300 INTRINSIC REGULATION OF HEART BEAT. [BOOK i. 



of the moment, and at the same time brings the two into mutual 

 dependence ; but the central nervous system is not the only means 

 of government : there are other modes of regulation, and so other 

 safeguards. 



166. Let us first consider the heart. The object, if we may 

 use the expression, of the systole of the ventricle is to secure 

 the needed arterial pressure ; it is this, as we have seen, which 

 drives the blood through the capillaries back to the heart. To do 

 this the ventricle must deliver at the stroke a certain quantity of 

 blood, exerting the pressure required to lodge the blood in the 

 arteries, and repeating the stroke at appropriate intervals. Hence 

 the work done will, in part, depend on the quantity of blood 

 collected in the ventricle during the diastole, that is, on the inflow 

 from the venous system. If the quantity brought be too small, 

 then though the whole contents of the ventricle be ejected with 

 adequate force at each stroke, and the stroke repeated regularly, 

 the ventricle will fail in its object ; speaking generally we may 

 say that a lessened venous inflow will tend to lessen, and an 

 increased venous inflow will tend to increase the work of the heart. 

 This venous inflow is dependent on various causes, and may be 

 variously modified by various events. 



The blood in filling the ventricle distends its walls, and this 

 distension, especially the fuller distension resulting from the 

 auricular systole, also influences the ventricular stroke ; for the 

 contraction of the cardiac fibre, like that of the skeletal muscular 

 fibre, is increased up to a certain limit by the fibre being put on 

 the stretch ( 140). This influence, however, is more distinctly 

 seen on the arterial side. The greater the arterial pressure 

 against which the ventricle has to deliver its contents, the greater 

 the tension of the ventricular walls ; and hence, a high arterial 

 pressure tends of itself to enforce the ventricular systole. As in 

 the skeletal muscle, however, this beneficial influence soon reaches 

 its limit. 



167. The spontaneous beat of the heart is the outcome of 

 the nutrition of the cardiac tissues. In the absence of all inter- 

 ference by inhibitory or auginentor fibres, the heart will continue 

 beating with a certain rhythm and force, determined by the 

 metabolism going on in its muscular and nervous elements. The 

 beat therefore will be influenced by anything which affects that 

 metabolism. And the obvious and direct cause of changes in the 

 nutrition and so in the behaviour of the heart lies in changes in 

 the quantity and quality of the blood supplied to the cardiac tis- 

 sues. In the mammal this means the quantity and quality of the 

 blood flowing through the coronary arteries. 



If in a mammal the coronary arteries be tied or otherwise 

 occluded the heart in a few seconds comes to a standstill ; this, 

 which always results if both arteries be tied, sometimes if one 

 only be tied, is preceded by an irregularity or by changes in the 



