THE ANIMAL MACHINE 



of the nerve impulse in the transaction is easily de- 

 monstrable; for if the nerve cord is severed, as often 

 happens in accidents, the muscle supplied by that 

 nerve immediately loses its power of voluntary con- 

 traction. It becomes paralyzed, as the saying is. 



THE NATURE OF MUSCULAR ACTION 



Paying heed, now, to the muscle itself, it must be 

 freely admitted that, in the last analysis, the activities 

 of the substance are as mysterious and as inexplicable 

 as are those involved in the nervous mechanism. It is 

 easy to demonstrate that what we have just spoken of 

 as a muscle fibre consists in reality of a little tube of 

 liquid protoplasm, and that the change in shape of 

 this protoplasm constitutes the contraction of which 

 we are all along speaking. But just what molecular 

 and atomic changes are involved in this change of form 

 of the protoplasm, we cannot say. We know that the 

 power to contract is the one universal attribute of living 

 protoplasm. This power is equally wonderful and 

 equally inexplicable, whether manifested in the case 

 of the muscle cell or in the case of such a formless 

 single-celled creature as the amoeba. When we know 

 more of molecular and atomic force, we may perhaps 

 be able to form a mental picture of what goes on in 

 the structure of protoplasm when it thus changes the 

 shape of its mass. Until then, we must be content to 

 accept the fact as being the vital one upon which all 

 the movements of animate creatures depend. 



But if, here as elsewhere, the ultimate activities of 



VOL. vn. 4 [49] 



