sec i urn ox. 253 



tion is emptied from the cells by filtration. Van der Stricht believes that the 



vesicles rupture and thus empty into the lumen. In longitudinal sections 

 various stages in the process may be seen scattered along the length of a 

 single tubule. 



Secretion of the Water and Salts. — There seems to be no question that the 

 elimination of water together with inorganic salts, and probably still other 

 soluble constituents, takes place chiefly through the glomerular epithelium. 

 This supposition is made in both the general theories that have been men- 

 tioned. It has, however, long been a matter of controversy, in this as in 

 other glands, whether the water is produced by simple filtration or whether 

 the glomerular epithelium takes an active part of some character in setting up 

 the stream of water. The problem is perhaps simpler in this case than in the 

 salivary glands, since the direct participation of secretory nerves in the process 

 is excluded. Ou the filtration theory the quantity of urine should vary 

 directly with the blood-pressure in the glomerulus. This relationship has 

 been accepted as a crucial test of the validity of the filtration theory, and 

 numerous experiments have been made to ascertain whether it invariably 

 exists. Speaking broadly, any general rise of blood-pressure in the aorta will 

 occasion a greater blood-flow and greater pressure in the glomerular vessels 

 provided the kidney arteries themselves are not simultaneously constricted to 

 a sufficient extent to counteract this favorable influence ; whereas a general fall 

 of pressure should have the opposite influence both on pressure and velocity of 

 flow. It has been shown experimentally that if the general arterial pressure 

 falls below 40 or 50 millimeters of mercury, as may happen after section of 

 the spinal cord in the cervical region, the secretion of the urine will be greatly 

 slowed, or suspended completely. Constriction of the small arteries in the 

 kidney, whether effected through its proper vaso-constrictor nerves or by par- 

 tially clamping its arteries, causes a diminution in the secretion and at the 

 same time in all probability a fall of pressure within the glomeruli and a 

 diminution in the total flow of blood. On the other hand, dilatation of the 

 arteries of the kidney, whether produced through its vaso-dilator fibres or by 

 section or inhibition of its constrictor fibres, augments the flow of urine and 

 at the same time probably increases the pressure within the glomerular capil- 

 laries, and also the total quantity of blood flowing through them in a unit of 

 time. From these and other experimental facts it is evident that the amount 

 of secretion and the amount of pressure within the glomerular vessels do often 

 vary together, and this relationship has been used to prove that the water of 

 the secretion is obtained by filtration from the blood-plasma. lint it will be 

 observed that the quantity of secretion varies not only with the pressure of 

 the blood within tin; glomeruli, but also with the quantity of blood (lowing 

 through them. Ileidenhain has insisted that it is this latter factor and not 

 the intracapillary pressure which determines the quantity of water secreted. 

 He believes that the glomerular epithelial cells possess the property of actively 

 secreting water, and that they are not simply passive fillers; that the forma- 

 tion, in other words, is not a simple mechanical process, bin a more complex 



