372 Professor E. H. Starling [May 20 r 



see that in any violent exercise involving the greater number of the 

 muscles of the body, the circulation must be increased in force seven 

 to ten times, and the heart, which is the pump maintaining the circu- 

 lation, must under these conditions do from seven to ten times as 

 much work as during rest. 



The Mechanism of Adaptation. 



What is the mechanism of these adaptations ? How is it that 

 the heart is able to carry on a circulation which may vary from a 

 passage of 3 litres of blood per minute up to 30 litres of blood 

 per minute ? (these figures representing the extreme limits between 

 which the output of the heart-pump may vary according to the con- 

 dition of the body). It might be thought that we are dealing here 

 simply with the influence of the central nervous system, which adapts 

 the activity of the muscles of the body to the requirements of the 

 environment, and that the heart being a muscle would be stimulated 

 to contract more strongly at the same time as the nervous system 

 calls into activity the voluntary muscles of the body. There is no 

 doubt that the heart is under the control of the central nervous 

 system, so that its action can be altered, increased or diminished by 

 the brain in accordance with the needs of the economy, but in the 

 heart we find also a wonderful power of adaptation to the varying 

 requirements of the organism which is quite independent of the 

 central nervous system. 



This can be shown quite easily either in the cold-blooded or 

 warm-blooded animal. The heart of the frog and tortoise can be 

 cut out and will continue beating for hours or even days. It has 

 long been known that the heart of the mammal would beat for some 

 minutes after being cut out of the body, but if we take pains to 

 ensure that the muscles constituting the walls of the heart continue 

 to receive their supply of oxygenated blood, the mammalian heart 

 can be made to beat for 8 to 12 hours after the death of the animal 

 from which it is taken. In order to investigate this properly, we 

 want to make such a preparation that we can control at will all the 

 conditions which may affect the action of the heart — viz. the amount 

 of blood flowing into the heart from the big veins, the resistance 

 which the heart has to overcome when it drives the blood out into 

 the arteries, and the temperature at which the heart contracts. We 

 must be able to measure at any time the output of the heart, the 

 arterial pressure it maintains, its changes in volume during contrac- 

 tion, the pressure in all its cavities during contraction, the amount 

 of blood flowing through the blood vessels of its walls, and its 

 chemical exchanges, as measured by the amount of oxygen which it 

 takes up and the amount of carbonic acid which it produces. It is 

 these chemical changes which give the energy for the work of the 

 heart. 



