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, on the circulation are extremely sig- 
nificant. When the body is greatly impoverished in 
CO, 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 
