CHAPTEE II. 



NERVOUS IRRITABILITY AND ITS MODE OF 

 ACTION. 



THE property possessed by nerves of being called into excitement by 

 an appropriate stimulus is termed their " irritability." This property 

 is not confined to the elements of the nervous system, but exists also in 

 other tissues and organs. Each organ or anatomical element, when 

 subjected to the application of a stimulus adapted to its physiological 

 character, reacts in a way peculiar to itself and produces a visible result 

 of a definite kind. Thus a glandular organ, when excited, exhibits the 

 phenomena of secretion ; a muscle or a muscular fibre, those of contrac- 

 tion. The visible result of glandular activity is the accumulation and 

 discharge of the secreted fluids, that of muscular contraction is a change 

 of form in the muscle, and a movement of the parts to which it is at- 

 tached. The irritability of a nerve or a nerve fibre, on the other hand, is 

 not manifested by any perceptible change in its own substance, but by 

 the phenomena of sensation or motion in the organs to which it is 

 distributed. 



Irritability of Sensitive Fibres. 



The irritability of the sensitive nerve fibres is most directly mani- 

 fested during life by the production of sensation. This sensation, 

 however, does not exist in the nerve itself, but in the nervous centre 

 where its fibres terminate. The proof of this is that if the communica- 

 tion between any part of a sensitive nerve and the brain be cut off by 

 division of the nerve fibres, no stimulus subsequently applied to the 

 separated trunk or branches of the nerve will produce any perceptible 

 sensation. If, however, the connection between the nerve and the 

 nervous centre be retained, while that with the external integument be 

 cut off, stimulants of various kinds applied to the nerve itself will pro- 

 duce a sensation which is more or less acute according to the stimulus 

 employed. Pinching or pricking the nerve, variations of temperature, 

 or the passage of an electric current, will all have the effect of bringing 

 into action the nervous irritability, and thus producing the effect of a 

 sensation. 



In order to accomplish this result, however, two conditions are essen- 

 tial. First, the nerve must be, as above mentioned, in communication 

 with the nervous centre where the sensation is to be perceived ; and 

 secondly, the nerve fibres themselves must retain their power of 

 irritability. The irritability of a sensitive nerve may be so deadened 



(417) 



