464 A MANUAL OF PHYSIOLOGY 



amount than any other salt, causes the smallest diuresis of all 

 (Haake and Spiro). Such facts suggest that the secreting cells 

 of the kidney are stimulated or inhibited by the contact of blood 

 or lymph in which the normal constituents are present in too 

 great or in too small amount, and that the intensity of the action 

 is proportional to the degree of deficiency or excess. The greater 

 the velocity of the circulation in the kidney, the more effective will 

 be the stimulation produced by any given substance present in 

 excess, and therefore the greater the total amount of it eliminated 

 in a given time. For in making the round of the renal circula- 

 tion the concentration of the substance in any given portion of 

 blood will fall less, and therefore the average stimulation exerted 

 by it during the round will be greater the faster the blood 

 flows. It is quite in agreement with this that when plethora 

 is occasioned by transfusion of blood there is little or no diuresis, 

 although the increase of arterial, capillary, and venous pressure, 

 and the dilatation of the kidney, are evident. For the rapid 

 passage of liquid out of the vessels would lead to a great increase in 

 the relative proportion of corpuscles to plasma that is to say, 

 to an abnormal condition of the blood. On the other hand, 

 when plethora is produced by injection of serum diuresis occurs 

 (Cushny). This, again, is what we should expect, since the 

 elimination of the superfluous liquid will restore the normal pro- 

 portion. The diminished viscosity of the blood (p. 22) produced 

 by the excess of serum will aid the flow through the kidney and 

 therefore increase the diuresis, while in the case of the plethora 

 produced by injection of blood the elimination of liquid will at 

 once increase the viscosity, diminish the velocity of the renal 

 flow, and tend to lessen diuresis. 



There is, then, little more reason to assume that the copious 

 flow of urine which follows the absorption of a large quantity 

 of water is due to a mere process of filtration than there is to 

 believe that filtration, and not selective secretion, is the cause 

 of the gush of saliva which precedes vomiting, or the sudden 

 outburst of sweat on sudden and severe exertion. In addition, 

 there are the positive proofs already mentioned that the ' rodded ' 

 epithelium of the tubules, which no one supposes to be abandoned 

 more to mere physical influences than the epithelium of the 

 salivary glands, plays a part in the secretion of some of the 

 urinary constituents. 



As to the nature of the mechanism set in motion, and the 

 series of events that take place as the constituents of the urine 

 journey from the interior of the bloodvessels to the lumen of 

 the tubules, we know no more than in the case of other glands. 

 This alone is clear, that the separation of the urine from the 

 blood implies the performance of a large amount of work by 



