806 THE NERVOUS SYSTEM. 



acting. Thus a toxin affecting directly the cell body may cause the 

 earliest and most pronounced changes far out in its dendritic or axomil 

 processes, i.e., in those parts of the neurone farthest removed from the 

 trophic centre. 



To the action of a specific poison certain neurones seem less resistant 

 than others. Thus, to the action of lead those neurones governing the 

 extensor muscles of the wrist seem especially susceptible. The cells of 

 the motor nucleus of the trigeminus seem to be less resistant than other 

 motor cells to the poison of tetanus. Again, the frequency with which 

 tabes dorsalis is associated with a syphilitic history seems to indicate a 

 special susceptibility to the syphilitic poison ou the part of the peripheral 

 sensory neurone. 



Comparing the changes in the nerve cell in toxemias (Plate XIII., 3 and 4) with 

 those aiduced by lesions to its axone axonal degeneration (Plate XIII., 6) \ve note 

 that while in the latter the chromatolysis is central in character, beginning in the region 

 of the axone hill and nucV^us, in the former the changes begin at the periphery, 1h 

 portion of the cell in most direct relation to the surrounding lymph. In axonal de- 

 generation the nucleus is usually eccentric. In toxaemia it usually remains central, at 

 least until the process of degeneration is far advanced. While this distinction holds 

 good in general and while the phenomena of central chromatolysis and eccentricity of 

 the nucleus regularly follow injury to the axone, we are not yet warranted in stating 

 that such changes never occur except after some injury, or that the presence of such 

 changes in the cell can be explained only on the basis of injury to its axis-cylinder 

 process. 



IV. Effects of Fatigue upon Neurones. Studies upon the effect of fa- 

 tigue upon the neurone have been made in animals after prolonged inns 

 cular activity and after electrical stimulation. While the results are not 

 in complete accord, there seems to be little doubt that definite morpho- 

 logical changes occur in the neurone as a result of fatigue. These changes 

 consist in a decrease in the size of the cell body, a decrease in the size of 

 the nucleus, often with distortion, and a marked decrease in the amount 

 of chromatic substance with more or less diffuse staining of both cell 

 body and nucleus. 



It seems not at all improbable that the clinical pictures presented by certain psy- 

 choses and neuroses are the expression of the effects upon the neurones of prolonged 

 fatigue. Certain local expressions of neurone exhaustion, as, for example, writer's 

 cramp, may possibly also be placed in the same category. 



As to the significance of those changes in the nerve cell, which are marked by dim- 

 inution in the chromatic substance alone, and to which the term " chromatolysis " has 

 been given, there is a considerable difference of opinion. Marinesco 1 considers them of 

 the nature of degeneration. Van Gehuchteu 2 is inclined to look upon the phenomenon 

 as conservative in character, a means by which the neurone assumes a condition most 

 advantageous for self-defence. It has been shown that even an extreme degree of 

 chromatolysis is not incompatible with function. Marinesco insists that so long as the 

 changes are confined to the chromatic substance recovery is possible, no matter how 

 extensive the alterations, while changes in the achromatic elements are always per- 

 manent in character. It seems quite probable that a comparatively easy method of 

 investigation has led to an overestimation of the significance of changes in the chro- 

 matic substance of the neurone, and to a neglect of probably more important but less 

 easily studied changes in the achromatic element. 



1 Marinesco, G., See Van Gehuchten's "Anatomic du Systeme Nerveux," p. 339. 



2 Van Gelmditen, loc. cit., p. 339. 



