THE SECRETION OF THE URTNE 493 



was formed could be greatly influenced by changes in the blood- 

 pressure. 



On such considerations, Ludwig founded the ' mechanical ' theory 

 of urinary secretion, which, although in a much' modified form, still 

 divides with the ' vital ' theory the allegiance of physiologists. 

 It is impossible here to enter in detail into a controversy that has 

 extended over more than half a century and produced an extensive 

 literature. The result of the discussion has been, in our opinion, 

 to establish in its essential principles the ' vital ' theory of Bowman, 

 or at least to show that no purely physico-chemical theory as yet 

 constructed will account for all the facts. 



Ludwig supposed that the urine, qualitatively complete in all its 

 constituents, was simply filtered through the glomeruli, the work 

 done in this filtration being performed entirely at the expense of 

 the energy of the heart -beat represented as lateral pressure in the 

 vessels of the tufts. But as the proportion of salts, and especially 

 of urea, is very far from being the same in urine as in blood, it had 

 further to be assumed that the liquid which passes into Bowman's 

 capsule is exceedingly dilute, and that absorption of water, and 

 perhaps of other constituents, takes place in its passage along the 

 renal tubules. This process of reabsorption he pictured as a purely 

 physical diffusion between the dilute urine in contact with the free 

 ends of the epithelial cells lining the tubules and the much more 

 concentrated lymph with which their deep ends are bathed. The 

 great length of these tubules, as compared with those of most other 

 glands, might indeed seem to indicate a long sojourn of the urine 

 in them, and the probability of important changes being caused in 

 its passage along them. But if we consider the immense length 

 (60 to 70 cm.) of the seminal tubules and of their gigantic ducts 

 (epididymis 6 metres), where, of course, absorption of water on a 

 large scale is out of the question, it will be granted that little can 

 be built upon the mere length of the renal tubules. On the other 

 hand, the salivary glands, where there are no glomeruli, secrete as 

 much water as the kidneys are supposed to filter ; and the pancreas, 

 whose capillaries form the first of a double set, and therefore in this 

 respect correspond to the renal glomeruli, secretes less water than 

 the liver, whose capillaries correspond to the low-pressure plexus 

 around the convoluted tubules of the kidney. So that deductions 

 drawn from the anatomical relations of the bloodvessels are not in 

 this case of much value, unless supported by physiological results. 



It is somewhat unfortunate that systematic writers have fallen 

 into the habit of discussing the mechanism of urinary secretion as 

 if the Ludwig theory and the Bowman theory presented an exact 

 antithesis, as if the one offered a complete ' mechanical ' explana- 

 tion of a process, which the other viewed as entirely ' vital,' and 

 therefore withdrawn from physical explanation. 



We need not concern ourselves here with the historical develop- 



