THE RATE OF THE HEART BEAT. 589 



was formerly much discussion as to whether this relationship is 

 brought about by reflexes through the extrinsic nerves of the 

 heart or whether it is due to some direct, perhaps mechanical, 

 effect upon the heart. The experiments of Newell Martin upon 

 the isolated heart seem to have settled the matter satisfactorily.* 

 By a method devised by him he kept dogs' hearts beating for 

 manv hours when isolated from all connections with the body 

 except the lungs. Under these conditions it was found that 

 even extreme variations in blood-pressure did not affect the 

 heart rate. Consequently, the variation that does take place 

 under normal conditions must be due to a stimulation of the 

 cardiac nerves. A rise of pressure in the arteries may affect 

 directly the cardio-inhibitory center or it may affect afferent 

 fibers in the heart or arteries, and thus reflexly stimulate the 

 cardio-inhibitory center. This point has been the subject of 

 a number of investigations, but Eyster and Hookerf appear 

 to have demonstrated that both methods of stimulation occur. 

 High arterial pressure affects the medullary center directly 

 and thus slows the rate, but it affects also certain sensory fibers 

 in the aorta at or beyond the arch, and through them causes a 

 reflex slowing. The slowing under these conditions tends to 

 diminish the output from the heart and in this way may serve 

 to prevent a further rise in arterial pressure. On the other hand, 

 it is stated (Bainbridgef) that a rise in venous pressure causes a 

 reflex acceleration of the heart rate, the afferent impulses for the 

 reflex arising in the sensory fibers in the walls of the veins or in 

 the right heart. Here also we have an adaptive mechanism since 

 it may be assumed that the more rapid rate tends to increase the 

 output from the heart. 



Variations with Muscular Exercise. — It is a matter of every-day 

 experience that the heart rate increases with muscular exercise. 

 A simple change in posture, in fact, suffices to affect the heart 

 rate. The rate is higher when standing (80) than when sitting 

 (70), and higher in this latter condition than when lying down 

 (66). Even light muscular work, such as tapping a telegraph key 

 as rapidly as possible, may raise the heart rate from 60 to 70 to 

 over 100 per minute (Bowen|), while the effect of moderate or 

 heavy work is correspondingly greater, the pulse rate rising to 

 150, or even 180 per minute. When the muscular work is con- 



* Martin, "Studies from the Biological Laboratory, Johns Hopkins Uni- 

 versity," 2, 213, 1882; also "Collected Physiological Papers," p. 25, 1895. 



t Eyster and Hooker, "American Journal of Physiology," 21, 373, 1908; 

 Bainbridge, "Journal of Physiology," 50, 65, 1915. 



t Bowen, "Contributions to Medical Research," dedicated to V. C.Vaughan, 

 1903. 



