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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



and face are widely separated from 

 each other that the arm-area lies be- 

 tween them, and that the area for the 

 trunk, though less schematically placed, 

 is located between the arm and leg. 

 This arrangement is more typically 

 represented on the mesial (Fig. 185) 

 than on the convex surface of the 

 hemisphere. 



The muscle-groups when enumer- 

 ated cephalo-caudad being those for 

 the head, arm, trunk, and legs, the 

 serial order of the cortical areas is 

 thus in correspondence with the order 

 of the muscle-groups which they con- 

 trol. 



The Size of the Cortical Areas. 

 Evidently there is no direct relation 

 between the extent of a cortical area 

 and the mass of muscles which it con- 

 trols ; certainly in man the mass of 

 muscles in the leg is five times greater 

 than that in the arm, and this many 

 times greater than that in the face and 

 head ; yet it is for the last area that 

 the greatest cortical extension is found. 

 Mass of muscle and extent of cortical 

 area do not therefore go together. 



When the movements effected by 

 the muscles in these several areas are 

 considered, we find that such move- 

 ments become more complex and more 

 accurate as we approach the head, and 

 it therefore accords with the facts to 

 consider the extension of the motor 

 areas as correlated with the refinement 

 of the movements which they control 

 a relation which may be expressed ana- 

 tomically as an increase in the number 

 of cortical cells controlling the related 

 cell-groups in the cord. 



Subdivision of Areas. The areas 

 which have been described are further subdivided, the subdivisions in the arm- 

 area being the clearest. Here it is found that the stimulation of the upper 

 part of the arm-area gives rise to movements which start at the shoulder, 



FIG. 188. Horizontal section of the human cere- 

 brum, showing the internal capsule on the left 

 side : F, frontal region ; O, knee of the capsule ; 

 JVC, NC, caudate nucleus ; NL, lenticular nucleus ; 

 0, occipital lobe ; TO, thalamus ; X, X, lateral ven- 

 tricle. In the internal capsule the letters indicate 

 the probable position of the bundles of fibres which 

 upon stimulation give rise to movements of the 

 parts named or which convey special sets of in- 

 coming impulses ; E, eyes ; H, head ; T, tongue ; M, 

 mouth ; L, shoulder ; B, elbow ; D, digits ; A, abdo- 

 men ; P, hip ; K, knee ; U, toes ; S, temporo-occip- 

 ital tract ; OC, fibres to the occipital lobe ; OP, optic 

 radiation (based on Horsley). 



