576 THE URINARY APPARATUS [CH. XXXVIII. 



vascular origin, and they furnish the best index we have of the 

 pressure of the blood in the capillaries. They give no indication of 

 the rate of blood-flow through the organ, which is an altogether 

 different thing. 



Finally, from the medical point of view, the activity of the 

 kidney may be invoked by the administration of drugs. Such drugs 

 are called diuretics. They act in different ways, some locally on the 

 kidney, such as caffeine, and certain saline diuretics ; others, such 

 as digitalis, act upon the general blood-pressure. It is important, in 

 prescribing these drugs, not to lose sight of the fact that whilst the 

 greatest benefit may attend their action, it is doubtful whether any 

 of those commonly administered through the alimentary canal (the 

 digitalis group excepted) can be regarded as doing their work without 

 throwing a greater or less strain upon some portion of the renal 

 epithelium. 



Having in this way considered the kidney as a whole, we must 

 next consider the function of the different types of cell found in the 

 kidney tubule, including the capsule of Bowman in that expression. 



A complete statement of the case would involve for each type a 

 description of (1) which of the urinary constituents traverses its 

 protoplasm; (2) the mechanism, whether secretory or physical 

 diffusion or filtration, by which the constituent is propelled ; and 

 (3) the direction in which it goes, whether from the blood to the 

 urine, or from the urine to the blood. 



Fortunately, however, it is not necessary to discuss the matter quite 

 so exhaustively, for certain possibilities have never been advanced, 

 and so can be put upon one side. For instance, no one has ever 

 suggested that the thick glandular cells of the tubules allow materials 

 to pass through them by physical diffusion; here, undoubtedly, we 

 have to deal with secretory action alone. Again, the direction of 

 flow through the capsule of Bowman is undoubtedly from the blood 

 to the urine, and the passage of water in the opposite direction, if it 

 does occur at all, is limited to certain regions of the tubule itself. 



Our problem, therefore, is simplified, and the questions remaining 

 are (1) whether the flow in the glomeruli is due to physical or to 

 physiological (secretory) processes; (2) the evidence of secretory 

 action in the tubules ; and (3) the question whether reabsorption of 

 fluid occurs in the tubules. In relation to the first of these points 

 we may ask the preliminary question. Is there any evidence of 

 physical diffusion or filtration in the kidney ? or, to put it another 

 way, is there any evidence of a urinary flow without the performance 

 of work on the part of the kidney cells ? 



The answer to this is as follows : such a flow cannot take place 

 as the result of any agency which alters the composition of the 

 fluid ; nevertheless, the appropriate injection into the circulation of 



