ORGANS OF THE ENERGIZING SYSTEM 91 



where the heat of the blood-stream is the more easily conducted 

 to the open air and so the exercising man feels warm — because 

 he is losing heat through his skin. He also sweats more and this 

 is because the secretion from the skin glands increases, since the 

 blood flows more rapidly through these organs. 



In violent bodily exertion CO 2 tends to accumulate in the 

 blood. But there is a respiratory centre in the medulla and this 

 is aflfected by the tension of CO 2 in the blood-stream — in this 

 way : increased COs-tension stimulates the nerves that accelerate 

 the respiratory movements — thus CO 2 in the blood is eliminated 

 more rapidly than normal. Conversely decreased COa-tension 

 retards the same muscular movements and the rate of 

 gaseous interchange in the lungs is decreased. On the pro- 

 longed scale this regulation proceeds in the cases of hibernating 

 mammals. 



24.C. Chemical Regulations. The secretion of pancreatic 

 juice, for instance, appears to be regulated apart from nervous- 

 muscular mechanisms. The entrance into the duodenum, from 

 the stomach, of partially digested food materials appears to be 

 accompanied by the elaboration of a substance called secretin. 

 This goes into the blood-stream and stimulates the pancreas to 

 secretion. Such substances are called hormones, and it is believed 

 that they have a general role in chemical adjustments, and even 

 in developmental processes. 



The regulatory mechanisms are doubtless physico-chemical 

 ones and the conception of automatic workings of such mechanisms 

 is an easy one. Models can be devised and there are obvious 

 mechanical analogues — in the throttle-gear of a steam, or gas 

 engine, for instance. (But the evolution of the automatisms is 

 not so easy to conceive, for the throttle-gear of a heat engine is 

 designed.) 



And all the fundamental problems involved in such automatic 

 regulations are unsolved : w^e do not know in what physical or 

 chemical ways CO 2 in the blood-stream affects the ganglionic 

 cells of the respiratory centre so that the latter now sends out 

 inhibitory impulses, and again acceleratory ones. And, of course, 

 we do not know how, precisely, a nervous impulse entering a 

 muscle-fibre can alter the state of tension of the latter. Yet 

 this is the most common and fundamental of all physiological 

 activities. 



