CHAP, iv.] THE VASCULAR MECHANISM. 305 



heart) through the coronary artery. 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 

 with an increase of the pulse-rate. This however only holds good 

 if the vagi 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 that 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 pulse, when the vagi 

 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 to a certain extent inhibited. 



20 



