58 ELECTRO-PHYSIOLOGY CHAP. 



muscle and striated muscle. The contractions of smooth muscle 

 are incomparably slower than those of striated muscle, so that we 

 could never speak of a " twitch " when a single stimulus was 

 acting on the elements of smooth muscle. The entire course of 

 contraction is, so to speak, macroscopic, since the latent period, as 

 well as all the phases of contraction and elongation, can be con- 

 veniently followed by the eye without artificial assistance. 



Midway between these sluggishly contracting smooth muscles 

 and the " twitching," striated muscles of invertebrates and verte- 

 brates stand the uninuclear, cross-striated elements of cardiac 

 muscle, where contraction is neither so sluggish as in smooth, nor 

 so rapid as in most striated skeletal muscles. For this reason 

 it was long a matter of dispute whether the single contraction 

 of the heart, discharged by a momentary excitation (which 

 in no way differs from a natural " heart-beat "), is really com- 

 parable with the elementary single twitch of a skeletal muscle. 

 But that it is so cannot now be doubted. It is clear that its 

 longer duration is no sort of proof that the single contraction of 

 cardiac muscle does not correspond with a simple twitch. Even 

 among the striated skeletal muscles of different animals, or the 

 muscles of the same individual, considerable differences may 

 occur, as we have seen, in regard to rapidity of contraction, and 

 it is easy to produce these artificially to a much greater extent 

 than is the case in normal cardiac contraction. 



We shall therefore assume that every single contraction of 

 cardiac muscle (vertebrate or invertebrate}, whether natural or pro- 

 duced l>y a brief artificial stimulus, is an elementary " twitrli* 

 although retarded and protracted in all its phases. 



If, under approximately equal conditions, the contractions in 

 the heart and skeletal muscle of the frog, excited by a single 

 induction shock, are recorded by the graphic method with a 

 lightly attached lever, it will be found, as Marey pointed out (1), 

 that the heart-curve and muscle-curve exhibit in respect of form 

 the same characteristic peculiarities of rapid rise, and more 

 gradual sinking. 



The latent period is, however, without exception, longer in cardiac 

 than in skeletal muscle under the same conditions, and the more 

 conspicuously so in proportion with the difference in rapidity of 

 contraction betw r een the two kinds of striated muscle. As this 

 difference is greater in poikilo-thermic vertebrates than in the 



