-. 64 THE HEART 



Arguing from a comparison of the body-weight it is 

 estimated that in man the average output of each ventricle 

 per beat at rest is 60 c.c. 



The same figure has been arrived at by another method 

 due to Krogh. This method is apphcable to man. The 

 subject breathes a certain volume of nitrous oxide and an 

 estimation is made of the amount which is absorbed in a 

 certain time. The rate of absorption of the gas at the 

 same pressure as it exists in the lungs is then determined 

 in vitro. From this is calculated the volume of the blood 

 passing through the lungs in a given time. 



ADAPTATION OF THE HEART 



It is estimated that the output of the heart per minute 

 varies from 3 htres during rest to 21 Htres during violent 

 exercise. The heart therefore has a very considerable 

 power of responding to the demands made upon it. Varia- 

 tions in the output can be brought about in two ways — 



1. By an increase in the rate of the beat, and 



2. By an increase in the output per beat ; that is to say, 

 by alteration in the capacity oi the heart at each diastole. 



In considering how the heart thus adapts itself it will 

 be most convenient to inquire first how far the capacity 

 for adaptation is inherent to the heart itself, and expresses 

 itself independently of nervous connections, and secondly 

 how this inherent tendency, if it exists, is modified or 

 supplemented by the agency of the central nervous system. 



The Isolated Heart 



The behaviour of the heart when freed from its nervous 

 connections is best studied by means of the heart-hmg 

 ^ preparation invented by Starling. Here is Starhng's 

 description of the apparatus. 



" Artificial respiration being maintained, the chest is opened 

 under an anaesthetic. The arteries coming from the arch of the 

 aorta — in the cat, the innominate and the left subclavian — are 



