MESENCEPHALIC HEMISECTION IN THE PRIMATE

Relatively little information is available about the subacute and chronic condition of hemidecerebrate monkeys. The present communication deals, not with ablations of one-half of the supratentorial neuraxis, but with the effects of four different types of mesencephalic hemisections—cuts above and below the tectum, and cuts passing through the rostral and caudal parts of the magnocellular part of the red nucleus. None of these hemisections produced extensor rigidity on either side of the body. Rigidity, in the sense of resistance to passive movement, was only encountered as a flexor phenomenon. This appeared contralaterally when a high mesencephalic hemisection passed through the rostral part of the red nucleus. Examination of the evidence seems to suggest that in order for such a phenomenon to appear a subthalamically crossing tegmental mechanism needs to be severed in addition to the projection fibers in the pes pedunculi (which ultimately cross in the medulla). The tegmental crossing mechanism seems to exert a flexor inhibitory effect, in contrast to the extensor inhibitory influence which originates in the cortex.