The purpose of the present undertaking is to classify the mature, normal, myelinated fibers found within the pyramid of the human medulla. Although this region offers an exceptional opportunity to examine the morphology of a well known and isolated nerve tract of the central nervous system, few investigators have been interested in its anatomical content.
The approaches employed for investigation of the pyramidal tract have been principally the following: study of secondary degeneration with the Marchi method, retrograde reaction, functional defects resulting from lesions and finally stimulation experiments. A knowledge of the pyramids might have influenced the conclusioiis drawn from these experiments because each method appears to have its limitations, based largely upon anatomical considerations. The Marchi method, for instance, acts ineffectively on small myelinated, not at all on unmyelinated and masks normal fibers which might be unaffected by the lesion. Retrograde degeneration may also not tell the complete story in respect to the origin of pyramidal tract fibers because the phenomenon is difficult to evaluate on small neurons ; also, Tower ( '40) in the monkey and Davison ('37) in man found fibers intact and apparently normal in the pons long after destruction of the pyramids. The pyramidal investigators studying the physiological defects proclncccl from cortical ablation experiments have in very few 2lT