THE CONTROL OF THE CIRCULATION 235 



distinct rise in temperature will be observed when the sciatic nerve of the 

 corresponding limb is cut. The application of this principle in deter- 

 mining the mass movement of blood by the amount of heat given off from 

 the hands or feet has already been explained. 



Other methods depend upon observation of the outflow of Hood from 

 the veins of the part. A simple application of this method can be used in 

 the case of the ear of the rabbit. If the tip of the ear is cut off, bleeding 

 under ordinary circumstances is only very slight, but if the cervical 

 sympathetic is cut, it becomes quite marked, slowing down again or 

 even stopping entirely when the peripheral end of the nerve is stimu- 

 lated. By making measurements of the volume of the outflow of blood 

 from a vein by this method, the extent of constriction or dilatation can 



tube to recorder 



oil enclosed 

 by membrane 



Fig. 72. Roy's kidney oncometer. (From Jackson.) 



be followed quantitatively. Vasodilatation also causes changes in the 

 character of the venous flow, the usually continuous flow becoming pul- 

 satile and the color of the blood brightening. Comparison of the pressures 

 in the arteries and the veins of a part is also often of value in the detec- 

 tion of changes in the caliber of the blood vessels, for, of course, the 

 greater the difference in pressure between the two manometers, the 

 greater must be the resistance offered to the flow. 



For experimental purposes, however, the standard method is that 

 known as the plethysmo graphic. For this purpose the organ or tissue is 

 enclosed in a so-called plethysmograph or volume recorder, the prin- 

 ciple of which will be clearly seen by consultation of the accompanying 

 diagram of one adapted for the kidney (Fig. 72). Any increase de- 

 tected by this means in the volume of the part must be due either to 



