EXCRETION 401 



investigation soon showed him that the rate at which urine 

 was formed could be greatly influenced by changes in the 

 blood-pressure. 



On such considerations, Ludwig founded the * mechanical* 

 theory of urinary excretion, 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 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 mechanical theory as yet con- 

 structed will account for all the facts. 



Ludwig supposed that the urine, qualitatively complete 

 in all its constituents, was simply filtered through the 

 glomeruli ; but as the proportion of salts, and especially of 

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

 he further assumed that the liquid which passes into Bow- 

 man's capsule is exceeding dilute, and that absorption of 

 water, and perhaps of other constituents, takes place in its 

 passage along the renal tubules. The great length of these 

 tubules, as compared with those of most other glands, 

 might 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 there- 

 fore in this respect correspond to the renal glomeruli, 

 secretes less water than the liver, whose capillaries corre- 

 spond 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, 



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