426 PHYSIOLOGY CHAP. 



so that the arterial blood from the renal artery, although it meets 

 with an enormous resistance in the glomeruli and still more in the 

 capillary network, does not remain long enough in the kidneys to 

 acquire the characteristics of venous blood. 



This peculiar rapidity of circulation in the renal vessels 

 evidently depends on the great difference in pressure between the 

 blood of the artery and that of the efferent vein ; the former comes 

 from the aorta, where pressure is maximal, the latter opens direct 

 into the vena cava, where pressure is almost nil, owing to the 

 proximity of the thorax. But for a complete explanation of the 

 phenomenon, it must also be remembered that the renal artery is 

 exceptionally large in comparison with the size of the kidney, so 

 that the arterial blood circulates in the kidneys not only with 

 greater velocity, but also in a larger amount t than in other organs. 

 Even, therefore, on the assumption that the renal tissue consumes 

 no less oxygen during its secretory activity than the other 

 secretory organs (as proved by the fact that there is a marked 

 development of heat in the kidneys, since the temperature of the 

 urine, according to Grijns, is 0*4 C. higher than that of the blood 

 which flows into it) we can see why the blood that has passed 

 through it retains all the characters of arterial blood. 



In regard to the process of Urinary Secretion there are two 

 rival theories, the physiological or vitalist theory of Bowman 

 (1842), and the mechanical theory of Ludwig (1844). Since all the 

 existing data derived from innumerable experiments in this much- 

 contested field relate either to the one or to the other of these, it 

 will be well to summarise them briefly. 



Bowman's theory was founded upon considerations derived 

 from the morphological structure of the kidney. As we have seen, 

 he discovered in the Malpighian corpuscles the intimate relation 

 between the blood circulating in the kidneys and the commence- 

 ment of the uriniferous tubules, and therefore held them to be the 

 apparatus by which the salts and water of the urine are excreted. 

 On the other hand, the secretion of the specific constituents of 

 urine (urea, uric acid, etc.) must be effected by the physiological 

 activity of the epithelial cells that line the uriniferous tubules. 

 As it passes through these tubes, the water which is expelled by 

 the glomeruli must dissolve and carry off the urinary constituents 

 actively withdrawn from the lymph by the epithelial cells, and 

 secreted at their free surface. 



To Bowman, therefore, the uriniferous tubules were the true 

 secretory apparatus, similar to that of any other gland, while the 

 glomeruli were a contrivance peculiar to the kidney, which 

 principally served to regulate the water content of the blood and 

 to facilitate the expulsion of the secretion from the tubules. 



Ludwig, on the other hand, considered the glomeruli to be an 

 apparatus for filtration, through which not merely water, but also 



