A thirty-five-year-old taxi-driver was involved in a head-on collision at slow speed. Soon afterwards he underwent cardiorespiratory arrest and subsequently attacks of unconsciousness. He refused treatment and died four days later.
At autopsy an os odontoideum was found. A short peg arose from the body of the axis and had compressed the spinal cord on flexion of the head. The histological structure of the os odontoideum is described and compared with the developing normal odontoid process. On this basis it is suggested that the os odontoideum represents the whole odontoid found as usual in relation to the anterior arch of theatlas, but which has failed to unite with the axis.