with coatings. It is not stretching analogy too far to say that nerve force may probably escape unless properly insulated.
In consequence of the fibers being covered with these white sheaths, they form what is called the white matter of the brain; while the nerve-centers are grayish, and therefore form what is called the gray matter of the brain, so that the gray matter receives and records the messages conveyed to it by the white insulated fibers.
From the sensory corpuscle, which is a small mass of protoplasm provided with branches connecting it to neighboring 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 contraction. These two corpuscles form what is called a nerve-center.
Not only are the motor corpuscles fewer as well as much larger than the sensory ones, but also the nerve-fibers which go out from 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 fiber terminates in a protoplasmic mass which is firmly united to a muscle-fiber, and which enables the muscle-fiber 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) the 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-center, 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 commissures. To such a center come sensations or impressions from the nerve-endings, and from such a center go out impulses which set the muscles in action.
I have dwelt thus at length on this most elementary point, because it appears to me that, in consequence of the rapidity with which function is being demonstrated to be definitely localized in various portions of the cerebral hemispheres, we are in danger of losing sight of Dr. Hughlings-Jackson's grand generalizations on nerve-function, and that we are gradually inclining to the belief that the function of each part is very distinct, and therefore can most readily act without disturbing another part. In fact, we are perhaps drifting toward the quicksands of spontaneity, and disregarding entirely the facts of every-day
