378 THE NERVOUS SYSTEM. 



ture of the primitive nerve-fibres, deprives the centripetal 

 nerves of their power of producing sensations when irritation 

 is again applied at a point more distant from the brain than 

 the injured spot; and in the same way, no irritation of a 

 motor nerve will excite contraction of the muscle to which it 

 is distributed, if the nerve has been compressed and bruised 

 between the point of irritation and the muscle ; the effect of 

 such an injury being the same as that of division. 



The action of nerves is also excited by temperature. Thus, 

 when heat is applied to the nerve going to a muscle, or to the 

 muscle itself, contractions are produced. These contractions are 

 very violent when the flame of a candle is applied to the nerve, 

 while less elevated degrees of heat, for example, that of a piece 

 of iron merely warmed, do not irritate sufficiently to excite 

 action of the muscles. The application of cold has the same 

 effect as that of heat. The effect of the local action of excessive 

 or long-continued cold or heat on the nerves is the same as that 

 of destructive mechanical irritation. The sensitive and motor 

 power in the part is destroyed, but the other parts of the 

 nerve retain their excitability ; and, after the extremity of a 

 divided nerve going to a muscle has been burnt, contractions 

 of the muscle may be excited by irritating the nerve below 

 the burnt part. 



Chemical Stimuli excite the action of both afferent and ef- 

 ferent nerves as mechanical irritants do; provided their effect 

 is not so strong as to destroy the structure of the nerve to which 

 they are applied. A like manifestation of nervous power is 

 produced by electricity and by magnetism. 



Some of these laws regulating the excitability of nerves, and 

 their power of manifesting their functions, require further 

 notice, with several others which have not yet been alluded 

 to. Certain of the laws and conditions of actions relate to 

 nerves both centrifugal and centripetal, being dependent on 

 properties common to all nerve-fibres; while of others, some 

 are peculiar to nerves of motion, some to nerves of sensation. 



It is a law of action in all nerve-fibres, and corresponds 

 with the continuity and simplicity of their course, that an im- 

 pression made on any fibre, is simply and uninterruptedly 

 transmitted along it, without being imparted or diffused to any 

 of the fibres lying near it. In other words, all nerve-fibres are 

 mere conductors of impressions. Their adaptation to this pur- 

 pose is, perhaps, due to the contents of each fibre being com- 

 pletely isolated from those of adjacent fibres by the membrane 

 or sheath in which each is inclosed, and which acts, it may be 

 supposed, just as silk, or other non-conductors of electricity do, 



