Chap, iv.] THE VASCULAR MECHANISM. 261 



An interesting combination of direct mechanical effects and 

 indirect nervous effects is seen in the relation of the heart's 

 beat to blood pressure. When the blood pressure is high, not 

 only is the resistance to the ventricular systole increased, but, 

 other things being equal, more blood flows (in the mammalian 

 heart) through the coronary arteries. Both these events would 

 increase the activity of the heart, and we might expect that the 

 increase would be manifest in the rate -of the rhythm as well as in 

 the force of the individual beats. As a matter of fact, however, 

 we do not find this. On the contrary, the relation of heart beat to 

 pressure may be put almost in the form of a law, that " the rate 

 of the beat is in inverse ratio to the arterial pressure ; " a rise of 

 pressure being accompanied by a diminution, and fall of pressure 

 by an increase of the rate of the rhythm. This however only holds 

 good if the vagus nerves be intact. If these be previously divided, 

 then in whatever way the blood pressure be raised — whether by 

 injecting blood or clamping the aorta, or increasing the peripheral 

 resistance, through an action of the vaso-motor nerves which we 

 shall have to describe directly — or in whatever way it be lowered, 

 no such clear and decided inverse relation between blood pressure 

 and pulse-rate is observed. It is inferred therefore that increased 

 blood pressure causes a slowing of the beat, when the vagus nerves 

 are intact, because the cardio-inhibitory centre in the medulla is 

 stimulated by the high pressure, either directly by the pressure 

 obtaining in the blood vessels of the medulla, or in some indirect 

 manner, and the heart in consequence more or less inhibited. 



