The comparitive anatomy of the intervertebral joints

Abstract

1. A comparative study of the joints between the vertebral bodies in different vertebrate animals has been made. 2. The natures of the intervertebral joint in different -vertebrate groups is dependent on its inherent pattern of self-differentiation. 3. In Elasmobranchs the cartilaginous composition of the vertebrae contributes to the flexibility of the column, but the notochord remains as a well-defined structure throughout life, as it does also in the vertebral column of Teleostei. In Fish, special fibrous joints between the amphicoelous vertebrae allow an effective side to side movement, and the persisting notochord imparts an additional flexibility to the column. 4. Regional variation of the vertebral column is a characteristic feature of land animals; it is first encountered in Amphibia and is greatest in Mammals. In Anura, shortening of the vertebral column, associated/associated with absence of a tail, is a characteristic feature correlated with the jumping gait. In them, synovial ball and socket joint between the procoelous vertebrae are described, and an account is given of lateral intervertebral and central ligaments, to which there is no reference in the literature. The central ligament a degenerate oranio-caudally with advance in age, except whore functional, demand requires their persistence at the joints of the amphicoelous eighth vertebra of the frog. Degenerated vestiges of notochord are present within the vertebrae throughout life, and in the central ligaments as long as they persist. 6. Fibro-cartilagnious disks are a feature of many intervertebral joints of land animals and are first encountered between the procoelous vertebrae of Lizards. In them the solid fibrous, non-laminated nature of the disk is remarkably characteristics. The vestigeal remnant of notochord in the middle of the disk of Lacertilia is a noteworthy feature. The major part of the disk is considered homologous with the annulus fibrosus/fibrosus of the Mammalian disk. 7. In Birds, a description has been given of the synovial saddle joints between the free vertebrae of the cervical and thoracic regions. The articular cartilage, in fowls of over two years, is characterised by a heterogeneous matrix, fibrillation and a diminution in cellularity with occasional fissures and clefts. The central ligaments of the synovial joints degenerate and disappear within ten days of hatching. Secondary cartilaginous joints in the caudal region are described; in them the notochord degenerates and is replaced by an extension from the surrounding fibre-cartilage of the disk. The functional significance of alternating regions of free -and fused vertebrae has been discussed. The moniliform appearance and the curvatures of the notochord are peculiar to each animal species, at different periods of growth, and this has been discussed following observations on pre- and post- hatched chicks. The notochord possesses and inherent quality of local cell aggregation in the intervertebral/intervertebral regions resulting in segmental self-differentiation. 8. In Mammals, the annulus fibrosus and the nucleus pulposus of the intervertebral disk remain distinct for a short time after birth: later, the line of demarcation between them is lost. The fibre-architecture of the annulus fibrosus in the Rat shows a basic pattern, characterised by an intricate laminated fibre-system. Initially, the nucleus pulposus is formed by an intervertebral aggregation of notochordal cells, which later undergo gradual senescence: a progressive invasion of fibres occurs from the fibro-cartilaginous zone of the annulus into the peripheral part of the degenerating notochordal tissue.