640 PHYSIOLOGY OF THE CEREBRUM 



C. DIRECT AND CROSSED EFFECTS OF STIMULATION OF THE MOTOR 



CORTICAL AREAS 



As already noted at page 632, the movements induced by stimulation of 

 the cerebral cortex occur mainly in the opposite half of the body. But move- 

 ments can be obtained also in the muscles of the same side. Of these bilateral 

 movements some can be obtained even with a very weak stimulus. With the 

 great majority of muscle groups, however, the movements on the same side 

 can only be induced with relatively strong stimuli. 



The eye movements are to be classed as bilateral movements since stimula- 

 tion of one hemisphere causes both eyes to be rotated toward the opposite 

 side. But the bilaterality in this case is only an apparent one ; for the internal 

 rectus of one eye contracts at the same time as the external rectus of the 

 other. Inhibition of the antagonistic muscle is also an important factor. 

 When all the nerves except the abducens to the eye muscles of one side, say 

 the left, are cut in the monkey, the left eye naturally is deflected to the left, 

 because the tonus has been destroyed in all but the external rectus muscle. 

 But if movement of the eyes to the right is then induced by appropriate 

 stimulation of the cerebral cortex, the left eye will turn back to the right 

 as far as the median line even though the internal rectus has been paralyzed. 

 That is, the stimulation has caused the tonus of the left external rectus to 

 be intermitted. Since this experiment succeeds also when the corona radiata, 

 the internal capsule, etc., are stimulated, the inhibition in question must be 

 started from some center below the cortex ( Sherrington, cf. page 636). 



The action induced in the face muscles is really bilateral, although those of 

 the opposite side contract the more powerfully. This is true of the buccinator 

 as well as of the muscles of the tongue and vocal cords. 



With regard to other muscular contractions induced from the same side of 

 the brain, it is to be remarked: (1) that their latent period is longer than that 

 of muscles on the opposite side (Franck and Pitres) ; (2) that they require a 

 stronger stimulus; and (3) that the muscles of the same side of the body never 

 make coordinated movements as do those of the opposite side, but show instead a 

 tonic contraction more or less like an extended tetanus. 



We see therefore that considerable differences exist between the movements 

 of the same and of the opposite side, and, as Gotch and Horsley especially 

 have emphasized, it is probably to be assumed that the muscles of the same 

 side are not so immediately dependent upon the cortical areas as are those 

 of the opposite side. 



It is conceivable that the excitation is conveyed to the muscles of the same 

 side by first crossing in the brain to the corresponding motor areas of the oppo- 

 site hemisphere. But if this be the case, it is not the only course the excitation 

 can take, for contractions on the same side have been obtained by a number of 

 authors even after the removal of these opposite areas, or of the entire opposite 

 hemisphere. 



The crossing therefore must take place in the lower centers. Lewaschew 

 obtained movements in the left hind leg by stimulating the left hemisphere 

 after hemisection of the spinal cord on the left side. In this case the excita- 

 tion had crossed to the right side of the cord and had crossed back again 

 below the level of the section (twelfth thoracic segment) to the left half. But 



