160 



HABIT AND INTELLIGENCE. 



[chap. 



through 

 nervous 

 agency. 



Nervous 

 mecha- 

 nism. 



Two sets 

 of nerves. 



the application of some poisons, as well as to the stimulus 

 of a flow of nervous energy. But where there is a nervous 

 system all muscular action appears to be normally pro- 

 duced by nervous agency. This is true even of the action 

 of the heart, which has a nervous system of its own. 

 When motion takes place in response to a stimulus and 

 through nervous agency, the mechanism is as follows : — 

 Every nerve-fibre is connected — at least at one extremity 

 — with a ganglion. Different nerve-fibres have different 

 functions, according to the organs with which they are 

 connected at their outer terminations (their ganglia being 

 called their inner terminations) : some are centri2oetal, and 

 transmit stimuli from without inwards to their ganglia ; 

 others are centrifugal, and transmit motor impulses from the 

 ganglia outwards to the muscles.^ All motor action which 

 is determined by nervous agency is a complex fact, in- 

 volving the participation of at least two nerve-fibres and a 

 ganglion. When motion is caused by a stimulus, the 

 stimulus — which may consist, for instance, in the contact 

 of something that irritates the skin, or in the presence of 

 food in the mouth — produces a flow of nervous energy 

 along the nearest centripetal fibre to its ganglion. The 

 ganglionic cell that receives the stimulus communicates it 

 to another cell in the same ganglionic mass, or to another 

 ganglion. Some action takes place among those cells, 

 which determines the flow of a current of nervous energy 

 outwards from the ganglion, along a centrifugal or motor 



^ Centripetal and centrifugal are, I think, better words than afferc/vt and 

 efferent, which Dr. Carpenter uses. 



It is doubly inaccurate to use the words sensory and motor in this sense. 

 All the sensory nerves are no doubt centripetal, but the facts of reflex 

 action stated in the text show that there are centripetal nerves which are 

 not sensory. And though centrifugal nerves are necessarily motor, yet 

 they may be sensory nerves also. I agree with Mr. Lewes in thinking it 

 most probable that the motor nerves are the seat of the muscular sense, 

 or sense of muscular action. Thus some centripetal nerves are sensory, 

 and others are not ; while all centrifugal nerves are motor, but some are 

 sensory and others are not ; for the action of a voluntaiy muscle is 

 accompanied by the muscular sense in its nerves, but the action of the 

 muscles of the heart and stomach, in health at least, produces no 

 sensation. 



