THE MOTOR CENTERS AND THE WILL. loi 



the same become highly complex when we arrive at the top of the 

 brain. 



At the same time, while we have this simple plan of structure, we 

 find that there is also a fundamental mode of action of the same — a 

 mode which is a simple exposition of the principle, no effect without a 

 cause — a mode of action which is known as the phenomenon of simple 

 reflex action. 



The general plan of the whole nervous system is illustrated by this 

 model. Imbedded in the tissues all over the body, or highly special- 

 ized and grouped together in separate organs, such as the eye or ear, 

 we find large numbers of nerve-endings — that is, small lumps of proto- 

 plasm from which a nerve-fiber leads away to the spinal cord and so 

 up to the brain. These nerve-endings are designed for the reception 

 of the different kinds of vibration by which energy presents itself to 

 us. As the largest example of these nerve-endings, let me here show 

 you one of the so-called Pacinian bodies, or, more correctly, Marshall's 

 corpuscles, for Mr. John Marshall discovered these bodies in England 

 before Pacini published his observations in Italy. Here you see one 

 of these small oval bodies arranged on the ends of one of the nerves 

 of the fingers, and here you see the nerve-fiber ending in the little 

 protoplasmic bulb which is protected by a number of concentric 

 sheaths. Pressure or any form of irritation of this body at the end 

 of the nerve-fiber causes a stream of nerve-energy to travel through 

 the spinal cord to the brain, and so we become conscious that some- 

 thing is happening to the finger. 



Here in this section of the sensitive membrane of the back of the 

 eye, the retina, you see a similar arrangement, only more complicated 

 — namely, nerve-fibers leading away from small protoplasmic masses 

 which possess the property of absorbing light and transforming it into 

 nerve-energy. It is this transformation of nerve-energy into heat, 

 light, pressure, etc., which it seems to me should alone be called a sen- 

 sation, irrespective of consciousness. And, in fact, we habitually say 

 we feel a sensation. The terms " feeling " and " sensation," however, 

 are frequently used as interchangeable expressions, although, as I shall 

 show you directly, " feeling " is the conscious disturbance of a sensory 

 center in the surface of the brain, and in fact feeling is the conscious 

 perception of sensations. This distinction between feeling and sensa- 

 tion, 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-fiber which conveys the energy of the sensation is 

 a round thread of protoplasm which in all probability connects the 

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

 fibers running in nerves are white, whereas, as you know, protoplasm 

 is gray. They are white because each is insulated from its fellow by 

 a white sheath of fatty substance, just as we protect telegraph-wires 



