304 REGULATION BY NUTRITION. [BOOK i. 



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 relaxation 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 them- 

 selves into groups. This intermittence 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 abnormal 

 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 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. 



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. 



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 



