CH. XVIII.] TROPHIC NERVES 207 



disease of the cervical sympathetic, the disordered vaso-motor 

 phenomena which ensue do not lead to the disorders of nutrition we 

 have described. Nevertheless in trophic disorders, it is very difficult 

 to be sure that the disordered metabolism is not in part due to 

 vascular disturbances. 



There can, therefore, be but little doubt that we have to deal 

 with the trophic influence of nerves ; * but the dust, etc., which falls 

 on the cornea must be regarded as the exciting cause of the ulceration. 

 The division or disease of the nerve acts as the predisposing cause. 

 The eyeball is more than usually prone to undergo inflammatory 

 changes, with very small provocation. 



The same explanation holds in the case of the influence of the 

 vagi on the lungs. If both these nerves are divided, the animal 

 usually dies within a week or a fortnight from a form of pneumonia 

 called vagus pneumonia, in which gangrene of the lung substance is 

 a marked characteristic. Here the predisposing cause is the division 

 of the pneumogastric nerves ; the exciting cause is the entrance of 

 particles of food into the air passages, which on account of the loss 

 of sensation in the larynx and neighbouring parts are not coughed 

 up. Another trophic disturbance that follows division of the vagi is 

 fatty degeneration of the heart. 



Many bedsores are due to prolonged confinement in bed with 

 bad nursing ; these are of slow onset. But there is one class of bed- 

 sores which are acute; these are especially met with in cases of 

 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 ; the nutrition of the skin is so greatly impaired 

 that the mere contact of it with the bed for a 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 an early 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. 



* The proof, however, that there are distinct nerve-fibres anatomically is not 

 very conclusive. 



