252 Mr. Victor Ilon^leij [March 27, 



conscious perception of sensations. This distinction between feeling 

 and sensation, if dogmatic, will save us from dispute as to the meaning 

 of the word sensation ; and further, the distinction is one, as I have 

 just shown, which is justified by custom. 



Now the nerve fibre which conveys the energy of the sensation is 

 a round thread of protoi)lasm which in all probability connects the 

 nerve-ending with a sensory corpuscle in the spinal cord. These 

 nerve fibres running in nerves are white, whereas, as you know, 

 protoplasm is grey. They are white because each is insulated from 

 its fellow by a white sheath of fatty substance, just as we protect 

 telegraph wires with coatings. It is not stretching analogy too far 

 to say that nerve force may probably escape unless properly insulated. 



In consequence of the fibres being covered with these white 

 sheaths, they form what is called the white matter of the brain ; while 

 the nerve centres are greyish, and therefore form what is called the 

 grey matter of the brain, so that the grey matter receives and records 

 the messages conveyed to it by the white insulated fibres. 



From the sensory corpuscle, which is a small mass of protoplasm 

 provided with branches connecting it to neighbouring corpuscles, the 

 nerve energy if adequate passes along a junction thread of protoplasm 

 to a much larger corpuscle, which is called a motor corpuscle, and 

 the energy of which when liberated by the nerve impulse from the 

 sensory corpuscle is capable of exciting muscles into active contrac- 

 tion. These two cv:)rpuscles form what is called a nerve centre. 



Not only are the motor corpuscles fewer as well as much larger 

 than the sensory ones, but also the nerve fibres wdiich go out fi-om 

 them are larger too. In fact, it would seem as if we had another close 

 analogy to electrical phenomena ; for here, where we want a sudden 

 discharge of a considerable intensity of nerve force, we find to hand a 

 large accumulator mechanism and a large conductor, the resistance of 

 which may justly be supposed to be low. Finally, the motor nerve- 

 fibre terminates in a protoi^lasmic mass which is firmly united to a 

 muscle fibre, and which enables the muscle fibre to contract and so 

 cause movement of one or more muscles. Now, with this idea of the 

 general plan on which the whole nervous system is constructed, you 

 will understand that muscular action, i. e. movement, will occur in 

 proportion to (1) t! e intensity of the stimulation of the sensory 

 corpuscle, and (2) the resistance in the different channels. When a 

 simple flow through the whole apparatus occurs, it is called a simple 

 reflex action, and this was discovered in England by Dr. Marshall 

 Hall. 



To recapitulate : a nerve centre, theoretically speaking, we find to 

 consist of a sensory corpuscle on the one hand and a motor corpuscle 

 on the other, both these being united by junction threads or com- 

 missures. To such a centre come sensations or impressions from the 

 nerve-endings, and from such a centre go out impulses which set the 

 muscles in action. 



I have dwelt thus at length ou this most elementary point, because 



