456 A MANUAL OF PHYSIOLOGY 



work done in this nitration 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 pro- 

 cess 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 concen- 

 trated 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 ' 

 explanation 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 ' vital/ and therefore withdrawn from physical explanation.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- 

 ment of this discussion. Three main questions require our atten- 

 tion : 



1. Is there any evidence that reabsorption actually occurs in 

 the tubules ? If reabsorption on an important scale does take 

 place, it follows at once that there must be a difference of function 

 between the two parts of the renal apparatus, through which 

 urinary constituents pass in opposite directions. 



2. But if there is no reabsorption, or none of importance, it 

 may still be asked whether, the direction of movement of the 

 urinary constituents through the glomeruli and the tubular 



