CH. LVII.} TROPHIC NERVES 815 



paralysis, due to disease of the spinal cord ; they come on in three or 

 four days after the onset of the paralysis in spite of the most careful 

 attention ; they cannot be explained by vaso-motor disturbance, nor 

 by loss of sensation ; there is, in fact, no doubt they are of trophic 

 origin ; the nutrition of the skin is so greatly impaired that the mere 

 contact of it with the bed for e, few days is sufficient to act as the 

 exciting cause of the sore. 



It will be noticed that in some instances of trophic disorder the nerves which are 

 injured are efferent ; the muscular wasting that occurs when a muscular nerve is cut 

 is the best marked example of this. In nerve itself Wallerian degeneration follows 

 the direction of growth, which, as a rule, is the direction in which the nerve transmits 

 impulses. The acute Wallerian change does not actually leap synapses, still the 

 trophic influence of one set of neurons upon a second set among which the axons of 

 the first set terminate is shown by a slow wasting process, of which chromatolysis 

 is the principal visible sign. In the peripheral axons of the cells of the spinal and 

 corresponding cranial ganglia, the trophic disorder follows a peripheral direction, 

 while impulses are carried in the opposite or afferent direction. The trophic influence 

 here travels against the stream of impulse. It cannot fail to be a striking fact that 

 the most marked trophic disorders with which we are acquainted, herpes, acute 

 bedsores, Charcot's disease, eye changes after division or injury to the fifth nerve, 

 vagus pneumonia, etc., are due to interference with sensory channels. Loss of 

 sensation is the great predisposing cause of nutritive mischief. 



