inserted in the course of an arterial twig. Dog's 

 kidney (after Golubew). 



leave the blood through the glomeruli, entering the uriniferous 

 tubules as a urine containing a large percentage of water, which 

 is concentrated in its passage through the uriniferous tubule by 

 the absorption of water ; while according to the theory of Bowman, 

 and later Heidenhain, only the water and inorganic salts leave the 

 blood through the glomerulus, and that in the proportion found in 

 the urine, while the urea is secreted by the epithelial cells of the 

 uriniferous tubules, and mainly in those portions of the tubules 

 possessing a striated epithelium. The majority of writers who 

 have considered the question of urinary excretion have directly or 

 indirectly expressed themselves as adherents to one or the other 

 of the above theories. A number of recent observers have departed 

 somewhat from either of the above theories, and of these we may 



