1G0 SECTIONAL ADDRESSES. 



of the changes in his metabolism under different circumstances. Here 

 then are the demands : how does the circulation actually accomplish its 

 task ; what are the facts of the case ? 



Thirty years ago Zuntz and Hageniann succeeded in making observa- 

 tions on the output of blood from the heart in a horse by a method which 

 involved operative procedure, though this did not interfere with the 

 animal"s capacity for drawing a load, and these experiments afforded the 

 first reasonably definite information as to the degree of alteration of the 

 cardiac output caused by vigorous muscular work. More recently methods 

 have been developed for determining the circulation rate in man, and 

 with the help of these we are beginning to find out under different conditions 

 of natural bodily activity the actual quantitative variations in the amount 

 of blood expelled from the heart, the extent of the changes in gas content of 

 the venous blood entering the lungs, and the relative parts played by the 

 two factors of increase in pulse rate and alteration in the systolic- 

 discharge at each beat which we have already identified as the potential 

 means by which the heart can respond to alteration in the demands 

 made upon it. 



This is, however, only the beginning of our task. To ascertain the 

 full facts of the regulation of the circulation and its adaptation to the 

 varying needs of the body is a more difficult problem than the regulation 

 of the breathing. We may know a little about the. behaviour of the heart, 

 but the heart is, after all, only the pump. We have still got to deal with 

 the question of the distribution of the blood to different regions of the 

 body. In spite of all the work that has been done in thepast, our ignorance 

 on this side of the question is still profound, and the darkness will not be 

 dispelled until we know much more about what happens in the normal 

 animal. I am not going to assert that human physiology has succeeded 

 in this case in giving us a solution where the method of direct experimenta- 

 tion on lower animals has proved inadequate. It certainly has not as 

 yet, but I do maintain that the knowledge that we have gained from a 

 broad study of the whole respiratory function in man has at least empha- 

 sized the nature of the problem that confronts us, and has given us some 

 conception of the difficulties that we have to face. Suppose we expose 

 and stimulate some nerve and find that this leads either directly or reflexly 

 to contraction of the blood vessels in some region of the bod} 7 . All we have 

 done is to show up a route along which it is possible for vaso-constrictor 

 impulses to travel. We must go much farther than this : we must ascer- 

 tain whether such impulses do follow this route during normal life, and if 

 so, when ; what variation may be exhibited in the frequency or strength of 

 the impulses and in the magnitude of the resultant effect ; what is the 

 natural stimulus which initiates these impulses and causes them to vary 

 from time to time, and what this variation may mean to the tissues in the 

 area supplied by the blood vessels in question. 



What we have in fact got to find out are the actual quantitative changes 

 which occur under natural conditions in the general circulation, and then 

 we must try to interpret these in the light not only of our knowledge of 

 the potentialities of the circulatory system itself but in relation also to 

 the varying activities and demands of the tissues. 



What matters to the tissues of the bodv is that each and all of them 



