210 THE VASCULAR MECHANISM. 



very great length of time. In treating of the skeletal muscles we saw that 

 in their case the exhaustion following upon withdrawal of the blood-stream 

 might be attributed either to an inadequate supply of new nutritive mate- 

 rial and oxygen, or to an accumulation in the muscular substance of the 

 products of muscular metabolism, or to both causes combined. And the 

 same considerations hold good for the nervous and muscular structures 

 of the heart, though the subject has not yet been sufficiently well worked 

 out to permit any very definite statements to be made. It seems prob- 

 able, however, that an important factor in the matter is the accumulation 

 in the muscular fibres and in the surrounding lymph of carbonic acid, and 

 especially of the substances which give rise to the acid reaction. 



When the frog's heart is thus " fed " with various substances the interest- 

 ing fact is brought to light that some substances, such, for instance, as very 

 dilute lactic acid, lead to increased expansion, and others, such, for instance, 

 as very dilute solutions of sodium hydrate, to diminished expansion, that is 

 to continued contraction, of the quiescent ventricle. It would appear that 

 the muscular fibres of the ventricle over and above their rhythmic contrac- 

 tions are capable of varying in length, so that at one time they are longer, 

 and the ventricle when pressure is applied to it internally dilates beyond the 

 normal, while at another time they are shorter, and the ventricle, with the 

 same internal pressure, is contracted beyond the normal. Further, in the 

 frog at least, when the pause between two beats is lengthened the relaxa- 

 tion of the ventricle goes on increasing, so that apparently the ventricle 

 when beating normally is already somewhat contracted when a new beat 

 begins. In other words, the ventricle possesses what we shall speak of in 

 reference to arteries as tonicity or tonic contraction, and the amount of 

 this tonic contraction, and in consequence the capacity of the ventricle, 

 varies according to circumstances. We have, moreover, evidence that 

 inhibitory impulses diminish and augmenting impulses increase this tonic 

 contraction. 



When the frog's ventricle is thus artificially fed with serum or even with 

 blood, the beats, whether spontaneous or provoked by stimulation, are apt to 

 become intermittent and to arrange themselves into groups. This intermit- 

 tence is possibly due to the serum or blood being unable to carry on nutrition 

 in a completely normal manner, and to the consequent production of abnor- 

 mal chemical substances ; and it is probable that cardiac intermittences seen 

 during life have often a similar causation. Various chemical substances in 

 the blood, natural or morbid, may thus affect the heart's beat by acting on 

 its muscular fibres or its nervous elements, or both, and that probably in 

 various ways, modifying in 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 distention of its 

 cavities. The contractions of cardiac muscle, like those of ordinary muscle 

 (see 79), 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 maybe passed, and an overfull ventricle will 

 fail 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 distention of the ventricle is (at the commencement of 

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



