CHAP, iv.] THE VASCULAR MECHANISM. 323 



ways, modifying in different directions the rhythm, or the individual 

 contractions, or both. 



Concerning the effect on the heart of blood which has not been 

 adequately changed in the lungs we shall speak when we come to 

 treat of respiration. 



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 81), 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 fail to beat at all. Hence an increase in the 

 quantity of blood in the ventricle will augment the work done in 

 two ways; the quantity thrown out will, unless antagonistic 

 influences intervene, be greater, and the increased quantity will be 

 ejected with greater force. Further, since the distension of the 

 ventricle at the commencement of the systole at all events is 

 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. 



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. 



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