CHAP, in.] ELIMINATION OF WASTE PEODUCTS. 525 



rapid stream of blood through the organ. The short and rela- 

 tively broad renal artery comes off direct from the abdominal 

 aorta, where the blood-pressure is extremely high ; the renal 

 vein opens directly into the vena cava, where the blood-pressure 

 is extremely low. Between the mouth of the renal artery and 

 the mouth of the renal vein the difference of pressure is very 

 great indeed ; and as we have seen in treating of the vascular 

 system it is the difference of pressure between two points of 

 the vascular tract which is the actual cause of the flow of blood 

 from the one point to the other. The difference of pressure 

 indeed which drives the blood through the limited area of the 

 kidney is the same difference of pressure which drives the 

 blood along the abdominal aorta down to the foot and back 

 again to the vena cava. 



This free and abundant supply of blood is regulated, is either 

 increased or diminished, according to the needs of the moment, 

 by the vaso-motor system ; this is shewn by experimental and 

 other results, which it will be profitable to study in some detail. 

 Before entering into these details, however, it will be well to 

 call attention to the fact that when vaso-motor events modify 

 the flow of blood through an organ they produce their effects in 

 one direction or another by working on arterial blood-pressure. 

 Thus, as we shall see, when stimulation or section of a nerve 

 increases the flow of blood through the kidney it does so by 

 increasing the pressure in the small vessels of the kidney, includ- 

 ing the capillary loops of the glomeruli. In such a case the 

 walls of the glomerular loops, through which the passage of 

 materials to form (part of) the urine takes place, are subjected 

 to two influences ; on the one hand to a fuller, more rapid flow 

 of blood past them, and on the other to an increase of the pres- 

 sure which that blood as it passes along exerts on them. We 

 shall have subsequently to discuss the share taken by these two 

 influences in determining and modifying the passage of material 

 through the walls of the glomerular loops ; and this will bear 

 on the question of filtration to which we have above alluded ; 

 but for the present it will be convenient to deal with the effects 

 of variation in blood-pressure apart from this secondary question. 



330. The vaso-motor mechanisms of the kidney. It may 

 be shewn experimentally that the kidney is supplied with a vaso- 

 motor mechanism as well developed perhaps as that of any other 

 part of the body. By means of a modification of the plethysmo- 

 graph (Figs. 102, 103), we can readily observe the variations 

 which take place in the volume of the kidney. 



The instrument consists of two parts, one of which (Fig. 102), 

 called the oncjometer, 1 is applied to the organ about to be studied, 

 while the other (Fig. 103), called the oncograph, is the recording part 



1 From oncos, bulk. 



