270 SOME CHAPTERS ON THE 



originated must be intact, for if this is excised no regeneration 

 follows (Lugaro 36 ). 



(d) In the case of the posterior nerve roots no regeneration 

 takes place after the nerves have entered the spinal cord, and this 

 corresponds with the anatomical fact that here the nerves lose 

 their neurilemma. The inference is that the presence of this 

 structure is therefore necessary in some way for the growth of 

 new fibres. 



(e) There must be a fresh section of both ends of the nerve, 

 for if a cut end is sutured to an uninjured longitudinal surface, 

 no regeneration takes place (Kilvington 28 ). 



If these various conditions are fulfilled, the evidence that down- 

 growth takes place from the central end is very good. 



Ramon y Cajal and Marinesco ( 33 ) figure the new fibres growing 

 downwards ; these are at first seen only in the neighbourhood of 

 the suture, and gradually grow towards the periphery. The fibres 

 are often quaintly twisted, and the bulbous end shows curious 

 shapes which are probably the result of amoeboid movements. 

 The new fibre seems to seek the old path by some force of chemio- 

 tactic attraction ( 35 ). The medullary sheath is formed first at 

 the central end, later at the periphery, so that the former is the 

 oldest part of the new fibre. If a second cut is made, either 

 central to the line of suture (Langley and Anderson x ) or peri- 

 pheral to this (Halliburton), degeneration again takes place, but 

 only in the peripheral part of the new fibres. 



Finally, the embryological evidence is in favour of the central 

 origin of the fibres, though it must be remembered that this 

 evidence is only available by making use of the assumption that 

 the regeneration is carried out by the same process as began 

 the growth in the embryo. Ross Harrison ( 39> 40 ) has verified 

 on various species of Rana, the original observations of His, 

 that all the parts or the nerve fibre grow outwards from 

 the cells of the epiblast that form the neural ridge, and that 

 this is true not only for the axis cylinder, but also for the 

 medullary sheath and neurilemma, in this latter particular con- 

 troverting Bethe's statements, referred to in the next section. 

 Harrison actually observed under the microscope in parts of 

 embryos kept alive the outgrowth of new fibres from the neural 

 crest. Each fibre had a swelling at the free end like that of a 

 regenerating nerve, and this swelling showed amoeboid movements. 



