THE CIRCULATION OF THE BLOOD AND LYMPH 147 



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 accelerat- 

 ing 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 asserted 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 (Van de Velde). 



As an example of the direct action of a chemical stimulus 

 on a cardiac centre, we may cite the marked inhibition pro- 

 duced by injection of an extract of the suprarenal capsule 

 into a vein (p. 475), and as an instance of the direct action 

 of a physical change, the slowing of the heart in asphyxia as 

 the blood-pressure rises (p. 163). The variation in the pulse- 

 rate associated with changes in the position of the body, 

 to which we have already referred (p. 96), has been attri- 

 buted to direct stimulation of the inhibitory 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 may also be due in part to 

 changes in the amount of muscular contraction. 



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 



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