994 PHYSIOLOGY 



pressure, while the lower is the tracing of the oncograph lever. At the 

 beginning of the experiment the lower dorsal nerve-roots had been dissected 

 out and prepared for stimulation. The peripheral end of the anterior root 

 of the tenth dorsal nerve was excited by means of an interrupted current 

 at the point marked with a cross on the tracing. This stimulation was 

 followed by a rise of blood-pressure together with a diminution in the kidney 

 volume. The increased blood-pressure would by itself tend to force more 

 blood into the kidney and so increase its volume. The fact that the kidney 

 volume diminished shows that there must have been active contraction of 

 the arterioles of the kidney, emptying this organ of blood and so causing it 

 to decrease in size. This contraction of the vessels would tend to cause a 

 rise in general blood-pressure and must have taken some part at any rate 

 in the rise actually observed. If the oncometer in this experiment had been 

 used alone, it would have been impossible to have determined whether the 

 shrinkage of the kidney might not have been due to a lowering of general 

 blood-pressure, in consequence of vaso- dilatation occurring elsewhere, or in 

 consequence of the failure of the heart's activity. On the other hand, 

 without the oncometer it would only have been possible to determine 

 that there was increased peripheral resistance somewhere or other in the 

 body. 



Instead of taking the volume of the kidney we might have determined the blood- 

 flow through its vessels either directly by means of a cannula in the renal vein, or by 

 the indirect method of Brodie. This method depends on the fact that under normal 

 conditions the amount of blood leaving an organ is equal to that entering it during any 

 short space of time. If the efferent vein be clamped for five or ten seconds, the blood 

 entering the organ during this time cannot escape, and therefore accumulates in the 

 organ and increases its volume. If the organ be in a plethysmograph the increase of 

 volume during this period may be measured and is exactly equal to the volume of blood 

 passing through the artery into the organ during the five or ten seconds of the closure. 

 The vein must not be obstructed too long, otherwise the increasing distension of the 

 organ will appreciably increase the resistance to the entry of blood, and so diminish the 

 velocity of the blood in the artery. 



The direct determination of the venous outflow is not well adapted to large organs 

 on account of the very rapid loss of blood which occurs through the open vein. The 

 method is, however, of great value in dealing with the circulation through small organs 

 such as the submaxillary glands. In such a case it is usual to hinder or prevent the 

 clotting of the blood by the preliminary injection of leech extract, and then, after 

 placing a cannula in the efferent veins of the organ, to allow blood from the cannula 

 to drop on to a mica disc attached to a Marey tambour. This tambour is connected 

 by a tube with a registering tambour, every drop on the disc giving rise to a small 

 elevation of the lever of the second tambour. 



COURSE OF THE VASO-CONSTRICTOR FIBRES 



In investigating the course of the vaso-constrictor fibres we have to 

 determine : 



(1) The origin of the fibres from the central nervous system ; 



(2) The course of the fibres on their way to their peripheral distribution 

 in the blood-vessels ; 



(3) Their connections with nerve-cells. 



