502 t EXCRETION 



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. 23) 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 

 nitration, 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. 



Cushny has recently stated more clearly than had previously been 

 done a theory which he designates as the ' modern theory ' of urine 

 formation. He assumes that blood-plasma is filtered through the 

 glomeruli under the hydrostatic pressure of the blood, only the colloid 

 proteins being kept back. The filtrate contains the non-colloid con- 

 stituents approximately in the proportions in which they exist in 

 plasma. It is therefore very poor in urea and very rich in sugar as 

 compared with urine. In the tubules some of the constituents, which 

 he terms ' threshold bodies ' are reabsorbed. These are the sub- 

 stances like sugar, the sodium and chlorine ions, etc., which are only 

 excreted when they exceed a certain threshold value in the plasma. 

 Other constituents of the filtrate, like urea, are not reabsorbed. These 

 are called 'no-threshold bodies,' and are excreted in proportion to 

 their absolute amount in the plasma. The cells of the tubules are 

 supposed in some way at present unknown to take up from the filtrate 

 the threshold bodies, and always in a definite concentration namely, 

 that in which they normally exist in blood. As Cushny puts it, the 

 filtrate is deproteinized plasma, from which ' Locke's fluid ' (p. 66) 

 is reabsorbed by the tubule cells. Apparently he thinks it simpler to 

 make the assumption that the renal cells are organized to absorb from 

 the lumen of the tubules a solution of invariable composition, leaving 

 a variable residue to be excreted, than to make the assumption, under- 

 lying the Bowman-Heidenhain theory, that they are organized to 

 leave behind in the blood or lymph an invariable residue by absorbing 

 from them a solution of variable composition. In reality, however, the 

 two assumptions are precisely on the same footing : they are equally 



