In a recent attempt to trace the anatomic connections of the frontal lobe of the monkey, degenerations have consistently been found in the spinal cord following ablations restricted to the premotor area. As the existence of nonpyramidal corticospinal fibers has not hitherto been recognized, a systematic examination of their course, as well as of their cells of origin, has been undertaken, and this article records the results of this study.
Histologically, the premotor and motor areas of the cortex, which are designated as areas 6 and 4, respectively, in Brodmann's architectonic map1 (fig. 1), are identical except for the absence in the premotor area of the giant pyramidal cells of Betz. Ablation experiments2 had suggested that the pyramidal tracts originated solely from the motor area, and Holmes and May,1b who studied retrograde cellular degeneration in the cortex after lateral semisection of the cord, concluded that the Betz