The quantitative cortical origin of pyramidal axons of Macaca rhesus

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Abstract
Seventy Macaca rhesus [Macacus rhesus] monkeys were subjected to partial unilateral cerebral ablations or hemidecortication. From silver-impregnated sections, the number of pyramidal axons remaining after various time intervals was estimated by a statistically sound sampling method. The pyramid contralateral to the cerebral ablation served as a paired control. The number of pyramidal axons was determined without the observer knowing the type of operation performed, and nonoperated controls were randomly included. The cytoarchitectural accuracy of the cerebral lesions was estimated without knowledge of the condition of the pyramid. Axolysis of pyramidal fibers in the monkey requires 6 to 12 months to approach completion. The curve of axolysis seemed to follow the familiar exponential law of decay. On the basis of animals surviving 1 year after operation, we conclude that all or virtually all descending pyramidal axons arise in area 6, area 4, and the parietal lobe. Insofar as the surgeon''s eye and hand and the light microscope can define cytoarchitectural areas, approximately 29% of the descending pyramidal axons arise in area 6, 31% in area 4, and 40% from the parietal lobe. On observational and statistical grounds, the occipital lobe probably can be eliminated as a source of pyramidal fibers. As a consequence of temporal lobectomy or ablation of the frontal pole, a few degenerating axons can be found in the ipsilateral pyramid. The fiber loss is not significant statistically, but because of the small number of animals, statistical insignificance is insecure evidence. These few degenerating fibers appear to result from involvement of the paracentral areas by secondary pathogenic factors, such as shift of the brain or operative interference with the anterior and middle cerebral arteries, but small contributions from the temporal lobe or frontal pole cannot be completely excluded. After hemidecortication, 1.32% of axons in the pyramid remain in animals surviving for 1 year. A similar number of axons survive complete mesencephalic crusotomy. These axons are grouped almost exclusively at the margins of the medullary pyramids. They would appear to represent merely the degree of intermingling of pyramidal fibers with surrounding tracts, rather than an ascending system. Within the depths of the pyramid proper, degeneration of longitudinal axons is virtually 100% complete at 1 year. If an ascending system is present, it degenerated retrogressively after hemidecortication or crusotomy. The number of axons in a pyramid cannot be defined accurately by simple inspection. Although 30% to 50% of the fibers may be missing, the pyramid may look nearly normal on inspection. The inaccuracies of subjective appraisal, inadequate histologic methods (particularly reliance on the Marchi technic or retrograde degeneration), and failure to recognize the slow rate of axolysis explain the failure of previous anatomic studies to define the major cortical fields of origin for the pyramidal fibers.