XXIX.] THE PHYSIOLOGY OF MIND. 35 



organs of sense, and producing sensation as tliey enter the 

 sensory ganglia. 



Sensation sometimes determines the flow of a downward Consen- 

 current from the motor ganglia along their nerves to the ^^^^'^^^^'^ 

 muscles, causing muscular motions. This is what is called 

 consensual action. 



Sensation sometimes determines the flow of an upward Conscious- 

 current from the sensory ganglia along the nerves that sensation. 

 connect them with the ganglionic substance of the cere- 

 brum, producing consciousness. This is original conscious- 

 ness, or consciousness of sensation. 



Consciousness generally determines the flow of currents ThougM. 

 along the nerves that connect the different parts of the 

 ganglionic substance of the cerebrum with each other, 

 producing thought. 



Thought is not itself conscious ; but it generally, though Conscious- 

 not always, becomes so by producing currents in the nerves jucecrb " 

 of consciousness. Consciousness thus produced by thought thought. 

 sometimes takes the form of the revival of the conscious- 

 ness of a sensation, in which case it constitutes the recol- 

 lection of the sensation. 



Thought is capable of acting on the motor ganglia by wui. 

 means of currents sent down the nerves of will. The 

 motor ganglia respond to the stimulus exactly as if it 

 were a stimulus to consensual action coming from the 

 sensory ganglia : they send the proper current down the 

 motor nerves to the muscles, causing voluntary muscular 

 motion. 



Even as a very brief outline, the foregoing list of the 

 various kinds of mental interactions is incomplete. It 

 leaves out all but what may be called the primary classes 

 of interactions. When we come to the laws of association, 

 we shall have to speak of secondary classes of interactions, 

 which are governed by those laws. 



The phenomena of sleep, dreaming, and somnambulism 

 depend on the partial or total suspension of the activity 

 of various nervous centres of the mental life, and on the 



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