668 Dynamic Theory. 



neither set would ever be or become operative without the other. A few 

 muscular movements are performed at birth, and even before, but the 

 greater part, even of reflex movements, have to be acquired b;y experi- 

 ence ; that is, under stimulation from external sources of energy, by 

 wny of the organs of sense. 



If you touch the naked foot of a sleeping child gently with a feather, 

 the limb will probably be withdrawn, perhaps without waking the child. 

 The stimulus has formed a nerve current, which has gone up to the gan- 

 glions in the spinal cord, and has been deflected thence back to the mus- 

 cles of the same limb, and their contraction withdraws it. But although 

 this action is performed in unconsciousness and without involving the 

 cerebral cells at all, it must nevertheless be learned. For if the exper- 

 iment be performed upon a very young infant the foot will not be with- 

 drawn, and if the irritation be persisted in, the result will be simply 

 pain or uneasiness, indicated by its crying. The crying is reflex, in- 

 deed, but it arises from a reflection from the medulla oblongata instead 

 of the spinal cord. This reflex action of crying, like that of sucking, 

 does not have to be learned. These were so thoroughly practiced by the 

 infinite line of our ancestry that through habit and heredity the parts 

 involved in these reactions are perfect at birth, and begin to operate as 

 soon as the proper external stimuli are applied. But the muscle move- 

 ments of the limbs, even those which are reflex, and much more those 

 which are purposive, must be acquired, although the hereditary machin- 

 ery is so nearly complete at birth that it is soon made operative by a lit- 

 tie practice ; or, as we might say, a little more practice, since whatever 

 approach to perfection it has, results from ancient practice. It is only 

 after repeated applications of the stimulus to the skin of the new baby, 

 that the current is able to make its way entirely around the circuit. The 

 conducting line, consisting of the afferent nerve, the cells of the gan- 

 glion, and the efferent nerve, offers resistance probably in all parts, and 

 certainly in the cells. But this conductor is composed of plastic mate- 

 rials which yield more or less at every assault of the polar current, and 

 finally become its facile pathway. It is obvious that every attack of 

 the- stimulating current upon the cells, differentiates them more and 

 more, and is able to penetrate further, and affect a greater number of 

 them. Further habit makes them more easily moved, so that a con- 

 stantly decreasing proportion of the stimulation is required to move the 

 cells, and a correspondingly increasing amount of it is transmitted down 

 the efferent nerve, increasing the force of muscle excitement and con- 

 traction. Thus, the difference between an inexperienced and a differen- 

 tiated cell is, that the latter is readily moved by the nervous stimula- 

 tion, while the former is not. This difference constitutes the physiol- 

 ogy of memory. While the cell remains readily movable, it has memory. 



