TONICITY OF THE CARDIAC MUSCLE. 195 



contraction wave. Therefore the time between the normal and inter- 

 polated ventricular contractions will be longer than the time between 

 the normal and interpolated contractions of the sinus ; in other words, 

 any irregularity in the beat at headquarters will tend to be compensated 

 by the time the ventricle contracts, so that the contractions of the 

 ventricle will occur at much more regular intervals than those of 

 the sinus. Seeing that the main importance to the economy is to 

 ensure a regular ventricular beat, it is clear that this more or less 

 complete obliteration of any irregularity in the rhythm, by means of the 

 varying rates of conduction at different phases of the beat, is a very 

 important factor in maintaining the efficiency of the heart. 



All these different properties are possessed, not only by cardiac 

 muscle, but also by all other contractile tissue, so that the differences of 

 the cardiac muscle are differences of degree rather than of kind. In 

 addition to these properties, other muscular tissues are said to possess 

 tonicity, a property which is especially well marked in the muscles of 

 the vascular system ; by analogy, the cardiac muscle ought also to 

 possess tone, and it would not be unreasonable to expect that different 

 parts of the heart should vary in the extent of their tonicity, just as 

 they vary in the extent of their rhythmicity, in their power of rapidity 

 of contraction, and their other properties. 



The question whether cardiac muscle does or does not possess tone is 

 by no means easy to answer off-hand, owing to the beating of the heart; for 

 it is clear that the extent of relaxation between the contractions depends, 

 not only upon the tone of the muscular tissue, but also upon the rate of 

 beat : with a slower beat there is more time for complete relaxation to 

 take place than with a quicker beat, and it is very doubtful whether, 

 with the normal rate of beat of the cold-blooded heart, complete 

 relaxation does take place between the beats. It is therefore difficult to 

 speak of an alteration of tone in the cardiac muscle, if such apparent 

 alteration is accompanied with the corresponding change in the rate of 

 beat. Still there is evidence which points distinctly to a power of 

 tonicity in the cardiac muscle. This evidence is especially dependent 

 upon the action of various drugs and solutions upon the nature of the 

 cardiac contraction. Such solutions may be divided into two groups : 

 those which tend to produce a tonic contraction of the ventricle or apex 

 of the ventricle, so that a less and less complete relaxation takes place 

 between the beats, and finally the ventricle remains in a condition of 

 systolic standstill ; and those which tend to produce an atonic condition 

 of the ventricle or apex, characterised by the greater and greater 

 feebleness of the contractions, until finally the ventricle stands still in 

 the condition of complete relaxation or diastolic standstill. The type 

 of the first group is given by the action of alkaline sodium compounds, 

 and of the second by the action of acid substances, such as lactic acid. 1 



The nature of the action of sodium hydrate in very weak solution 

 (1 part to 20,000 normal saline) is shown in Fig. 106, which represents 

 samples a, b, c, d, e, taken at intervals from a continuous curve, 2 and the 

 corresponding action of lactic acid (1 part to 10,000 normal saline) in 

 Fig. 107. In both cases the time when the solution was sent in is repre- 

 sented by a vertical arrow. We see from the figures that not only is 

 the relaxation after each beat hindered more and more by the alkaline 

 salt, but also the muscle remains at each beat a longer time in the 

 1 Gaskell, Journ. PhysioL, Cambridge and London, 1880-82, vol. iii. p. 53. 2 Ibid. 



