476 THE NERVOUS SYSTEM. 



by some force generated in the nervous centres. But 

 almost all things that can disturb the nerves from their 

 passive state act as stimuli, and agents the most dissimilar 

 produce the same kind, though not the same degree of 

 effect, because that on which they act possesses but one 

 kind of excitable force. Thus all stimuli chemical, 

 mechanical, and electric, when applied to parts endowed 

 with sensation, or to sensitive nerves (the connection of the 

 latter with the brain and spinal cord being uninjured) pro- 

 duce sensations ; and when applied to the nerves of muscles 

 excite contractions. Muscular contraction is produced by 

 such stimuli as well when the motor nerve is still in con- 

 nection with the brain, as when its communication with the 

 nervous centres is cut off by dividing it : nerves, therefore, 

 have, by virtue of their excitability, the property of exciting 

 contractions in muscles to which they are distributed ; and 

 the part of the divided motor nerve which is connected 

 with the muscle will still retain this power, however much 

 we may curtail it. 



Mechanical irritation, when so violent as to injure the 

 texture of the primitive nerve-fibres, deprives the centri- 

 petal 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 con- 

 tractions 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. 



