434 PHYSIOLOGY CHAP. 



to draw water from the tissues, and thus produce a hydraemic 

 plethora which raises arterial pressure and favours glomerular 

 nitration, is legitimate. 



But this interpretation of the action of diuretics does not 

 correspond with the facts, because they are able to produce 

 diuresis without causing any rise of pressure. Thus Heidenhain 

 noted diuretic effects with intravenous injection of sodium nitrate 

 mixed with chloral hydrate, although no rise of arterial pressure 

 was visible on the manometer. Paneth afterwards saw that on 

 reducing urinary secretion by compression of the vas efferens, 

 transfusion of sodium nitrate solution or any other diuretic caused 

 a, free flow of urine from the ureter, even if arterial pressure was 

 lowered. 



According to Albertoni a marked secretion of urine also occurs 

 after the spinal cord has been divided on injection of sugar into 

 the blood. In the rabbit intravenous injection of sugar increases 

 the secretion from the kidneys without increasing arterial pressure. 



Stefani and Cavazzani demonstrated that urea, when injected 

 into the blood, has a dilatator action on the vessels in. general, but 

 acts more particularly upon those of the kidney. By artificial 

 circulation of isotonic salt solution to which 2 per cent urea had 

 been added, at constant pressure, the flow from the renal vessels 

 was increased by 78 per cent, from the vessels of the head by 37 

 per cent, of the liver by 32 per cent, of the limbs by 20 per cent. 



These facts contradict the mechanical, and favour the secretory 

 theory, indicating that diuretics are specific stimuli of the activity 

 of the kidney cells. This view is strengthened by the fact which 

 Thompson ascertained in 1894, viz. that atropine (which suspends 

 pr checks the activity of all glandular tissues) acts upon the 

 kidneys by diminishing urinary secretion and the amount of urine 

 excreted, i.e. it acts in the opposite sense to diuretics, although it 

 produces no fall in arterial pressure. Morphine acts in the same 

 way, but by lowering arterial pressure. On administering atropine 

 and morphine simultaneously, urinary secretion is arrested for a 

 certain time (15 to 45 minutes), although the renal circulation 

 continues, for when the flow of urine recommences it is absolutely 

 devoid of protein. 



Many other serious objections might be raised to the second 

 part of Ludwig's theory, which assumes that the glomerular filtrate, 

 in passing through the uriniferous tubules, condenses gradually 

 by reabsorption of water, which is effected by an endosmotic 

 process in the walls of the tubules, until the filtrate assumes all 

 the chemical characters of urine. 



If the whole of the urea, and, generally speaking, all the con- 

 stituents of urine, were really eliminated from the blood by 

 filtration through the glomeruli, as assumed by this theory, two 

 impossible consequences would ensue, as Heidenhain pointed out : 



