548 CIRCULATION OF BLOOD AND LYMPH. 



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 

 many 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 reflex stimulation of the cardiac nerves. 

 The origin of the sensory stimulus in this reflex is not clearly known; 

 possibly the afferent nerves of the heart itself are stimulated or 

 afferent fibers distributed to the walls of the aorta. 



Variations with Muscular Exercise. It is a matter of everyday 

 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). Unusual exertion, as in running, causes a very marked and 

 long-lasting increase in the pulse rate. The purposeful charac- 

 ter of this adaptation is very evident. Increase in muscular activity 

 calls for a more rapid circulation to supply the oxygen and other 

 elements of nutrition, but the physiological mechanism by which 

 this adaptation is obtained is not explained satisfactorily. Johans- 

 son,! who has studied the matter carefully, concludes that the 

 effect is due mainly to two causes: First, to the effect of the chem- 

 ical products of metabolism in the active muscle, which are given 

 off to the circulation and are then carried to the nerve centers 

 where they affect the cardiac nerves, or possibly to an effect of 

 these metabolic products on the heart directly. He considers this 

 factor as of relatively subordinate importance. Second, the chief 

 factor is found in an associated activity of the accelerator nerves. 

 That is, the discharge of impulses along the voluntary motor paths 

 (pyramidal) sets into activity at the same time and proportionally 

 the center of the accelerator nerve fibers. Hering, } on the con- 

 trary, gives experimental evidence for the view that the 

 increase in heart rate after exercise is due to a reflex stim- 

 ulation of the accelerator nerves of the heart. After pro- 

 longed or excessive muscular exertion the heart rate remains 

 accelerated for a considerable period after cessation of the work, a 

 fact which would indicate some long-lasting influence, such as is im- 



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

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

 f Johannson, "Skandinavisches Archiv f. Physiologic," 5, 20, 1895. 

 j "Centralblatt f. Physiologic," 8, 75, 1894. 



