THE REPAIR OF A NERVE 415 



with another piece of similar wire ; the two segments of wire 

 which are united are not injured throughout their length, 

 and conduct the electrical impulse as well as ever. In the 

 case of a nerve it is different ; the stretch of nerve still united 

 to the central nervous system is spoken of as the central end ; 

 this consists of long threads which are the outgrowths of 

 protoplasmic masses, termed nerve cells, situated in the grey- 

 matter of the central nervous system. The central end of a 

 divided nerve remains healthy, or approximately so. The other 

 segment, which is called the peripheral! end, consists of nerve 

 fibres also, but after division they soon lose their power of 

 responding to stimuli and of conducting nerve impulses. They 

 simultaneously undergo a degenerative change, which can be 

 easily recognised by the microscope, and ultimately atrophy. 

 These fibres having been cut off from the nerve cells — of which 

 they are outgrowths— are no longer capable of receiving that 

 nutritive control which it is the special object of these proto- 

 plasmic masses to accomplish. This was first pointed out by 

 the elder Waller, and the term " Wallerian degeneration," by 

 which it is known, is a permanent record of this historical fact. 



Even the most rapid suturing of the divided ends together is 

 ineffective to prevent Wallerian degeneration from occurring in 

 the distal segment. 



This shows the difficulties attendant on the process of nerve 

 regeneration ; for the new fibres have not only to bridge a gap, 

 but they are also necessary to replace the degenerated fibres 

 in the peripheral segment right down to its ultimate and final 

 ramifications in skin, muscles, or other organs. 



In the elucidation of the problem as to how this is 

 accomplished, experiments upon animals have been rendered 

 necessary, so as to extend and complete the imperfect 

 information, which is all that can be obtained by clinical 

 observation in man. 



But in order to pave the way to this, it is first necessary 

 to allude to the way in which nerve fibres find their way 

 to the periphery from the central nervous system in the 

 embryo. The central nervous system (brain and spinal cord) 

 is originally formed by an infolding from the surface of the 

 most external embryonic layer known as the epiblast, the 

 remainder of the epiblast appearing in the fully formed animal 

 as the epidermis. This infolding is cut off from the surface 



