CHAPTER II. 

 NERVOUS IRRITABILITY AND ITS MODE OF ACTION. 



THE property possessed by nerves of being called into excitement 

 by a stimulus is termed their "irritability." Such a property 

 exists in other tissues and organs ; and each one, when subjected to 

 a stimulus adapted to 'its character, reacts in a way peculiar to itself, 

 and produces a definite effect. Thus a gland, when excited, exhibits 

 the phenomena of secretion ; a muscle, those of contraction. 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 attached. 

 The irritability of a nerve, on the other hand, is not manifested by any 

 perceptible change in its own tissue, but by the phenomena of sensation 

 or motion in the organs to which it is distributed. 



Irritability of Sensitive Fibres. 



The irritability of sensitive nerve fibres is manifested by sensation. 

 .This sensation, however, is not felt in the nerve, but in the nervous 

 centre where it terminates. For if the communication between a sen- 

 sitive nerve and the brain be cut off, no stimulus subsequently applied 

 to its trunk or branches will give rise to a sensation. But if this con- 

 nection be retained, while that with the external integument is cut off, 

 stimulants applied to the attached portion of the nerve will cause sen- 

 sations more or less acute, according to the stimulus employed. Pinch- 

 ing or pricking the nerve, variations of temperature, or the passage of 

 an electric current, will all bring into action its irritability, and thus 

 produce a sensation. 



For this result, however, two conditions are essential. First, the 

 nerve must be, as above mentioned, in communication with its nervous 

 centre ; and secondly, the nerve fibres must retain their irritability. 

 The irritability of a sensitive nerv 7 e may be so deadened by compres- 

 sion, contusion, or cold, that no stimulus applied to the part will 

 produce a perceptible effect. In the observations of Mitchell,* the 

 application of extreme cold, in man, to the region of the ulnar nerve 

 at the elbow, produced, when the chilling process had reached a certain 

 stage, complete loss of sensibility in the parts to which the nerve is 

 distributed. The irritability of a sensitive nerve may also be sus- 

 pended by injuries not involving its own fibres. Thus division of 

 certain parts in the brain or spinal cord may produce a loss of sen- 



* Injuries of Nerves and their Consequences. Philadelphia, 1872, p. 59. 



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