230 



CHAPTER IX. 



VITAL PROPERTIES OF NERVES AND NERVOUS CENTRES. CLASSIFI- 

 CATION OF NERVES ACCORDING TO THE VITAL ENDOWMENTS OF 

 THEIR FIBRES. STIMULI OF NERVOUS ACTION, MENTAL AND PHY- 

 SICAL. VIS NERVOSA ITS NATURE; IS IT ELECTRICAL? 



Of the Vital Properties of Nerves and Nervous Centres. There 

 are no textures which exhibit such proneness to molecular change, 

 under the influence of their proper stimuli, as nerve and muscle. 

 It has already been stated in the first chapter (p. 56), that each of 

 these tissues manifests its vital action in a different, although a very 

 analogous way. Muscles, while they are capable of responding to 

 other stimuli, almost invariably act in obedience to that of nerve; 

 and the changes which muscular contraction produces are obvious 

 to our unaided senses in the shortening of the muscle, and in its 

 greater thickness and hardness. Even the alterations in the con- 

 dition of its sarcous elements may be discerned by the microscope, 

 and have been described at page 179. 



The changes, however, which take place in nerve, when in ac- 

 tion, are known to us only by the effects which they produce on 

 the sentient mind or on muscular parts. There is no alteration 

 in the physical appearance of the nerve or its fibres, which can 

 be detected by our aided or unaided vision. Yet, from the ra- 

 pidity with which stimuli applied to nerves produce their effects 

 on distant muscular parts, from the instantaneous cessation of these 

 effects on the removal of the stimulus, and the speedy renewal of 

 them on its reapplication, we can refer the phenomena to nothing 

 so well as to a molecular change, rapidly propagated along the 

 course of the nerve from the point of application of the stimulus. 

 And in the instantaneousness of its production, and the velo- 

 city of its propagation, we may compare it to that remarkable 

 change in the particles of a piece of soft iron, in virtue of which 

 it acquires the properties of a magnet so long as it is maintained 

 in a certain relation to a galvanic current; these properties being 

 instantaneously communicated when the circuit is completed, and 

 as instantaneously removed when it is broken. A state of polarity 



