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 specialized and grouped together in separate organs, such as the eye or ear, we find large numbers of nerve-endings—that is, small lumps of protoplasm 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 bis 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 something 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 sensation, 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 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-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
