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TEXT-BOOK OF PHYSIOLOGY 



cranial nerves. This interpretation is supported by the experiments of Serial er, 

 which showed that the contraction of the eye muscles which followed stimula- 

 tion of the occipital lobe took place between 0.2 and 0.3 second later than when 

 the frontal lobe was stimulated; and that as the motor reaction takes place 

 after extirpation of the frontal region, the route of the efferent impulse can- 

 not be to and through the frontal lobe, but probably through some lower 

 center. The same facts hold true for the reactions of the ear muscles follow- 

 ing stimulation of the temporal lobe. 



|| For these reasons it came to be believed that each sense area is associated 

 with a motor area, though the two are not identical but separate in theii 

 distribution. The associated motor areas assist in the formation of a 

 mechanism by which reflex movements are executed when sense organs 

 are stimulated. 



The view that the cortex of the cerebrum can be divided into separate and 

 independent though physiologically related motor and sensor areas has,.how- 



FIG. 249. DIAGRAM OF THE MOTOR AND SENSOR AREAS ON THE MESIAL SURFACE OF 

 THE MONKEY BRAIN. (After Horsley and Schafer.} 



ever, been questioned in recent years, and a somewhat different interpreta- 

 tion given to the facts. It is believed by many physiologists and neurologists 

 that the so-called motor and sensor areas are so closely related that it is 

 almost impossible to distinguish one from the other either anatomically or 

 physiologically. Thus the Rolandic region is believed to be both motor and 

 sensor in function, the former, however, being more predominant in the pre- 

 central, the latter in the post-central, convolution. As these two functions 

 are so intimately blended and their anatomic substrata so difficult of separa- 

 tion, it is thought the term sensori-motor should be employed as more descrip- 

 tive and more in accordance with the facts to the entire Rolandic region. 



This view has been strengthened by the results of the embryologic 

 investigation of Flechsig, which show that different nerve-tracts become 

 medullated or receive their myelin investment at successively later periods and 

 that the tracts which first become myelinated and are hence first functionally 

 active, belong to the afferent system. Among the first to undergo myelini- 



