SummaryTranslational acceleration head injury experiment was performed on 12 Japanese monkeys and one baboon using a HYGE sled and a slider-impactor system. All the monkeys became concussed by force ranging 241 to 366G in slider average acceleration and two of them died of concussion (fatal or irreversible concussion). The fatal concussion animals as well as the delayed death ones showed almost unexceptionally rib and clavicle fractures and mediastinal hemorrhage. Subarachnoid hemorrhage seen in six animals were not correlated with the outcome. We were unable to produce visible brain lesions, such as brain contusion, subdural hematoma, or intracerebral hematoma, by using pure trans lational acceleration impacts to the head. It should be misleading to draw a human or primate head injury tolerance limit from the fatal concussion level.Interestingly seven monkeys showed spontaneous eyelid blinking and four showed pupillary light reaction immediately post-impact and at least six animals showed "nor mal" EEG activity ten seconds after the impact. The cause of concussion was discussed and the role of cardiorespiratory supression by the primary shock is suggested.

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