RESPIRATION 281 



dog could be varied from 215 to 2,000 cc. per minute by simply 

 regulating the supply of blood to the right side of the heart. 19 



The heart is thus a pump which is capable of adjusting its out- 

 put without any variation in rate of stroke ; and we might imagine 

 a heart working quite efficiently on this principle, without any 

 -regulation by the nervous system. The circulation would adjust 

 itself automatically in accordance with the rate at which blood 

 was allowed to pass through the systemic capillaries; and the 

 resistance in the arterioles and capillaries would automatically 

 maintain a sufficient arterial blood pressure. 



It is possible that in certain cases of heart disease, where the 

 physiological connection between auricles and ventricles through 

 the bundle of Kent and His is broken, the circulation is main- 

 tained in this way, since in these cases the pulse rate does not 

 change during the very limited amount of muscular exertion 

 which is possible. In normal persons or animals, however, the 

 pulse rate increases very markedly during muscular exertion ; and 

 in persons in whom, owing to some nervous or cardiac abnormality 

 this increase does not occur, the capacity for exertion is very small. 

 We must infer, therefore, that under normal conditions the ca- 

 pacity of the heart for increasing the circulation rate without 

 increase of the rate of heartbeat is very limited far more so than 

 might be inferred from study of a heart-lung preparation. In 

 other words the output of the heart during systole is usually 

 pretty constant under normal conditions, as Henderson was the 

 first to point out. 



We must now consider in more detail how the distribution of 

 blood is regulated. It has been known since the discovery by 

 Claude Bernard of vasomotor nerves that the distribution of 

 blood in the body is regulated through the nervous system. Vaso- 

 constrictor nerves are known to be widely distributed in all parts 

 except the central nervous system, and vasodilator nerves have 

 also been discovered at certain points. There is also a main vaso- 

 motor center in the medulla from which vasoconstrictor impulses 

 radiate, and subsidiary vasomotor centers in the spinal cord. 

 Another and much more direct means of regulating the distribu- 

 tion of blood has recently been discovered by Krogh. 20 He has 

 found by microscopical examination of living capillaries, and by 

 injection of Indian ink, that under resting conditions the great 

 majority of capillaries in muscular and other tissues are firmly 



19 Patterson and Starling, Journ. of PhysioL, XLVIII, p. 357, 1914. 



20 Krogh, Journ. of Physiol., LII, p. 457, 1919. 



