CHAP, ii.] THE CONTRACTILE TISSUES. 169 



moves into a new position, several particles thus moving towards 

 the same point cause a bulging at that point, and several particles 

 moving away from the same point cause a retraction at that 

 point ; but no two particles get nearer to each other so as to 

 occupy together less space and thus lead to condensation of sub- 

 stance, or get farther from each other so as to occupy more space 

 and thus lead to increase of bulk. 



In this respect, in that there is no change of bulk, but only a 

 shifting of particles in their relative position to each other, the 

 amoeboid movement resembles a muscular contraction ; but in 

 other respects the two kinds of movement seem different, and 

 the question arises, have we the right to speak of the substance 

 which can only execute amoeboid movements as being contractile ? 



We may, if we admit that contractility is at bottom simply the 

 power of shifting the relative position of particles, and that 

 muscular contraction is a specialized form of contraction. In a 

 plain muscular fibre (which we may take as simpler than the 

 striated muscle) the shifting of particles is specialized in the sense 

 that it has always a definite relation to the long axis of the fibre ; 

 when the fibre contracts a certain number of particles assume a 

 new position by moving at right angles to the long axis of the 

 fibre, and the fibre in consequence becomes shorter and broader. 

 In a white blood corpuscle, amoeba, or other organism executing 

 amoeboid movements^ the shifting of the particles is not limited 

 to any axis of the body of the organism ; at the same moment one 

 particle or one set of particles may be moving in one direction, and 

 another particle or another set of particles in another direction. 

 A pseudopodium, short and broad, or long thin and filamentous, 

 may be thrust out from any part of the surface of the body and 

 in any direction ; and a previously existing pseudopodium may 

 be shortened, or be wholly drawn back into the substance of the 

 body. 



In the plain muscle fibre the fact that the shifting is specialized 

 in relation to the long axis of the fibre, necessitates that in a 

 contraction the shortening, due to the particles moving at right 

 angles to the long axis of the fibre, should be followed by what we 

 have called relaxation due to the particles moving back to take 

 up a position in the long axis ; and we have several times 

 insisted on relaxation being an essential part of the total act of 

 contraction. If no such movement in the direction of relaxation 

 took place, the fibre would by repeated contractions be flattened 

 out into a broad thin film at right angles to its original long 

 axis, and would thus become useless. A spherical white blood 

 corpuscle may, by repeated contractions, i.e. amoeboid movements 

 transform itself into such a broad thin film ; but in such a 

 condition it is not useless. It may remain in that condition for 

 some time, and by further- contractions, i.e. amoeboid movements, 

 may assume other shapes or revert to the spherical form. 



