848 THE CENTRAL NERVOUS SYSTEM 



arc becoming reciprocally inhibited, the other muscles participating 

 to a slight degree. If all the nerves to the extraocular muscles of the 

 right eye are cut except the sixth, which supplies the external rectus, 

 it will be found that this eye looks permanently toward the right side; 

 that is, an external strabismus is produced. If now the right cortex is 

 stimulated, both eyes will, as before, move to the left, although the 

 right eye will not move farther than the middle Line. Us movement as 

 far as this, however, niusi evidently be due to an active inhibition of Ihe 

 externa] red us muscle, for none of Ihe other muscles can act since the 

 nerves are cut. 



The experiment of conjugate deviation brings out another point re- 

 garding cerebral localization — namely, that the muscles which ordinarily 

 acl along with muscles on opposite sides of the body, are innervated 

 from both sides of the cerebral cortex. This applies not only to the 

 movements of the eyes, but to the respiratory and other movements of 

 the neck and trunk. Destruction of the trunk center of the cerebral 

 cortex on one side does not produce any paralysis, while stimulation in 

 this region produces an equal movement on both sides. We may say 

 therefore that bilaterally acting muscles are innervated from both sides 

 of the cerebral cortex. 



The movements produced by stronger stimulation of the cerebrum do 

 not remain localized, and they persist for some time after the stimulus 

 has been removed. Still further increase in the strength of the stimulus 

 may cause the contraction to spread until it affects all parts of the 

 body, giving rise to a convulsion. There are two types of contraction 

 during this convulsion, the first being a strong tonic contraction, which 

 outlasts the stimulus for some time and then gives way to a series of 

 clonic contractions, occurring at intervals of from six to ten per second, 

 and gradually getting slower as the fit dies away. The convulsion al- 

 ways starts in the muscle group represented by the cortical center that 

 is being stimulated. Thus, if the hand area is the seat of stimulation, 

 the convulsions begin in the muscles of the hand; then they spread to 

 the muscles of the forearm and shoulder on the same side, and then to 

 the face, the trunk, and the leg; and if the stimulus is strong enough, 

 they may spread to the opposite side and thus involve the whole body. 

 This "march" of the convulsion depends upon the overflow of the stimu- 

 lus on to the various centers of Ihe brain, and the pathways through which 

 it occurs seem to be located in the subcortical centers, for the spread 

 is not prevented by isolating the cortical centers from one another by 

 cuts encircling them, or by division of the corpus callosum. Never- 

 theless, 1he centers do in some way become involved in the spread, as 

 is indicated by 1hc fact that Ihe complete excision of one of them 



