440 THE HUMAN BODY. 



circumstances the secretion of urine is suppressed. If the 

 lower end of the cut cord be stimulated the vessels all over 

 the body of the animal contract, and blood-pressure every- 

 where becomes very high. But the kidney vessels being 

 constricted with the rest allow very little blood to enter 

 the glomeruli in spite of the high aortic pressure, and little 

 or no urine is secreted. If, however, the vasoconstrictor 

 nerves of the kidney be cut before the stimulation of the 

 cord, we get a dilatation of the kidney vessels with a con- 

 striction of vessels elsewhere, and abundant blood flows 

 through the glomeruli under high pressure : the whole 

 kidney swells and abundant urine is formed. When the 

 skin vessels contract on exposure to cold, more blood flows 

 through internal organs, the kidneys included, and the blood- 

 pressure in these is if anything increased, the expansion of 

 internal arteries not at the most more than counterbalancing 

 the constriction of the cutaneous. Hence the greater secre- 

 tion of urine in cold weather. Injection of a little water 

 into a vein of an animal causes a very transient constriction 

 of the kidney vessels followed by a dilatation; and general 

 blood-pressure not being at the same time lowered, pressure 

 in the renal glomeruli is high and the secretion of urine 

 increased. Urea introduced into the blood acts in a similar 

 way, but more markedly , so that this substance causes 

 diuresis not merely, as we have seen, by stimulating the cells 

 of the tubuli, but also by exciting the vaso-dilator nerves of 

 the glomerular arteries. Solution of sodium acetate or even 

 of common salt injected into the veins causes very marked 

 local vascular dilatation in the kidney, and hence great flow 

 through the organ under high pressure and a marked in- 

 crease in the quantity of urine excreted. Even if the nerves 

 going to the kidney be first cut, the above results follow, 

 these salts appearing to act directly on a local renal vaso- 

 dilator mechanism. They may of course also, like urea, 

 directly stimulate the cells of the contorted tubules, but this 

 is not proved. The increased secretion of urine after drink- 

 ing much water is probably produced by the dilution of the 

 blood by the liquid absorbed by the alimentary canal, essen- 

 tially in the same manner as the extra secretion caused by 

 direct injection of water into the blood-vessels. That the 

 central nervous system may influence the renal secretion is 

 well known, fear, for example, stimulating it. Probably 



