PECULIARITIES OF CARDIAC MUSCLE. 449 



Physiological Peculiarities of Cardiac Muscle as 

 compared with skeletal, 



There are two respects in which the excitatory properties of cardiac 

 muscle appear to differ from those of skeletal, namely — (1) The existence 

 of periodic • variations of excitability; (2) the characteristic relation of 

 response to stimulus known as the " all or none " principle, according to 

 which the vigour of the response evoked by a stimulus adequate to 

 produce an effect, is independent of the strength of the stimulus. 



1. Periodic variations of excitability. — There can be no doubt 

 that any part of the heart tends to respond rhythmically to a stimulus 

 which acts continuously. The continuous stimulus may be mechanical, 

 e.g., an increase of intracardiac pressure, or it may be electrical. A 

 notable instance of the effect of the former may be seen in the snail's 

 heart, which, when quite quiescent, can be made to beat rhythmically 

 by distending it. The same thing happens in nearly as striking a way 

 in a motionless apex preparation of the ventricle of the heart of the 

 frog, when similarly treated. The rhythmical effect of a continuous 

 electrical stimulus, such as that of a succession of alternating induction 

 currents following one another with very great frequency, can be well 

 seen in a strip of muscle cut from the border of the ventricle of the 

 heart of the tortoise. It is, I think, usual to explain such instances 

 of the apparent transformation of a constant excitatory action into a 

 recurrent one, by saying that cardiac fibres can only respond rhythmically 

 — that they are essentially incapable of tetanus, etc. According to a 

 similar mode of explanation, this rhythmicality is attributed to the 

 liability of the heart to be " refractory " for a certain period after each 

 excitation. Neither of these modes of expression brings us, however, 

 any nearer to an understanding of the phenomenon we have under con- 

 sideration ; they are merely different ways of stating it. There are so 

 many non-physiological examples (such as, e.g., the dropping of water 

 from a narrow aperture under a constant pressure) of the conversion of 

 a continuous action into a rhythmical one, that we need not be surprised 

 at the same thing happening in muscular tissue. The difficulty lies 

 rather in the fact that in skeletal muscle no such conversion appears 

 to take place, than that it occurs in cardiac muscle. Before, however, 

 we admit that it does not occur in the former, we must inquire what 

 form it would assume in such muscle, if it could be observed. In so far 

 as the response to a single stimulation is strictly limited in duration, 

 varying with the muscle employed and with temperature, skeletal 

 muscle has also its period. What the period is may be estimated either 

 from the mechanical or from the electrical response to an instantaneous 

 stimulation, but much more easily from the latter than from the former. 

 The monophasic response lasts less than one-fiftieth of a second ; so that 

 a " refractory period," if such there be, should, by analogy with the 

 heart, have a similar duration. In the heart the corresponding mono- 

 phasic variation, at a temperature of 12° C, lasts about two seconds, so 

 that where cardiac muscle responds once a second, we should expect, 

 under the same conditions, skeletal muscle to respond a hundred times 

 a second. It has not yet been proved, experimentally, that a con- 

 tinuously excited muscle does thrill mechanically at any such rate, but 

 there are indications, not yet investigated, of an electrical thrill of this 

 order of frequency produced by continuous stimulation. 

 vol. 11. — 29 



