ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 127 



similar observations in regard to the process of tetanus with 

 prolonged excitation, in the sluggish claw- and quick tail-muscle 

 of the crab. Complete tetanus of the latter is never of long 

 duration, the muscle quickly relaxes, and for some time exhibits 

 a marked diminution of excitability ; the tetanus of the claw- 

 muscles, on the other hand, increases gradually, and may persist 

 for a long time. The relation of this phenomenon to the normal 

 activity of both kinds of muscle is unmistakable. The powerfully 

 developed adductor of the claw has to remain uniformly con- 

 tracted for a long period with a great output of energy, while 

 the tail serves up quick movements (strokes) like a rudder, and is 

 concerned less with a prolonged yield of energy than with rapidity 

 of motion. 



These results are an additional confirmation of the conclusions 

 which we have shown to stand out re energy and duration, from 

 the distribution and presence of sarcoplasmic and non-sarcoplasmic 

 (light and dark, i.e. pale and red) muscles. The same facts assume 

 a still greater importance when it is remembered that in the 

 majority of cases one muscle contains lioth kinds of functionally 

 different fibres in varying quantitative relations. And if this 

 double composition appears sometimes in a single, simple twitch, 

 and is plainly expressed in the curve, the same also occurs, 

 and in a much more marked degree, in tetanus. Generally 

 speaking, we may expect that muscles, the bulk of which consists 

 chiefly of sluggish (dark, red) fibres, will exhibit properties in 

 conformity with these, while, if composed of quick fibres, they 

 will react like the latter. 



This is well indicated, according to Grlitzner (15), in the 

 relation between the height of the single twitches and the height 

 of tetanus. In loaded muscle the latter considerably out-tops the 

 former ; but under uniform conditions the difference is much 

 more marked in sluggish than in quick fibres. If, e.g., with direct 

 excitation, the height of tetanus in the mixed gastrocnemius in 

 frog and toad are compared, it will be seen that the muscle 

 of the latter, which mainly consists of sluggish fibres, will raise 

 the same weight much higher than the corresponding muscle 

 of the frog, although it is much smaller. The former almost 

 curls itself into a ball with strong electrical stimulation, while the 

 frog's muscle, even in the most pronounced tetanus, is far from 

 being rolled up. While in the " quick " muscles of the frog 



