344 INTRINSIC REGULATION OF HEART-BEAT. [BOOK i. 



through them with less or with greater ease, as well as by the 

 character of the circulating blood. 



188. These two chief variables, the beat of the heart and the 

 width of the minute arteries, are known to be governed and regulated 

 by the central nervous system, which adapts each to the circum- 

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

 mutual independence ; but the central nervous system is not the 

 only means of government ; there are other modes of regulation, 

 and so other safeguards. 



Thus while undoubtedly the two prominent governors of the 

 heart are the inhibitory fibres of the vagus, and the augmentor 

 fibres from the splanchnic system, the one slowing the rhythm and 

 weakening the stroke, the other quickening the rhythm and 

 strengthening the stroke, other causes may vary the beat, in the 

 absence of any action of either of these two nerves. Mere 

 distension of the ventricle, by increasing the tension of the 

 ventricular fibres, and so increasing the force of the contraction of 

 each fibre (see 162), brings about a more forcible beat. As we 

 shall see in dealing with respiration, a powerful inspiration leads 

 to a larger flow of blood into the heart, and forthwith the ventricle, 

 out of its very fulness, gives stronger beats for the time. So also 

 when by valvular disease or otherwise an unusual obstacle is 

 presented to the outflow from the ventricle, increased vigour in 

 the strokes of the distended organ strives to compensate the 

 mischief. As however in the case of the skeletal muscle, the 

 tension, if too great, may be injurious. In a similar manner the 

 auricle, by a stronger or a weaker contraction, may distend the 

 ventricle to a greater or to a less extent, and so produce a stronger 

 or weaker ventricular systole. 



189. Still more efficient perhaps as a direct governor of the 

 heart's beat is the quality and quantity of blood passing in the 

 mammal through the coronary arteries and regulating the nutrition 

 of the cardiac substance. In the absence of all interference by 

 inhibitory or augmentor fibres the heart will continue beating 

 with a certain rhythm and force, determined by the metabolism 

 going on certainly in its muscular, and possibly to a certain 

 extent also in its nervous elements. We have seen that the energy 

 set free in an ordinary skeletal muscle, in response to a stimulus, 

 may vary from nothing to a maximum according to the metabolism 

 going on, according to the nutritive vigour of the muscular fibres. 

 The spontaneous rhythmic beat of the cardiac substance may 

 be regarded as the outcome of a metabolism more highly pitched, 

 more elaborate, of a higher order than that which simply furnishes 

 the ordinary skeletal fibre with mere irritability towards stimuli. 

 All the more therefore may the beat be expected to be influenced 

 by any change in the metabolism of the cardiac substance, and so 

 by any change in the blood which furnishes the basis for that 

 metabolism. Hence the beat of the heart, quite apart from 



