■830 PHYSIOLOGY OF DIGESTION AND SECRETION. 



together, do not anastomose, and they unite to form a single efferent 

 vein of a smaller diameter than the afferent artery. The whole 

 structure, therefore, is not an ordinary capillary area, but a rete 

 mirabile, and the physical factors are such that within the capil- 

 laries of the rete there must be a greatly diminished velocity of the 

 blood-stream, owing to the great increase in the width of the stream 

 bed, and a higher blood-pressure than in ordinary capillaries, 

 owing to the narrow efferent vessel and the capillaries of the tubule 

 which form a resistance beyond the rete. Surrounding this 

 glomerulus is the double-walled capsule. One wall of the cap- 

 sule is closely adherent to the capillaries of the glomerulus; it 

 not only covers the structure closely, but dips into the interior 

 between the small lobules into which the glomerulus is divided. 

 This layer of the capsule is composed of flattened, endothelial- 

 like cells, the glomerular epithelium, to which great importance 

 is attached in the formation of the secretion. It will be no- 

 ticed that between the interior of the blood-vessels of the glomerulus 

 and the cavity of the capsule, wliich is the beginning of the urin- 

 iferous tubule, there are interposed only two very thin layers, — • 

 namely, the epithelium of the capillary wall and the glomerular 

 epithelium. The apparatus would seem to afford most favorable 

 conditions for filtration of the liquid parts of the blood. The epi- 

 thelium clothing the convoluted portions of the tubule, including 

 under this designation the so-called irregular and spiral portions 

 and the loop of Henle, is of a character quite different from that of 

 the glomerular epithelium (Fig. 298, B, C, D, E, F, G). The cells, 

 speaking generally, are cuboidal or cylindrical, protoplasmic, and 

 granular in appearance; on the side toward the basement mem- 

 brane they often show a peculiar striation, while on the lumen side 

 the extreme periphery presents a compact border which in some 

 cases shows a cilia-like striation. These cells have the general 

 appearance of an active secretory epithelium, and one theory of 

 urinary secretion attributes this function to them. 



The Secretion of Urine. — ^The kidneys receive a rich supply 

 of nerve fibers, but most histologists have been unable to trace any 

 connection between these fibers and the epithelial cells of the kidney 

 tubules. 



The majority of purely physiological experiments upon direct 

 stimulation of the nerves going to the kidney are adverse to the 

 theory of secretory fibers, the marked effects obtained in these ex- 

 periments being all explicable by the changes produced in the blood- 

 flow through the organ. Two general theories of urinary secretion 

 have been proposed. Ludwig held originally that the urine is 

 formed by the simple physical processes of filtration and diffusion. 

 In the glomeruli the conditions are most favorable to filtration, and 

 he supposed that in these structures water filtered through from the 



