REGULATION OF ENVIRONMENT — %5 
the veins is dependent inversely on the pressure of CO, 
in the blood. Accordingly deprivation of CO, leads 
to contraction of veins, with resulting congestion 
of capillaries and a decrease in the volume of the 
blood in active circulation equalling that induced by 
haemorrhage. On the other hand, any condition, such 
as muscular work, which is accompanied by increased 
pressure of CO, and diminished oxygen pressure in 
the blood leads to dilation of the veins, and consequent 
increased rapidity in return of blood to the heart, 
with increase of venous blood pressure. What part, 
if any, the nervous system plays in this process, or 
what other substances beside CO, are of influence, 
there are as yet no data to enable us to decide. From 
the circulatory phenomena in asphyxia due to breath- 
ing air deprived of oxygen (when there seems to be 
a great increase of both arterial and venous blood 
pressure) we may, however, infer that want of oxygen 
is one such factor. 
The state of tonic contraction of the unstriped 
muscle such as is found in the walls of blood vessels 
depends, doubtless, on many other conditions besides 
nervous control. Recent investigation shows that one 
of the most interesting of these,conditions is the supply 
to the blood of adrenalin, a specific product of the 
activity of the suprarenal glands. This discovery 
illustrates in a striking way the interdependence of 
different parts of the body—a subject to which I shall 
presently return. 
When we review what is known as to the regulation 
of the circulation it is evident that it is not primarily 
