74 ORGANISM AND ENVIRONMENT 



of the intestinal area, so that the pressure is relieved. 

 The accelerator or augmentor nerve fibres are, ac- 

 cording to recent investigations by Bainbridge, brought 

 reflexly into action by rise in the pressure in the great 

 veins opening into the heart. 



It is clear also that the amount of blood pumped 

 by the heart must depend on the supply of venous 

 blood, and there is experimental evidence, first 

 brought by Yandell Henderson, that fall in venous 

 blood pressure may actually limit the heart's output 

 of blood, so that the frequency of the heart beats is 

 no measure of the rate of circulation, just as the 

 frequency of breathing is no measure of the amount 

 of air breathed. In this connection the state of con- 

 traction or relaxation of the walls of the veins is a 

 factor of great importance. Yandell Henderson's 

 observations, part of which are not yet published, 

 though communicated to me verbally, seem to indi- 

 cate that contraction of the peripheral veins dams 

 back blood in the capillaries. Less blood passes on 

 to the great veins and the pressure in them becomes 

 insufficient for the adequate filling of the heart. 



The immediate causes of contraction of the walls 

 of the veins are not yet exactly known ; but the obser- 

 vations of Yandell Henderson on the influence of the 

 pressure of CO 2 on the circulation are extremely sig- 

 nificant. When the body is greatly impoverished in 

 CO 2 by excessive artificial respiration the circulation 

 fails, apparently from an inadequate supply of blood 

 to the heart. The simplest explanation of the facts 

 seems to be that the tonic contraction of the walls of 



