Variation in form of the pyramidal tract and its relationship to digital dexterity.

A morphometric analysis of the pyramidal tract's relation to digital dexterity was performed on data from 69 mammals. The results show that the variation in digital dexterity among mammals corresponds most closely to the variation in place of termination of pyramidal tract fibers within the spinal cord, corresponds less closely to the variation in the size of the tract itself and its constituent fibers, and does not correspond reliably with any other feature yet reported. Since the termination of pyramidal tract fibers on or very near spinal motor neurons is a prerequisite even for the peculiar kind of dexterity seen in some non-primates (e.g., raccoon, kinkajou), this one feature alone seems to be a critical factor.