ELECTROTONUS. 465 



which unites with the proximal portion of the old fiber still in 

 connection with its nutritive center, and hence has undergone no 

 degenerative change. Such a regenerated fiber has both conduc- 

 tivity and irritability, the former appearing as early as the third 

 week, but the latter not manifesting itself until afterward. At 

 this period of the regeneration there is neither myelin nor axis- 

 cylinder, and the fiber is responsive to mechanical stimuli, but 

 not to induction shocks, which latter property returns only after 

 the axis-cylinder is developed. The medullary substance later 

 appears and forms a tube ; and still later the axis-cylinder is 

 formed, having its origin in the central end of the nerve i. e. y 

 the portion which is still in communication with the cell from 

 which the original axis-cylinder was developed. The complete 

 regeneration of a nerve may take months. 



So far as known, regeneration does not occur in the central 

 nervous system. 



NERVE-IMPULSES. 



The function of nerve-fibers is to conduct impulses, and this 

 from centers to the periphery, from the periphery to centers, 

 or from one center to another. Although much study has been 

 given to the subject, exactly what a nerve-impulse is has never 

 been determined. Except an electrical change in the nerve itself, 

 and the results of the reception of the impulses at the termination 

 of the nerve, as motion in a motor nerve, secretion in a secretory 

 nerve, etc., there is no evidence of the fact that impulses have 

 travelled over the nerve. Chemical, mechanical, or thermic 

 changes, if they occur, have never been demonstrated. 



The stimuli which excite muscle will also stimulate nerves ; 

 these are electrical, mechanical, chemical, and thermic ; and what 

 has been said of these applies in general to nerves. It should be 

 called to mind, however, that induction-shocks stimulate nerves 

 more powerfully than a voltaic current j while in the case of 

 muscle it is the voltaic current which is the more powerful 

 stimulus. 



Velocity of Nerve-impulses (Fig. 267). In the motor 

 nerves of human beings nervous impulses travel at the rate of 

 33 meters a second, and in sensory nerves, from 30 to 33 meters 

 in the same length of time. 



Electrotonus. Although contraction of a muscle takes place 

 at the make and break of a constant current which is passed 

 through the nerve distributed to it, and although no apparent 

 change takes place in the nerve at either of these moments, or 

 indeed while the current is passing;, still during the latter period 

 important changes are actually taking place, though they are not 

 visible ; these are changes in the electrical condition of the nerve, 

 in its excitability and also in its conductivity, and are collectively 



30 



