THE INNER TISSUES OF ANIMALS. 361 



discharged along an efferent nerve or nerves, when excite- 

 ment of an afferent nerve has disengaged it. How such a 

 structure as this results, the hypothesis does not show. But 

 admitting these shortcomings it may still be held that we 

 are, in the way pointed out, enabled to form some idea of the 

 actions by which nervous tissue is differentiated. 



303. A speculation akin to, and continuous with, the last, 

 is suggested by an inquiry into the origin of muscular tissue. 

 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 or more of its component col- 

 loids. It is a feasible supposition that of the several isomeric 

 changes simultaneously set up among these component col- 

 loids, some may be accompanied by 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 

 isomeric 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 



