CHAP. VII.] 



MUSCULAR ACTION. 



181 



widened and distorted. These contractions are never stationary, 

 but oscillate from end to end, relinquishing on the one hand what 

 they gain on the other. When they are numerous along the same 

 margin, they interfere most irregularly with one another, dragging 

 one another as though striving for the mastery, the larger ones con- 

 tinually overcoming the smaller; Fig. 50. 



then subsiding as though spent, 

 stretched again by new spots of 

 contraction ; and again, after a 

 short period of repose, engaged 

 in their turn by some advancing 

 wave : this is the first stage 

 of the phenomenon (fig. 50). 



At a Subsequent Stage, the Border of an eleme ntary fibre of a young Crab.shew- 

 Pnd< of thp fihrp rnmrnnnlv ing a spot of contraction, and the sarcolemma elevated 



nmoniy m the form of bulle by the expressed water ._ Magni . 

 cease to be fixed, in conse- fied 30 ^ raeters - 



quence of the intermediate portions, by their contraction receiving 

 some of the pressure of the glass. The contractions, therefore, 

 increasing in number and extent, gradually engage the whole 

 substance of the fibre, which then is reduced to at least one-third 

 of its original length. 



The muscular tissue in these animals is comparatively tough ; 

 but, where it is more fragile, as in the 

 frog, it may give way in the intervals 

 between spots of contraction, and be- 

 come ruptured and disorganized in va- 

 rious degrees.* In fishes we have seen 

 a succession of phenomena similar to 

 what has been described in the crab ; 

 waves of contraction advancing and 

 receding, but gradually augmenting in 

 bulk, till the whole fibre was finally 

 contracted (fig. 51). 



In all these examples, as long as the 



pnrk nf tliP frno-mnnf HVA flvprl on/I 

 iragmeilt are fixed, ana 



will not viplfl to thp rnnvpllpnt fnrrp 

 1 J 1L ^OnVCl.ent lOrce, 



that force is seen to be exerted in a 

 momentary manner in successive por- 

 tionsofthcmass. In proportion as they 

 yield to it, the resistance which enabled 

 the contraction of new parts to stretch 



* Phil. Trans. 1840, p. 490, pi. xix. fig. 75. 



. Sta S es of contraction seen on one occa 

 8 ion in an elementary fibre of the Skate. 

 1 he u PP ermost state is tna * previous to 

 the commencement of active contrac- 



tlo a n a . a . successive -waves 



similar -waves- s tm moving 

 fibre ' but engaging its whole 



