CHAP, ii.] THE CONTRACTILE TISSUES. 147 



appeared. If no regeneration takes place these nuclei with their 

 bed eventually disappear. 



In the central portion of the divided nerve similar changes may 

 be traced as far only as the next node of Ranvier. Beyond this 

 the nerve usually remains in a normal condition. 



Regeneration, when it occurs, is apparently carried out by 

 the peripheral growth of the axis-cylinders of the intact central 

 portion. It would seem that when the cut ends of the nerve are 

 close together the axis-cylinders growing out from the central 

 portion run into and between the shrunken neurilemmas of the 

 peripheral portion, and new medulla, at first delicate and inter- 

 rupted, but subsequently becoming continuous and complete, 

 makes its appearance in the protoplasmic strands in a centrifugal 

 order. But, the complete history has not as yet been clearly made 

 out, and much uncertainty still exists as to the exact parts which 

 the proliferated nuclei and the protoplasmic material referred to 

 above respectively play in giving rise to the new structures of the 

 regenerated fibre. 



Such a degeneration may be observed to extend down to the very 

 endings of the nerve in the muscle, including the end-plates, but 

 does not at first affect the muscular substance itself. The muscle, 

 though it has lost all its nervous elements, still remains irritable 

 towards stimuli applied directly to itself: an additional proof of 

 the existence of an independent muscular irritability. 



For some time the irritability of the muscle, as well towards stimuli 

 applied directly to itself as towards those applied through the impaired 

 nerve, seems to be diminished ; but after a while a peculiar condition 

 (to which we have already alluded, 78) sets in, in which the muscle 

 is found to be not easily stimulated by single induction-shocks but to 

 respond readily to the make or break of a constant current. In fact it 

 is said to become even more sensitive to the latter mode of stimulation 

 than it was when its nerve was intact and functionally active. At the 

 same time it also becomes more irritable towards direct mechanical 

 stimuli, and very frequently fibrillar contractions, more or less rhythmic 

 and apparently of spontaneous origin, though their causation is ob- 

 scure, make their appearance. This phase of heightened sensitiveness 

 of a muscle, especially to the constant current, appears to reach its 

 maximum, in man at about the seventh week after nervous impulses 

 have ceased, owing to injury to the nerves or nervous centre, to reach 

 the muscle. 



If the muscle thus deprived of its nervous elements be left to 

 itself its irritability, however tested, sooner or later diminishes; but 

 if the muscle be periodically thrown into contractions by artificial 

 stimulation with the constant current, the decline of irritability 

 and attendant loss, of nutritive power may be postponed for some 

 considerable time. Bat as far as our experience goes at present 

 the artificial stimulation cannot fully replace the natural one, and 

 sooner or later the muscle like the nerve suffers degeneration, loses 



102 



