6o2 THE POPULAR SCIENCE MONTHLY. 



pal ones have their homologues in all animals. They have also their 

 special names. When the excitant is moved about over the different 

 convolutions, the existence of the motor zone is made manifest by the 

 successive movements of the animal ; the sensitive zone is known by 

 signs of sensation, while the indications of an intellectual zone are not 

 thus positive. Observe what passes when we irritate the so-called 

 motor region. We excite a certain point of the convolutions, and a 

 movement is produced. Groping about with our electrodes, we find 

 the effect limited to a small tract where at all the points the same 

 movement is provoked, and that alone. Next to this zone, we can in the 

 same way find the limits of others presiding over other movements. 

 The influence appears greatest at the center of the zone : exciting the 

 periphery sometimes produces a light supplementary movement belong- 

 ing to a neighboring zone. The special centers or zones being very 

 near together, our means of electrizing them are not perfect enough 

 to prevent a slight diffusion of the current into neighboring centers 

 and faintly exciting them. To get precise separate effects we must 

 excite the center of the special zone. 



As the same movement invariably follows the stimulation of a 

 special center (the name of a special zone), we conclude that the center 

 in question has charge of this movement, and, as a great number of 

 these centers, placed side by side, preside over most of the movements 

 of the body, we infer the existence of a motor region of the brain, the 

 seat of voluntary control of the body. But, since, as the galvanometer 

 shows, the electricity is diffused over neighboring centers, what right 

 have we, it may be asked, to assert that the movement observed, when 

 a certain center is excited, is due to that excitation, when neighboring 

 centers are also excited ? We reply that to these facts we can oppose 

 other facts. On removing the electrical conductors scarcely a centi- 

 metre, we excite perfectly definite unlike movements, although the 

 current is diffused as before. Why are not the movements last pro- 

 duced the same as those observed at the first position? Why such 

 distinct localization in spite of diffusion '? Although the occurrence of 

 diffusion is proved by means of sensitive apparatus, it is physiologically 

 insuflicient. Besides, it can be prevented by proper precautions. 



Again, it is asked if there may not be such a thing as diffusion 

 beneath the surface ? This is a grave question, for, under the motor 

 region, there is a large ganglion containing motor fibers going to the 

 muscles. If the current diffuses downward as far as this ganglion, we 

 can not assume that the convolution alone is excited. There are three 

 answers to this objection : 1. The excitation of the convolutions 

 nearest to this ganglion gives the least results ; sometimes no move- 

 ment at all. Unless we admit that the effects are directly as the re- 

 sistance, which is absurd, downward conduction can not be affirmed. 

 2. Braun has demonstrated that a section of the white fibers below 

 .the excited points, interrupting the physiological continuity, does not 



