AND ITS MODE OF ACTION. 425 



Bert, 1 by dividing a sensitive nerve and then reversing its position, 

 so that its peripheral extremity is brought into connection with the 

 nerve centres. The end of the tail, in a young rat, was deprived of its 

 integument for a length of five centimetres, and the denuded portion 

 inserted beneath the integument 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 con- 

 nection for its support, the tail was amputated at its base, and thence- 

 forward remained attached to the body of the animal only by what was 

 previously its peripheral extremity. In three months the signs of sensi- 

 bility again began to be manifested when the end of the tail, thus re- 

 versed, was subjected to compression ; and at the end of six months its 

 sensibility was re-established 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 impres- 

 sions from its base toward its point. 



There is no evidence, therefore, that nerve fibres are endowed with two 

 different modes of action, one for sensation, the other for motion. In 

 each case the condition of the nerve itself may be of the same nature. 

 But, being thrown into a state of excitement throughout its entire length, 

 it communicates a stimulus to the organ with which it is connected. If 

 this organ be a perceptive nervous centre, the effect produced is a sen- 

 sation ; if a muscle, it results in contraction and movement. These acts 

 cannot be interchanged with each other, because the muscle cannot feel 

 and the nervous centre is incapable of contraction ; but they are both 

 indirect effects of the nervous influence, and do not necessarily depend 

 upon any difference in its nature. 



Rapidity of Transmission of the Nerve Force, 



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

 ous system require a certain time for their accomplishment. The action 

 both of the senses and of the will is exceedingly rapid, but still is not 

 absolutely instantaneous. Between the mental decision to perform a 

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

 interval of time, during which the nervous mechanism is called into 

 activity. A certain period also intervenes between the contact of a 

 foreign body with the skin, and our complete perception of its existence 

 and qualities. There is even more or less difference between individuals 

 in the length of time required for the performance of nervous action ; 

 the quickness of the senses and the promptitude of the will frequently 

 varying to a perceptible degree. In the case of a voluntary movement, 

 the period consumed in its entire accomplishment may be occupied 

 by three different processes, namety : 1. The act of volition, taking 

 place in the brain ; 2. The transmission of the motor impulse, through 

 the spinal cord and nerves, to their peripheral terminations ; and 3. The 



1 La Vitalit6 propre des Tissues animaux. Paris, 1866, p. 12. 



28 



