1212 



HUMAN ANATOMY. 



In the lower one-third or fourth of the motor zone are found the motor centres 

 for the face and tongue, that is, for the facial and hypoglossal nerves. In the middle 

 third or half are the arm centres. In the upper part of the region and paracentral 

 lobe, are the centres for the lower extremity. Localized lesions of the motor zone 

 may therefore produce a paralysis limited to one part controlled by the affected por- 

 tion of the cortex, as of the face, arm or leg (monoplegia). The lesion is much 

 more likely to involve two adjacent areas, as of the face and arm, or of the arm 

 and leg, giving rise to a combined paralysis ; but no single lesion, unless it were 

 crescentic in form, could involve at the same time the leg and face areas without 

 including the intervening arm area. 



Within each of the larger areas a more specialized differentiation is possible, 

 although none of them can be sharply defined, not even the larger. That the facial 

 centre lies in the lower part of the anterior central convolution is certain, and it is 

 believed that the upper and lower muscles of the face are each represented by a sepa- 

 rate centre. In the upper and forward part of the face-area are represented the 

 movements of the cheek and eye-lids ; in the posterior part the movements of the 

 pharynx, platysma and jaws. 



FIG. 1042. 



Diagram illustrating probable relations of physiological areas and centres of lateral aspect of left cerebral 



hemisphere. (Mills.) 



In the arm-area it is considered as certain that the centre for the movements of 

 the thumb and index finger is below; above is that for the finger and hands; and 

 in the highest part is that for the shoulder. In the posterior parts of the second 

 frontal convolution and in a portion of the third frontal convolution are the centres 

 for the associated lateral movements of the eyes and lateral movement of the head 

 (Beevor and Horsley). 



Our knowledge of the more special localization within the leg centre is not at all 

 exact, and the many views held are very contradictory. It is believed that the 

 centres for the movements of the thigh, knee, foot, and toes, are arranged in the 

 order named, from before backward on the lateral border of the hemisphere and in 

 the paracentral lobe. 



A narrow zone for the movements of the trunk, as shown by (iriinbaum and 

 Sherrington, is located between the upper border of the arm-area and the lower 

 bonier of the leg-area. It is now considered probable, however, that the cutaneous 

 sensory centres are posterior to and in close contact with the motor centres in 

 the postceniral convolution, while other centres for stereognostic perception and the 

 muscular sense are located in the superior and inferior parietal convolutions. 



The speech centres are in the posterior part of the third left frontal convolution 

 (Brora's convolution), in right-handed people in the first left temporal convolution, 

 and perhaps in the left angular gyrus. 



