366 THE NERVOUS SYSTEM. 



inward ; and when it commences at the inner extremity, as in a motor 

 nerve, it must move from within outward. But under other conditions 

 it may be capable of moving in either direction. The following experi- 

 ment shows that this is possible, so far as regards the sensitive nerves. 



2. Sensitive impressions may pass, in the fibres of a sensitive nerve, 

 either from without inward or from within outward. 



This of course never takes place in the normal condition ; but its pos- 

 sibility has been demonstrated, in the experiments of Paul Bert,* by 

 dividing a sensitive nerve and then reversing its position, so that its 

 peripheral extremity is in connection with the nerve centres. The 

 end of the tail, in a young rat, was deprived of integument for a 

 length of five centimetres, and the denuded portion inserted beneath 

 the skin of the back of the same animal. At the end of eight days, 

 when the ingrafted portion had become adherent to the subcutaneous 

 tissues, and had contracted sufficient vascular connection for its support, 

 the tail was amputated at its base, and thenceforward remained attached 

 to the body of the animal only by what was previously its peripheral 

 extremity. In three months sensibility again began to be manifested 

 in the end of the tail, thus reversed ; and in six months it wa^ 

 reestablished to an unmistakable degree. The nerves of the tail, 

 which before the operation transmitted sensitive impressions from its 

 point toward its base, afterward transmitted the same impressions from 

 its base toward its point. In this instance the nerve fibres which thus 

 acted in a reverse direction were fibres of new formation, like those 

 which generally replace the degenerated fibres of divided nerves (page 

 355) ; but there is no evidence that such regenerated fibres are in any 

 way different from those originally existing in the same parts. 



Although the nerve fibres therefore may excite two different forms 

 of action, their own condition may be the same for both. If they com- 

 municate their stimulus to a perceptive nervous centre the effect is a 

 sensation ; if to a muscle, it is contraction. These acts cannot be inter- 

 changed with each other, because the muscle is not sensitive and the 

 nervous centre is not contractile ; but they are both indirect effects of 

 the nervous influence, and do not necessarily indicate any difference in 

 its nature. 



Rapidity of Transmission of the Nerve Force. 



It is a matter of conscious experience that the operations of the 

 nervous system require a certain time for their accomplishment. The 

 action both of the senses and of the will, though exceedingly rapid, 

 is not instantaneous. Between the mental decision to perform a 

 movement and its actual execution, there is a short but real interval of 

 time, during which the nervous mechanism is called into play. A cer- 

 tain period also intervenes between the contact of a foreign body with 

 the skin, and our perception of its existence and qualities. There is 

 even more or less difference between individuals in the time required 



* La Vitality propre des Tissues aniraaux. Paris, 1866, p. 12. 



