1114 PHYSIOLOGY 



active constriction of the blood-vessels of the limb (Fig. 462). On the 

 other hand, a fall of blood pressure causes relaxation of the arterial 

 wall, so that the primary passive iall of the plethysmograph lever is 

 succeeded, even during the maintenance of the low general blood 

 pressure, by a rise to its normal level (Fig. 463). Thus increased blood 

 pressure causes contraction, while diminished blood pressure causes 

 relaxation of the wall of the arterioles a state of things eminently 

 adapted to the maintenance of a continuous flow of blood through a 



Hind limb 



B.P. 



Signal 

 Time 10 sec. 



FIG. 463. Effect of temporary compression of the abdominal aorta on the 

 volume of the denervated hind limb. Two compressions, the second not 

 marked by the signal. Blood pressure taken in the femoral artery of one 

 hind limb, the other hind limb being in the plethysmograph. (BAYLISS. ) 



part, and to minimising the local effects of the alterations of general 

 blood, pressure which may be conditioned by changes occurring in other 

 parts of the body. 



THE COURSE AND DISTRIBUTION OF THE 

 VASO-MOTOR NERVES 



Since the blood-vessels, like the heart, are the seat of an automatic 

 activity, complete nervous control of these tubes can only be secured 

 by the provision of two sets of nerves : one set augmentor or motor 



