These are =amphiarthroses=, formed by the junction of the extremities of
the bodies of adjacent vertebræ. The =articular surfaces= in the
cervical region consist of a cavity on the posterior end of the body of
the anterior vertebra, and a corresponding convexity or head of the
succeeding vertebra. In the other regions the surfaces are much
flattened. The uniting media are:
1. The =intervertebralfibro-cartilages= (Fibrocartilagines
intervertebrales). Each of these is a disc which fits into the space
between the bodies of two adjacent vertebræ, to which it is intimately
attached. The discs are thinnest in the middle of the thoracic region,
thicker in the cervical and lumbar regions, and thickest in the
coccygeal region. Each consists of a dense fibrous peripheral part
(Annulus fibrosus), and a soft pulpy central part (Nucleus pulposus).
[Illustration:
FIG. 139.—SAGITTAL SECTION OF LAST TWO THORACIC AND FIRST LUMBAR
VERTEBRÆ, SHOWING LIGAMENTS AND SPINAL CORD (MEDULLA). (After
Schmaltz, Atlas d. Anat. d. Pferdes.)
]
The fibrous ring consists of laminæ of fibrous tissue and
fibro-cartilage, which pass obliquely between the two vertebræ and
alternate in direction, forming an X-shaped arrangement. The central
part of the ring is largely cartilaginous, and gradually assumes the
character of the pulpy center. The latter is very elastic and is
compressed, so that it bulges considerably from the surface of
sections; it consists of white and elastic fibers, connective-tissue
cells, and peculiar clear, transparent cells of various sizes. It is a
remnant of the notochord. There are joint cavities in the cervical
intercentral joints, and in those between the last cervical and the
first thoracic, and between the last lumbar and the sacrum. In the
latter the cavity is coextensive with the extremities of the bodies;
in the former, it is usually not so extensive.
2. The =inferior common ligament= (Ligamentum longitudinale ventrale)
lies on the ventral surface of the bodies of the vertebræ and the
intervertebral fibro-cartilages, to which it is firmly attached. It
begins about the fourteenth or fifteenth thoracic vertebra, and is at
first a narrow, thin band. Further back it becomes gradually thicker and
wider, and terminates on the pelvic surface of the sacrum by spreading
out and blending with the periosteum. It is strongest in the lumbar
region, where the tendons of the crura of the diaphragm fuse with it.
3. The =superior common ligament= (Ligamentum longitudinale dorsale)
lies on the floor of the vertebral canal from the dens or odontoid
process to the sacrum. It is narrow over the middles of the vertebral
bodies, and widens over the intervertebral fibro-cartilages, to which it
is very firmly attached.
This ligament is in relation with the spinal veins on either side, and
in the middle of each vertebra a transverse anastomotic branch passes
under the ligament.