196 CIRCULATION OF THE BLOOD 



brum, etc., as peripheral organs by which the cardiac centers are excited re- 

 flexly, just as they are roused to activity by afferent fibers coming from other 

 parts of the body (Franck). According to this conception the actual center 

 of the inhibitory nerves would lie only in the medulla. It can be acted upon 

 by a great many afferent nerves from the skin, from the heart itself, from 

 the abdominal viscera, the lungs, the sense organs, and from the different 

 parts of the brain. , 



The blood pressure also exercises an influence on the rate of the heart beat. 

 It is true that in an exsected heart, one observes no influence upon the pulse- 

 frequency by variations of the arterial pressure within the vital limits, and the 

 variations of the pulse produced by great variations of the venous pressure are 

 not especially large. But under normal conditions of the circulation, very evi- 

 dent changes in frequency often occur as the result of variations in pressure, 

 and this even if every possible connection of the heart with the central nervous 

 system is broken. Thus it is found that if an increase in blood pressure due to 

 a local vasoconstriction occurs, there often goes with it an acceleration of the 

 heart beat, the chief cause of which is probably to be sought in the suddenly 

 increased blood supply to the heart. By this means those portions of the heart 

 where the contraction starts are roused to more frequent action. 



In a heart completely isolated from the central nervous system an increase 

 of pressure may produce also a retardation. The inhibitory mechanism there- 

 fore as well as the motor mechanism can be excited by a rise of blood pressure. 

 The result will depend upon the relative irritability of the two mechanisms. 



When all the cardiac nerves are intact, the heart frequency decreases with 

 a rise in blood pressure and increases with a fall, whatever the order in which 

 the variations succeed each other. Since slowing of the heart is not the usual 

 result of increased arterial pressure when the vagi are cut, it is clear that the 

 above-mentioned phenomenon is an effect of the vagus center. 



This excitation of the vagus center is called out in part by the depressor; 

 but is probably also connected with a change in the circulation of the brain, 

 excitation of the center following an increase of intracranial pressure. 



Nothing certain is known as to how the accelerator center acts with a rise 

 of pressure. The increase of pulse frequency observed in an anaemic condition 

 of the brain might possibly be referred to an excitation of this center, but it 

 can be explained also by a fall in the tonic influence of the vagus center. 



From the facts just discussed, it cannot be looked upon as fully established 

 that effects on the efferent cardiac nerves, which can be obtained by stimulation 

 of afferent nerves, are caused exclusively by a reflex from the cardiac centers, 

 for it is not impossible that a change in the blood supply to the brain, brought 

 about by a reflex effect on the vascular system, has participated to some extent. 



14. THE RATE OF HEART BEAT 



Now that we have studied the influence of nerves upon the rate of the 

 heart beat, it remains for us to inquire what are the normal variations in man. 



If all disturbing influences be removed as far as possible i. e., if the 

 individual be resting quietly in bed and abstain from food only very slight 

 variations in the pulse frequency appear in the course of the day. But the 

 pulse rate is quickly affected by all sorts of influences. 



