CHAP, iv.] THE VASCULAR MECHANISM. 193 



different directions the rhythm, or the individual contractions, or 

 both. 



The physical or mechanical circumstances of the heart also 

 affect its beat ; of these perhaps the most important is the amount 

 of the distension of its cavities. The contractions of cardiac 

 muscle, like those of ordinary muscle (see p. 87), are increased up 

 to a certain limit by the resistance which they have to overcome ; 

 a full ventricle will, other things being equal, contract more 

 vigorously than one less full ; though, as in ordinary muscle, the 

 limit at which resistance is beneficial may be passed, and an over- 

 full ventricle will cease to beat at all. 



Under normal conditions the ventricle probably empties itself 

 completely at each systole. Hence an increase in the quantity of 

 blood in the ventricle would augment the work done in two ways ; 

 the quantity thrown out would be greater, and the increased 

 quantity would be ejected with greater force. Further, since the 

 distension of the ventricle is (at the commencement of the systole 

 at all events) dependent on the auricular systole, the work of the 

 ventricle (and so of the heart as a whole) is in a measure governed 

 by the auricle. 



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 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, as 

 Marey has insisted, 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 thereby stimulated, and the heart in consequence to a certain 

 extent inhibited. 



13 



