688 FLOW OF BLOOD THROUGH KIDNEY. [BOOK n. 



arrangements ; for these are extremely favourable to a full and 

 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 experi- 

 mental 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, including 

 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 pressure 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. 



410. 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. 88, 89), 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. 88), called 

 the oncometer 1 , is applied to the organ about to be studied, while 



1 From OJKOS, bulk. 



