THE PYRAMIDAL TRACT

Abstract

Pre- and postcentral areas were re- moved from the cortex of 6 monkeys by electrolysis. The lesions were shallow so as not to interfere with the underlying internal capsule. After allowing degeneration periods of 9-18 wks., paraffin sections 5[mu] thick were selected from mesencephalic, pontine and medullary levels and stained with protargol. Some sections were stained by Ranson''s pyridine Ag method for comparison. The extent of the lesion was checked by staining sections from the cortex with cresyl violet. The pyramidal tract in the monkey is made up primarily of small axis-cylinders, many of which are unmedullated; there are a few medium-sized and large fibers. Extirpation of area 4 (Betz cells) in 3 monkeys caused degeneration of only a portion of the fibers of the left or operated pyramid leaving 68%, 60% and 73% respectively; most of the large fibers were gone, though a few remained. Lesions in area 4 plus some of the post-central gyrus produced in 2 cases a 25% and 32% decrease on the side opposite the lesion and a 50% and 56% decrease on the same side. Gliosis was pronounced and many large fibers were gone. The parietal lobe adjacent to the post-central gyrus contributed few if any fibers to the pyramidal system. Degeneration was found in the crossed, lateral corticospinal tract of the spinal cord, but no ventral cortico-spinal, circumolivary or Pick''s bundle were found in the monkey on the evidence of degeneration.