CHAP. IX.] THE NEEVOUS FORCE. 235 



stimulus applied to parts which derive their nerves from it below 

 the separation. And the reason is obvious ; the solution of conti- 

 nuity of the nerve interrupts the propagation of the change which 

 the mental or physical stimulus excites in it. In the case of the 

 voluntary nerve, the mental stimulus is propagated no further 

 peripherad than the point of section ; and in that of the sensitive 

 nerve, the change travels no further centrad than the same point. 

 That the interruption is caused solely by the solution of continuity, 

 and not by any alteration in the properties of the nerve, is proved by 

 the fact that the lower segment of the motor nerve will still continue 

 to respond to a physical stimulus. Mechanical or chemical irri- 

 tion, or the passage of an electric current across it, will cause its 

 muscles to contract. Such a degree of injury to a nerve as will 

 destroy the continuity of the nervous matter within the tubular 

 fibres is likewise sufficient to destroy its power as a propagator 

 of nervous change. This effect will be produced by tying a liga- 

 ture very tightly round a nerve, or by pressing it very forcibly 

 between the blades of a forceps, The paralysis which results 

 from the compression of a nerve by a tumor, or in any other 

 way, is no doubt due to a similar solution of continuity in the 

 nervous matter. 



From these facts we draw the important inference that, in propa- 

 gating the influence of a stimulus, either from periphery to centre, 

 or vice versa, nerves are not mere passive conductors. The whole 

 extent of the fibre between the point stimulated and its peripheral 

 or central connexion is the seat of change. How necessary, then, 

 to the normal action of nerves must it be to preserve their physical 

 condition in a healthy state ! A morbid fluid impregnating a 

 nerve at any point may irritate it, or may suspend or destroy its 

 inherent property by modifying its nutrition. It is thus, likewise, 

 that nerves may be paralysed by soaking them in a solution of 

 opium, or of belladonna, aconite, tobacco, or other powerfully seda- 

 tive or narcotic substances, or that they may be unduly excited by 

 applying a solution of strychnia. The contact of a solid body with 

 a nerve may irritate and keep up a continual state of excitement, if 

 it do not destroy its properties. A spicula of bone in contact with 

 nervous fibres is often the cause of the severest forms of neuralgia. 

 That alteration of nutrition which we call inflammation may pro- 

 duce like effects. Various physical agents are followed by similar 

 consequences. The benumbing influence of cold is explained in this 

 way. Exposure to a continuous draught of cold air is a frequent 

 cause of facial paralysis. How instantaneously will the giving 



