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ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 59 



warm-blooded animals, the difference in the length of the latent 

 period is more marked also. Thus in the frog the latent period 

 of cardiac muscle may last 0'28 sec., while in the gastroc- 

 nemius of the same animal it is only O'Ol sec. according to 

 Helmholtz, and still shorter (0*005 sec.) according to .the latest_ 

 observations. The period of rising energy is, in the frog's heart, 

 2-3 sees, according to Marchand (2), while the same period 

 in skeletal muscle must be measured by fractions of a second. 

 The striated muscles of the medusae, which approximate to cardiac 

 muscle in other respects also, are characterised by a similar 

 sluggish contraction (Eomanes, 3). 



Similar, if less extensive, differences in the time-relations of 

 contraction have recently been shown to exist within striated 

 skeletal muscle itself, and that not merely in different animals, 

 but in one and the same individual, even within one muscle. 

 It may be said, speaking generally, that there are, in a physiological 

 sense, two kinds of multinuclear, cross-striated muscle-fibres, charac- 

 terised respectively ly rapid and ly sluggish contraction (" quick " and 

 " sluggish " muscles). Between the two there are innumerable 

 intermediate stages. 



It is, e.g., evident that the skeletal muscles of a tortoise or 

 chameleon, as a rule, contract much more slowly than those of the 

 frog or a warm-blooded animal ; while, on the other hand, certain 

 muscles of insects contract more quickly than the best-adapted 

 muscles of warm-blooded animals. This is practically obvious 

 from the respective movements of the creatures, taking only, e.g., 

 the slow sluggish movements of the tortoise in comparison with the 

 marvellously rapid wing-beats of many insects, whose muscle- 

 fibres must contract several hundred times in the second. The 

 contraction curve of such muscles must be immeasurably shorter 

 than that of the frog or tortoise. It is probable that, as Hermann 

 (4, p. 38) suggested, a continuous, graduated scale might be drawn 

 up in the animal kingdom, beginning, after Marey, at the excess- 

 ively rapid contractions of the wing-muscles of insects ; then 

 would follow the striped skeletal muscles of birds, fishes, mammals, 

 frogs, toads, and lastly of tortoises and hibernating dormice, then 

 cardiac muscles, and finally most of the smooth muscle-cells whose 

 ntraction process, as we have said, is macroscopic. In frog 

 muscles the single twitch lasts, at ordinary temperature, from 

 O'l to 0'3 sec., in the tortoise often more than 1 sec., while in 



