6 THE PRESIDENTIAL ADDRESS. 



for their whole length between the point of severance and the muscle 

 or skin they go to. And then at once the cut ends of the nerve-fibres 

 start re-growing from the point of severance, although for years they 

 have given no sign of growth. The fibre, so to say, tries to grow out 

 to reach to its old far-distant muscle. There are difficulties in its way. 

 A multitude of non-nervous repair cells growing in the wound spin 

 scar tissue across the new fibre's path. Between these alien cells the 

 new nerve-fibre threads a tortuous way, avoiding and never joining any 

 of them. This obstruction it may take many days to traverse. Then 

 it reaches a region where the sheath-cells of the old dead nerve-fibres 

 lie altered beyond ordinary recognition. But the growing fibre 

 recognises them. Tunnelling through endless chains of them, it 

 arrives finally, after weeks or months, at the wasted muscle-fibres which 

 seem tO' have been its goal, for it connects with them at once. It 

 pierces their covering membranes and re-forms with their substance 

 junctions of characteristic pattern resembling the original that had died 

 weeks or months before. Then its growth ceases, abruptly, as it 

 began, and the wasted muscle recovers and the lost function is restored. 

 Can we trace the causes of this beneficent yet so unaccountable 

 reaction? How is it that severance can start the nerve re-growing? 

 How does the nerve-fibre find its lost muscle microscopically miles 

 away? What is the mechanism that drives and guides it? Is it a 

 chemotaxis like that of the antherozooid in the botanical experiment 

 drawn towards the focus of the dissolved malic acid? If so, there 

 must be a marvellously arranged play of intricate sequences of chemi- 

 cally attractive and repellent substances dissolved suitably point to 

 point along the tissue. It has recently been reported that the nerve- 

 fibre growing from a nerve-cell in a nutrient field of graded electrical 

 potential grows strictly by the axis of the gradient. Some argue for 

 the existence of such potential gradients in the growing organism. 

 Certainly nerve regeneration seems a return to the original phase of 

 growth, and pieces of adult tissue removed from the body to ai'tificial 

 nutrient media in the laboratory take on vigorous growth. Professor 

 Champy describes how epithelium that in the body is not growing when 

 thus removed starts growing. If freed from all fibrous tissue its cells 

 not only germinate, but, as they do so, lose their adult specialisation. 

 In nerve regeneration the nerve-sheath cells, and to some extent the 

 muscle-cells which have lost their nerve-fibre, lose likewise their 

 specialised form, and regain it only after touch with the nerve-cell has 

 been re-established. So similarly epithelium and its connective tissue 

 cultivated outside the body together both grow and both retain their 

 specialisation. All seems to argue that the mutual touch between the 

 several cells of the body is decisive of much in their individual shaping 

 and destiny The severance of a nerve-fibre is an instance of the disloca- 



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