THE MOTOR CENTRE.
According to the newer investigations, the motor centre embraces especially the anterior central or precentral convolution, and, further, the posterior part of the frontal lobe and the lobulus paracentralis. It includes the following regions.
a. Upper region : lobulus paracentralis and the upper quarter of the precentral convolution—the centre for the movements of the lower extremity. A further separation into particular centres for certain groups of muscles is often made ; the data, however, are so far from accord, that a subdivision into definite subcentres may be here omitted.
The largest part of the superior frontal convolution, especially the region bordering the paracentral lobule and the upper fourth of the precentral convolution, constitutes the centre for the muscles of the trunk.
b. Middle region : the middle two-fourths of the precentral convolution—the centre for the movements of the upper extremity. The further delimitation within this centre of subcentres for movements of the fingers, the hand, the arms and the shoulder, is so ordered, that the centre for the fingers occupies the lowest position, and that for the shoulder the highest.
c. Lower region : the lower fourth of the precentral convolution—the centre for the musculature of the face, the tongue, mastication, the larynx and the pharynx. Small special centres for the upper and lower facial nerve are assumed to exist.
Within the posterior part of the middle frontal convolution lies the centre for the movements of the eyes and of the head, particularly for the direction of the head and the eyes towards the opposite side (conjugate deviation). According to other investi•
gators, a second projection centre for the winking movements of both eyes has its seat in the gyrus angularis.
In regard to the motor centres it is particularly to be noted, that stimulation within the centre calls forth contraction and movements of the corresponding muscle-area of the opposite half of the body, and that in like manner injuries lead to paralysis in the opposite side of the body. This will be further discussed in connection with the motor conducting paths (page 139).
This rule is not, however, without exception. From certain centres, not only the corresponding muscles of the opposite side are controlled, but also those of the same side; that is, there exists for certain muscles a bilateral innervation. This is true for those muscles whose action, as a rule, is not unilateral but symmetrically bilateral, as, for example, in the case of the frontalis, orbicularis oculi and corrugator supercilii muscles supplied by the upper facial branch, or the muscles of mastication, of the pharynx and of the larynx. This bilateral innervation explains the fact, that after unilateral destruction of such centres the paresis of the muscles concerned is not pronounced, since the necessary stimulus may still be supplied from the unaffected centres of the opposite hemisphere.