FUNCTIONS OF NERYE- FIBRES. 479 



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 sensitive and 

 motor 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 of sensation and motion, being de- 

 pendent 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 

 impression 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 purpose is, perhaps, due to the contents of each 

 fibre being completely isolated from those of adjacent 

 fibres by the membrane or sheath in which each is enclosed, 

 and which acts, it may be supposed, just as silk, or other 

 non-conductors of electricity do, which, when covering a 

 wire, prevent the electric condition of the wire from being 

 conducted into the surrounding medium. 



