NERVE INJURIES 193 



view that the new nen^'e fibres formed after suture 

 are budded out from the cut central end. It will 

 be found that new medullated fibres are present only 

 in the proximal part of the regenerating nerve at 

 first, whereas at a later date they reach the periphery. 

 Only a few millimetres may have regenerated in a 

 month. It has recently been shown, by Perroncito, 

 that the fine fibrils which constitute the axis cyhnders 

 of the central end commence to grow, curl, bud, and 

 branch within a few hours of the injury, apparently 

 " feeUng for " the old track. 



Mott and HaUiburton have shown that if a nerve 

 is cut and sutured, and time allowed for regeneration, 

 after a second section at the same place the new 

 medullated fibres peripheral to the injury all degener- 

 ate. Had they been developed in situ by the activity 

 of the sheath cells, one would not expect degeneration 

 after the second section, because they would not in 

 that case have been cut off from their centre of origin. 

 The deduction is that the new fibres were derived 

 from the central end. 



Convincing proof has been advanced by embry- 

 ologists that the nerves in the embryo are not formed 

 in situ, but are budded out from the nervous elements 

 of the brain and spinal cord. By removing the 

 medullary groove in frog embryos and planting it 

 in lymph clot, Ross Harrison has actually observed 

 the developing nerve cell grow out its axon at the rate 

 of 20 fi in twenty-five minutes. The outgrowing axon 

 is actively amoeboid. He was able also, by destroying 

 the ventral part of the developing spinal cord, to 

 obtain tadpoles in which the muscles had no motor 



13 



