352 PHYSIOLOGICAL DEVELOPMENT. 



Contractility as well as irritability is a property of protoplasm 

 or sarcode ; and, as before suggested (§ 22), is not improbably 

 due to isomeric change in one of its component colloids. It 

 is a feasible supposition that of the several isomeric changes 

 simultaneously set up among these component colloids, some 

 may be accompanied by decided change of bulk and some not. 

 Clearly the isomeric change undergone by the colloid which 

 we suppose to form nerve, must be one not accompanied by 

 appreciable change of bulk ; since change of bulk implies 

 " internal work," as physicists term it, and therefore ex- 

 penditure of force. Conversely, the colloid out of which 

 muscle originates, may be one that readily passes into an iso- 

 meric state in which it occupies less space : the molecular 

 disturbance causing this contraction being communicated to 

 it from adjacent portions of nerve-substance that are mole- 

 cularly disturbed ; or being otherwise communicated to it by 

 direct mechanical or chemical stimuli; as happens where 

 nerves do not exist, or where their influence has been cut 

 off. This interpretation seems, indeed, to be directly at 

 variance with the fact that muscle does not diminish in bulk 

 during contraction but merely changes its shape. That which 

 we see take place with the muscle as a whole, is said also to 

 take place with each fibre — while it shortens it also broadens. 

 There is, however, a possible solution of this difficulty. A 

 contracting colloid yields up its water ; and the contracted 

 colloid plus the free water, may have the same bulk as before 

 though the colloid has less. If it be replied that in this 

 case the water should become visible between the substance 

 of the fibre and its sarcolemma or sheath, it may be rejoined 

 that this is not necessary — it may be deposited interstitially. 

 Possibly the striated structure is one that facilitates its 

 exudation and subsequent re-absorption ; and to this may be 

 due the superiority of striated muscle in rapidity of contrac- 

 tion. Granting the speculative character of this 

 interpretation, let us see how far it agrees with the facts. If 

 the actions are as here supposed, the contracted or more inte- 



