REGULATION OF ENVIRONMENT 75 



the veins is dependent inversely on the pressure of C0 2 

 in the blood. Accordingly deprivation of C0 2 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 C0 2 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 C0 2 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 



