I02 THE POPULAR SCIENCE MONTHLY. 



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 sen- 

 sory 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 cor- 

 puscle ; and (2) the resistance in the different channels. When a sim- 

 ple 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 commis- 

 sures. 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 func- 

 tion is being demonstrated to be definitely localized in various portions 

 of the cerebral hemispheres, we are in danger of losing sight of Dr. 

 Ilughlings-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 life 



