CENTRAL NERVOUS SYSTEM. 635 



It is sometimes stated that degeneration takes place in the direction of the 

 nerve-impulse. In a general way this is true, since the impulses usually 

 travel from the cell-body along the neuron. In the case of the fibres arising 

 from the cells of the spinal ganglion it is not true, since the section at the 

 distal side of the ganglion causes degeneration away from the spinal cord, 

 while that on the proximal side of the ganglion causes degeneration toward 

 the spinal cord ; yet in both neurons the impulse is in the same direction 

 namely, always toward the cord. 



The distal portions of the nerve may be regenerated, or, under other con- 

 ditions, the remainder of the neuron together with the cell-body from which 

 it springs may atrophy, and this latter process may result in even the complete 

 destruction of the nucleated portion. 



Degeneration of Nucleated Portion. In any case the internodal seg- 

 ment of the peripheral nerve-fibre which has been directly injured by the 

 section degenerates centrally as far as the next node of Ranvier. Whether 

 beyond this point any marked change is to occur depends on several circum- 

 stances.' When regeneration is prevented, the younger the animals on which 

 the operation has been made the more marked are the involutionary changes. 

 These consist, first, in a stoppage of growth-processes in the elements affected ; 

 second, in a simple atrophy. Such, for example, are the changes taking place 

 in the cells of the spinal cord after the amputation of a limb. Sometimes also 

 true degeneration follows. That these effects may be very plain in man, the 

 amputation should be one near the trunk i. e. involving a great number of 

 nerve-fibres, and be of long standing i. e. more than one year. 1 



It was discovered by von Gudden 2 that when nerves in young animals 

 are pulled away from their attachment with the central system, they most fre- 

 quently break just at the point where they emerge from the cord or brain axis. 

 When an efferent nerve is thus broken, in animals just born or very young, 

 the remaining portion i. e. the cell-bodies with so much of their neurons as 

 lie within the central system atrophies to complete disappearance. The cause 

 of this complete disappearance in the case of very young animals thus injured, 

 seems to lie in the intense struggle for nutriment among the nerve-elements 

 themselves. Thus young cells meeting with injury are unable to compete 

 with those about them for nourishment, and so perish. The bearing of such 

 a fact is very direct. If in man there is reason to think that an injury was 

 suffered during fetal life, there is a possibility that the injury may not only 

 have prevented the further development of the cells involved, but may also 

 have caused the complete destruction of some of them, in which case, of 

 course, the architecture of the region is necessarily abnormal. 



Such complete disappearance as the result of early injury has not been 

 shown for cells which lie entirely within the central system, or for those form- 

 ing the spinal ganglia. In the case of those central cells which form the 

 sensory nuclei, like the sensory nucleus of the fifth nerve, or of the vagus, 



1 Grigoriew : Zeitschrift fur Heillcunde, 1894, Bd. xv. 

 , 2 Archiv fur Psychiatric, 1870, Bd. ii. 



