The same method of investigation as stated in part Ⅰ was used on 4 cases of the monkey brain (Macaca muletta). 1) The extent of the center for the hind limb responded to electric stimulation was almost as same as or a little narrower than the extent with the Betz cells, while the extent of the center for the fore limb was much wider than the extent with the Betz cells. 2) The extent with motor response first appeared like islands at the threshold voltage, became larger as the voltage increased and then the extents for the upper and lower limbs became doubled, thus making the mixed area. 3) This enlargement of the mixed area by increased stimulation had a tendency to move
towards the center for the lower limb. 4) The threshold of stimulation was the lowest at the center for the upper limb, then at that for the lower limb, and the highest at the mixed area. And even in the same center, the threshold was lower at the central part with more Betz cells and higher at the peripheral part with less Betz cells. From these facts, it would appear that the extent of the motor cortex determined physiologically is not always the same as the extent of the distribution of the Betz cells, but is due to the density and sensitivity of the original nerve cells of the pyramidal tract, regardless of the size of the cells.