CEREBRAL LOCALIZATION. 603 



arrest electrical conduction sufficiently to prevent movements. 3. 

 Ferrier has shown that the direct excitation of this ganglion pro- 

 duces a general muscular contraction of the other side of the body, 

 and not special isolated movements. So that we again conclude that, 

 though electrical diffusion toward the corpus striatum (the name of 

 the ganglion in question) may exist, it is physiologically insufficient. 



But, it may still be asked if electrical diffusion does not excite 

 the white fibers interposed between these convolutions and the gan- 

 glion beneath. This objection borrows a character of probability from 

 the slight thickness of the layer of gray matter of the convolutions, 

 and also from the alleged unexcitability of the gray substance, which 

 has been proved in the case of the spinal cord, but conclusive proof in 

 regard to the brain is yet wanting. Many physiologists claim that 

 these cells are only excitable by the will. For the theory of motor- 

 centers they substitute that of psycho-motors. Reserving this discus- 

 sion for another time and place, we may say that, whether we excite 

 the cells or the fibers that arise from them, the result is the same. 



All stimulation, whatever its nature or origin, acts upon a nerve 

 according to its functions. Excite a motor nerve at any point of its 

 coui'se, and you produce movement ; excite a sensory nerve, and the 

 subject will feel a sensation which will vary with the nature of the 

 nerve. Compress the eyeball, you excite the optic nerve and get the 

 sensation of light ; auditive nerves give sensations of sound, and so 

 for all the nerves of special or general sensibility. If, then, in the 

 brain we excite the motor region, the origin of the motor nerves is 

 irritated, and we get movements ; excite the sensitive region, and in 

 place of movement we have sensation owing to the connection of these 

 nerves with sensitive cells. So that the electrization of the gray mat- 

 ter of the convolutions acts in the same way as the electrization of a 

 nerve on any point of its track ; the only difference is that in one 

 case we excite them at a point near their origin. From the anatomi- 

 cal relations that exist between the white fibers and gray cells, we 

 infer that the cells play the role of center to the nerves. 



We come now to the details of Fen-ier's experiments. Ferrier op- 

 erated chiefly on monkeys, because in them the will is in the ascendant, 

 while in lower animals automatism preponderates. As the result 

 of his experiments, he affirms the existence of three zones the intel- 

 lectual, motor, and sensitive into which the surface of the brain can 

 be divided. The one best known, and to which least objection has 

 been made, is the motor zone. In passing the electrodes over its 

 surface, we soon find the little, well-defined centers that preside over 

 particular groups of muscles. Under the microscope there is seen at 

 these points a limited mass of large cells, called nests by Betz. The 

 functions of these centers are best shown when they are electrized at 

 the central point, the current being then less likely to spread, and so 

 produce more complicated movements that mask the true function of 



