602 CIRCULATION OF BLOOD AND LYMPH. 



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 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 $hem causes a 

 reflex slowing. 



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

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

 over 100 per minute (BowenJ), 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- 

 tinued the pulse rate rises rapidly to a certain maximum, which it 

 maintains more or less constantly during the work. After the 

 cessation of the muscular exercise the rate drops very rapidly, 

 reaching the normal in a few seconds if the work has been light, 

 but only after a long interval, an hour or more, in the case of ex- 

 hausting muscular work, such as long-distance runs. Speaking in 

 general terms, therefore, it may be said that there is an important 

 immediate effect of muscular exercise on the heart rate, and a 

 longer lasting or relatively permanent effect observed chiefly after 

 strenuous long-continued exercise. This latter effect is due, in all 

 probability, to some change in the composition of the blood. One 

 thinks of the possibility of a change in the reaction of the blood 

 or at least a change in its alkali reserve which may result from the 



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

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



f Eyster and Hooker, "American Journal of Physiology," 21, 373, 1908. 



JBowen, "Contributions to Medical Research," dedicated to V. C. 

 Vaughan, 1903. 



