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Figure 5. — Records of activity from red (R) and white (W) regions of the myotome of different rainbow trout swimming at various 

 speeds. A: 1 .8 body lengths per second; B: 4.6 body lengths per second; C: 2.2 body lengths per second. Note that during regular sustained 

 swimming involving red muscle activity, no activity is detected from the white zone of the myotome. Occasional spikelike activity from 

 the white zone of the myotome is seen in B and C (also picked up by the electrode lying in the overlying red zone of the myotome), 

 sometimes inhibits the red muscle bursts (B) and sometimes does not (C). Vertical bars: 1 mV; time marker: seconds. 



A single rapid tail beat (to the right of the rec- 

 ord) interrupted the red muscle for a single cycle. 

 At higher sustained speeds, above 2 BL/s (as in 

 Figure 5C) this inhibition of red activity following 

 single rapid movement no longer took place. 

 Rather, the behavior was similar to that of the 

 herring in that the fish fell gradually back despite 

 the regular activity of the red system, until driven 

 forward again by a few rapid beats; to drop back 

 again and repeat the cycle until the white system 

 was exhausted. Under these conditions, the fish 

 did not "coast" following rapid tail movements. 



No electrical activity was observed from the 

 white zone of the myotome (the so-called mosaic 

 zone) apart from the spikelike potentials shown in 

 Figure 5B and C, although particular pains were 

 taken to ensure that the electrodes were recording 

 satisfactorily. All the fish recorded fiom gave this 

 same result. We conclude from our observations 

 that this part of the motor system is not active at 



696 



speeds below 2-2.5 BL/s. Figure 3B summarizes 

 the structure of the system. 



Discussion 



Our observations have shown once again that 

 the lateral red musculature is used by fish for 

 sustained slow cruising, and that rapid move- 

 ments of the tail are brought about by the activity 

 of the white motor system, during which spikelike 

 potentials can be recorded from the white zone of 

 the myotome. At intermediate speeds, there are 

 manifest differences between different fishes. 



The simplest situation is shown by the Pacific 

 herring, where sustained activity depends only on 

 the activity of the red motor system of the 

 myotome: the white fibres play no part in any 

 activity except rapid movements of short duration. 

 It is true that such movements can "top up," as it 

 were, the sustained activity of the red motor sys- 



