92 VARIATIONS OF IRRITABILITY. [BOOK i. 



swollen axis-cylinder. Meanwhile the nuclei which occur, one in 

 each segment of the nerve between each two nodes of Ranvier, 

 divide and multiply rapidly. Lastly the axis-cylinder breaks up 

 and disappears so that nothing remains of the original fibre 

 but the sheath of Schwann enclosing a proteid mass with many 

 nuclei. If no regeneration takes place these nuclei 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. When the cut ends of the nerve are close together the 

 axis-cylinders growing out from the central portion run into and 

 between the sheaths of Schwann of the peripheral portion ; but 

 much uncertainty still exists as to the exact parts which the 

 proliferated nuclei referred to above, the proteid remnants of the 

 medulla, and the old axis-cylinders of the peripheral portion 

 respectively play in giving rise to the new structures of the 

 regenerated fibre. 



This 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 on p. 86) 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, 

 from injury to the nerves or nervous centre, have ceased 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. But as far as our experience goes at present 



