The head movement system has a number of intrinsic specializations which separates it from other motor systems. The musclcs that serve to move the head and the receptors of these muscles have distinctive characteristics. The upper cervical cord differs in essential aspects of its orgar~ization from the more familiar lumbosacral cord, and one specialized supraspinal structure, the superior colliculus, plays a major role in the organization of head movement. This review will touch upon all these areas. In primates, the position of the foramen magnum at the base of the skull means that many head movements are executed with little influence on the centre of gravity and therefore without the need for compensatory adjustments in other muscle systems. In quadrupeds, the head cannot be rnloved without some compensating postural adjustment. But in both primate and quadruped, head movements are executed about an extensible and retractable 'universal joint' so that there is the possibility of horizontal and vertical rotation, flexion, or extension (either alone or in combination).
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V. C. Abrahams (1977) studied this question.