172 THE CIRCULATION OF THE BLOOD AND LYMPH 



may be due to the prolonged excitation of the cardio-inhibitory 

 centre, as well as to the disturbance of other centres in the bulb by 

 the cortical storm. It is a remarkable fact, too, and one that can 

 only be explained by such a connection, that although in the vast 

 majority of individuals the will has no influence whatever on the 

 rate or force of the heart, except, perhaps, indirectly through the 

 respiration, some persons have the power, by a voluntary effort, of 

 markedly accelerating the pulse. In one case of this kind it was 

 noticed that perspiration broke out on the hands and other parts of 

 the body when the heart was voluntarily accelerated. A rise of 

 blood-pressure due to constriction of the vessels has also been 

 observed. The effort cannot be kept up for more than a short time, 

 and the pulse-rate quickly goes back to normal. It has been 

 recently shown that this peculiar power is more common than has 

 been supposed, and that where it is present in rudiment it can be 

 cultivated, although it is a dangerous acquisition. 



As an example of the direct action on a cardiac centre of a 

 changed chemical composition of the blood, we may cite the 

 inhibition produced by injection of bile into a vein and revealed 

 in the slow pulse of many cases of jaundice ; and as an instance 

 of the direct action of a physical change, the slowing of the heart 

 as the blood-pressure rises (p. 188) in asphyxia or on clamping the 

 aorta. The variation in the pulse-rate associated with changes 

 in the position of the body, to which we have already referred 

 (p. 107), is brought about by direct stimulation of the in- 

 hibitory centre by the increase of blood-pressure in the medulla 

 oblongata when a person who has been standing assumes the supine, 

 or even the sitting, posture. But it is also due in part to changes in 

 the amount of muscular contraction, since muscular exercise causes 

 acceleration of the heart either reflexly, through afferent muscular 

 nerves, or by a direct effect of waste products of the metabolism of 

 the muscles on the cardiac centres in the bulb or on the heart itself 

 (p. 280). 



Theoretically, quickening of the heart might be caused either by 

 a diminution in the inhibitory tone or by an increase in the activity 

 of the augmentor centre; and slowing of the heart might be due 

 either to a diminution in the augmentor tone, if such exists, or to 

 an increase in the activity of the inhibitory centre. So that it is 

 not always easy to interpret such results as we have quoted above. 

 But it would appear that under ordinary conditions the rate of the 

 heart is mainly regulated by the inhibitory centre, which, within a 

 considerable range, can produce variations in either direction. The 

 augmentor mechanism is perhaps merely auxiliary to the inhibitory, 

 being called into action only in emergencies. 



