590 CIRCULATION OF BLOOD AND LYMPH. 



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 hght, 

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

 hausting muscular work, such as long-distance runs. The faster 

 heart rate in muscular exercise is an important part of the mechan- 

 ism of adaptation which provides the working muscles with an 

 increased flow of blood and an increased supply of oxygen, and 

 ensures that the increase is proportional to the need. The increase 

 in the heart rate is brought about by several means. In the first 

 place, at the very beginning of the exercise, as stated above, there 

 is an anticipatory increase which may be explained as being due 

 to an associated activity of the cardiac centers in the medulla, 

 that is to say, when the motor impulses come down from the 

 brain on their way to the muscles they affect the activity of the 

 medullary centers, probably by means of collateral branches of 

 the fibers in action. Experimental evidence indicates that it is 

 the cardio-inhibitory center which is chiefly affected.* Its tonic 

 action is depressed and the rate of beat is correspondingly in- 

 creased. When the muscular activity is large or prolonged other 

 factors come into play. The reaction of the blood is changed 

 somewhat toward the acid side. While the amount of change is 

 small, owing to the regulating mechanisms in the blood, we know 

 that it is sufficient to excite the respiratory center, and there is 

 experimental proof f that it affects also the cardio-inhibitory 

 center in such a way as to cause an increase in heart rate. So 

 also the mechanical effect of the contracting muscles upon the 

 veins squeezes more blood toward the heart, so that the right 

 heart is filled more rapidly and under a greater pressure. This 

 increase in venous pressure, as was stated in the last paragraph, 

 brings about a reflex acceleration of the heart rate. Still other 

 influences that may have a similar effect are a shght rise in body 

 temperature or an increased secretion of adrenalin. 



The Variations in Pulse Rate Due to Changes in the Compo- 

 sition or Properties of the Blood. — The condition under this head 

 that has the most marked influence upon the heart rate is the 

 temperature of the blood. Speaking generally, the rate of beat 

 increases regularly with the temperature of the blood or other 

 circulating hquid up to a certain optimum temperature. On the 

 heart of the cold-blooded animal this relationship is easily demon- 

 strated by supplying the heart with an artificial circulation of 



* Gasser and Meek, "Amer. Journal of Physiology," 34, 48, 1914. 

 t Hooker, Wilson, and Connet, Ibid., 43, 351, 1917. 



