CHAP, in.] ELIMINATION OF WASTE PRODUCTS. 527 



in general depend on the constriction or dilation of the renal 

 arteries and their ramifications, for distension due to venous 

 obstruction will only occur in special cases. Hence variations 

 in the volume of the kidney may be taken as a measure of varia- 



FIG. 103. SEMI-DIAGRAMMATIC SECTIONAL VIEW OF ONCOGRAPH. Half natural 

 size. K, tube connecting instrument with oncometer. D, piston floating on oil 

 contained in the cavity M ; the oil is prevented from escaping by the side of the 

 piston by the delicate flexible membrane E, which does not interfere with the 

 movements of the piston. H, recording lever connected with the piston by a 

 needle G passing through the guides F, F'. The screw C is for the purpose of 

 clamping the edge of the membrane between the two ring-shaped surfaces at N, 

 while the side tube L is for the purpose of filling the instrument. 



tions in its vascular supply, increase of volume indicating dilated 

 renal vessels, and decrease of volume indicating constriction of 

 the renal vessels. 



When by means of the instrument just described a tracing is 

 taken of the volume of a kidney in what may be considered a 

 normal condition, some such result as that shewn in Fig. 104 is 

 obtained. 



The volume of the kidney is seen to be so delicately respon- 

 sive to changes in the mean arterial pressure that the curve 

 reproduces almost exactly a blood-pressure curve, shewing not 

 only the respiratory undulations, but even the rise and fall due 

 to the individual heart-beats. With each rise of mean arterial 

 pressure more blood is driven into the renal vessels and the kid- 

 ney swells : with each fall of pressure less blood enters and the 

 kidney shrinks. On other tracings taken in the same way may 

 often be seen (not shewn in Fig. 104) the wider variations corre- 

 sponding to the Traube-Hering curves ; but it will be observed 

 that in these the kidney shrinks with the rise of pressure and 



