THE CORTICAL REPRESENTATION OF MOTOR UNITS EY

pulses as stimuli, found that the most excitable area of the baboon's motor cortex was located in the middle third of the precentral gyrus. Stimulation of this region with weakest shocks (strength 1-0-1-6 ma., duration 5-0 msecs.) caused flick-movements confined to thumb, index and minimus fingers. Stronger shocks, even while still too weak to exoite movements of other parts of the body, elicited flick-movements of the same digits from a larger area. Evidently the ventral horn cells governing these distal movements are, for some reason, more accessible to stimulation of the brain's surface than are those concerned with proximal limb segments. Such differential accessibility could depend on a richer anatomical representation, possibly by cells of individually lower threshold, in the cortex, and on denser synaptic connexions within the cervical spinal grey matter. If it be supposed that some narrowly grouped colony of Betz cells forms the unique point of departure for the corticospinal pathway to th