74 CHANGES DURING A NERVOUS IMPULSE. [BOOK i. 



stimulated artificially at any part of its course, the nervous 

 impulse set going travels in both directions. 



We used just now the phrase ' tetanization of a nerve/ meaning 

 the application to a nerve of rapidly repeated shocks such as would 

 produce tetanus in the muscle to which the nerve was attached, 

 and we shall have frequent occasion to employ the phrase. It will 

 however of course be understood that there is in the nerve as far 

 as we know no summation of nervous impulses comparable to the 

 summation of muscular contractions. The matter perhaps needs 

 fuller investigation, but as far as we know at present, we may 

 say that the series of shocks sent in at the far end of the nerve 

 start a series of impulses; these travel down the nerve and 

 reach the muscle as a series of distinct impulses; and the 

 first changes in the muscle, the molecular latent-period changes, 

 also form a series the members of which are distinct. It is 

 not until these molecular changes become transformed into visible 

 changes of form that any fusion or summation takes place. 



Putting together the facts contained in this and the preceding 

 sections, the following may be taken as a brief approximate history 

 of what takes place in a muscle and nerve when the latter is 

 subjected to a single induction-shock. At the instant that the 

 induced current passes into the nerve, changes occur, of whose 

 nature we know nothing certain except that they cause a ' current 

 |of action' or 'negative variation of the natural' nerve-current. 

 These changes propagate themselves along the nerve in both 

 directions as a nervous impulse in the form of a wave, having 

 a wave-length of about 18 mm., and a velocity (in frog's nerve) of 

 about 28 m. per sec. Passing down the nerve-fibres to the muscle, 

 flowing along the branching and narrowing tracts, the wave at last 

 breaks on the end-plates of the fibres of the muscle. Here it is 

 transmuted into a muscle-impulse, with a shorter steeper wave, 

 and a greatly diminished velocity (about 3m. per sec.). This 

 muscle-impulse, of which we know hardly more than that it 

 : is marked by a current of action, travels from each end-plate 

 in both directions to the end of the fibre, where it appears to 

 be lost, at all events we do not know what becomes of it. As 

 it leaves the end plate it is followed by an explosive decomposition 

 of material, leading to a discharge of carbonic acid, to the appearance 

 of some substance or substances with an acid reaction, and probably 

 of other unknown things, with a considerable development of heat. 

 This explosive decomposition gives rise to the visible contraction- 

 wave, which travels behind the invisible muscle-impulse at about 

 the same rate, but with a vastly increased wave-length. The fibre, 

 as the wave passes over it, swells and shortens, bringing its two ends 

 nearer together, its molecules during the change of form arranging 

 themselves in such a way that the extensibility of the fibre is 

 increased. 



