260 REGULATION BY NUTRITION. [BOOK i. 



pressure, are contracted beyond the normal. In other words, the 

 heart possesses what we shall speak of in reference to arteries as 

 tonicity or tonic contraction, and the amount of this tonic contrac- 

 tion, and in consequence the capacity of the chambers, varies accord- 

 ing to circumstances. The presence of some substances appears to 

 increase, of others to diminish this tonicity and thus to diminish or 

 increase the capacity of the chambers during diastole. This of 

 course would have an effect, other things being equal, on the 

 output from the heart and so on its work ; and indeed there is 

 some evidence that the augmentor and inhibitory impulses may 

 also affect this tonicity, but observers are not agreed as to the 

 manner in which and extent to which they may thus act. 



Another fact worthy of notice is when the heart is thus artifi- 

 cially fed with serum, or other fluids 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 fluid employed 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 intdhnittences seen during life are in certain cases thus 

 brought about by some direct interference with the nutrition of the 

 cardiac tissue and not through extrinsic nervous impulses descend- 

 ing to the heart from the central nervous system. Various chemical 

 substances in the blood, arising within the body or introduced as 

 drugs, 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. 



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 distention of its cavities. The contractions of cardiac 

 muscle, like those of ordinary muscle (see 76), 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 distention 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. 



