ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 161 



of each muscle - fibre, present fundamental differences, in the 

 sense that the fibrils conduct much more quickly (contract by 

 " twitches "), while the less excitable sarcoplasm, like almost all 

 undifferentiated protoplasm, transmits the excitation - process 

 slowly. On histological grounds it is indeed impossible not to 

 regard the fibrils as co-operating generally in the slow waves 

 of contraction, but it is noticeable that the undulation in the 

 muscle-fibres is best exhibited under just those conditions which 

 have been found experimentally to favour the excitation of 

 non-differentiated, contractile plasma. Mechanical excitation is 

 particularly appropriate, so that, at least in vertebrate muscles, 

 the intensity of excitation must be much greater than is required 

 to provoke a twitch. The time-order of development in the 

 different forms of contraction is also to be noticed, since it 

 occasionally gives an opportunity of observing how the twitch 

 that immediately succeeds, and almost coincides with, the stimulus, 

 is followed by the idio-muscular swelling, from which again pro- 

 ceed the slowly-spreading undulations. This agrees with the 

 much greater latent period and slower development of contraction, 

 in purely plasmatic parts. The question touched on here will 

 only be decided when our knowledge of the functional relations 

 between sarcoplasm and fibrils has advanced much further than 

 at present. 



In the cases so far discussed we have been concerned exclu- 

 sively with conduction of the excitatory process within single, 

 mt'ltinuclcar, longitudinal cells, such as those of striated, skeletal 

 muscle-fibres. A wave of contraction either stops half-way or 

 spreads to the end of the fibre, from which it can eventually 

 be reflected back, or more frequently expires there. Every 

 tendinous intersection, however small, will entirely block the 

 transmission of even the strongest excitation, so that stimulation 

 of a polymerous muscle at one end only results in contraction 

 of the part directly implicated. It is equally impossible for 

 excitation to be transmitted transversely from one fibre to the 

 next adjacent, and any seeming exceptions (e.g. in drying muscle) 

 are, as we shall see, to be explained on other grounds. The 

 conduction of excitation in muscular organs consisting of uni- 

 nuclear muscle-cells is, on the other hand, fundamentally different. 

 Co-ordinated action of a number of muscle-cells in consequence 

 of local stimulation is obviously possible only where the excita- 



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