Having been lately engaged in examining the structure of the vertebrae in different animals, I have been particularly struck with the mechanism of the spine and spinal canal in birds, by which a most remarkable degree of motion is gained in the neck, without any injury or pressure on a part of such vital importance to the existence of animal life, as the spinal marrow; an extent of motion, so great indeed as completely to compensate for the deficiency of it in the dorsal and lumbar regions, as well as for the want of any prehensile power in the anterior extremities. In attempting to explain the nature of this peculiar mechanism, which tends to throw considerable light on the physiology and pathology of the spine, I believe that I have not been preceded by any author. The cervical vertebras in birds are very numerous, varying from nine to twenty-four. They differ considerably from one another, according to their situation, in the form and direction of their articulating surfaces, and in the number and shape of the different processes, which afford extensive means of attachment to the muscles concerned in the different motions of the neck. Unlike the vertebræ in man and most of the mammalia, they are articulated together by complicated joints, which bear a close resemblance to the articulation of the olecranon with the humerus in the human subject, but differing in some respects; the vertebræ in birds admitting of lateral motion as well as flexion and extension, whilst the elbow is strictly a hinge-like joint. The varying position of these articular surfaces is greatly favoured by the interposition of a cartilage, which is curiously adapted to the surface of each bone, and is enclosed between reduplications of synovial membrane; and thus each joint is double, consisting of two synovial cavities, and is analogous to the articulation of the lower jaw in man; a circumstance, I believe, not mentioned either by Cuvier, Blumenbach, or Macartney.
XXII. On the mechanism of the spine
