NERVOUS CONTROL OF THE BLOOD-VESSELS 1119 



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 fo 

 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 appre- 

 ciably 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 dis- 

 tribution 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 their course and observing the 

 effects produced 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 distally and belong to the group 

 of collateral or peripheral ganglia. These fibres, as they leave the 

 central nervous system, are small medullated nerves. They end in 

 the ganglion by arborising round ganglion-cells, whence a fresh relay 

 of fibres starts and carries the impulses on to the muscle fibres of 

 the blood-vessels. The post-ganglionic fibres differ from the pre- 

 ganglionic fibres in being non-medullated. 



The discovery of the ganglia, with which any given set of nerve 

 fibres is connected, is rendered easy by the fact that in many animals 



