B46 THE CENTRAL NERVOUS SYSTEM 



Lated, and found that the latent periods were very variable, the after- 

 effects indefinite, and inhibition more prominent than excitation. More- 

 over, the inhibition was more or less independent of the simultaneous 

 excitation of the antagonistic muscle, in which respect it therefore dif- 

 fered from the type exhibited in the reciprocal innervation of the spinal 

 reflexes (see page 814). Nor were the results obtained from a given 

 cortical center always the same; thus, if a point giving a certain re- 

 sponse was stimulated immediately after previous stimulation, the re- 

 sult was often reversed; if it was inhibition in the first instance, it 

 might be excitation immediately afterward. But if sufficient time was 

 allowed, then the response was always of the same kind. 



By comparing the effect of simultaneous stimulation of an afferent 

 spinal root and of a flexion or extension point on the cortex, it was 

 found that the stimulation of the afferent root Avhen a flexion point 

 was being stimulated augmented the flexion, but when an extension 

 point was stimulated, stimulation of the afferent root might change the 

 response to flexion, the exact result depending considerably on the rel- 

 ative strength of the two stimuli. The general conclusion that may be 

 drawn from these results is that the special function of the cortex is to 

 reverse the centers of purely spinal reflexes when such reversal is de- 

 sirable or necessary. The cortex dominates the spinal reflexes, and in 

 general it may be said that its main effect is inhibitory in nature. 



It is particularly by the use of the method of moderate electrical stimu- 

 lation that exact localization has been worked out on the cerebral cor- 

 tex. As would be expected, this localization is much less defined and 

 definite in the lower as compared with the higher animals. In the 

 higher apes, it has been found that the motor centers are limited to a 

 narrow strip of cortex immediately in front of the Eolandic fissure — 

 the precentral area, as it is called. (Fig. 219.) 



From the accompanying figure, it will be noted that the centers are 

 arranged from below upward, in the reverse order to that in which the 

 muscular groups occur in the body; that is to say, the face, neck, etc., 

 are located lowest on the cortex, and the leg highest up. It will further 

 be noted that the extent of the centers for the neck and tongue is very 

 much greater than for the body or leg, that for the arm being interme- 

 diate. Tt is not. therefore, the extent of the muscular tissue that de- 

 termines the size of the cortical area controlling its movements, but 

 the type or complexity of the movements that the muscles perform. 

 The complex movements of the tongue and the vocal cords evidently 

 require greater cortical representation than do the coarser movements 

 of the large mass of muscular tissue of the trunk. The centers extend 

 somewhat upon the mesial aspect of the brain, but occupy here only a 



