THE NERVOUS CONTROL OF THE BLOOD VESSELS 1037 



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 determine whether the 

 shrinkage of the kidney might not be due to a lowering of general blood 

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

 sequence of the failure of the heart's activity. On the other hand, without 

 the oncometer it would have been possible to determine only 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. 



The two first details can be found by stimulating various nerves and 

 nerve roots in different parts of tneir course and observing the effects pro- 

 duced on the local and general circulation. The importance of the third 

 heading is due to the fact that the vascular nerves, like the visceral nerves 

 generally, do not have their last cell station in the spinal cord. The fibres 

 carrying vaso-constrictor impulses, on leaving the cord, do not pass direct 

 to the blood vessels, but come to an end in a collection of ganglion cells, 

 which may belong to the main chain of the sympathetic, or be situated more 

 peripherally and belong to the group of collateral or peripheral ganglia. 



