To illustrate plan of structure of vertebræ.
]
The vertebræ in a given region have special characters by which they may
be distinguished from those of other regions, and individual vertebræ
have characters which are more or less clearly recognizable. All typical
vertebræ have a common plan of structure, which must first be
understood. The parts of which a vertebra consists are the =body= or
=centrum=, the =arch=, and the =processes=.
The =body= (Corpus vertebræ) is the more or less cylindrical mass on
which the other parts are constructed. The anterior and posterior
extremities of the body are attached to the adjacent vertebræ by
intervertebral fibro-cartilages, and are usually convex and concave
respectively. The dorsal surface is flattened and enters into the
formation of the vertebral canal, while the ventral aspect is rounded
laterally, and is in relation to various muscles and viscera. In the
thoracic region the body presents two pairs of demifacets (Foveæ
costales) at the extremities for articulation with the heads of two
pairs of ribs.
The =arch= (Arcus vertebræ) is constructed on the dorsal aspect of the
body. It consists originally of two lateral halves, each of which is
considered to consist of a pedicle and a lamina. The pedicles form the
lateral parts of the arch, and are cut into in front and behind by the
=vertebral notches= (Incisura vertebralis cranialis, caudalis). The
notches of two adjacent vertebræ form =intervertebral foramina= for the
passage of the spinal nerves and vessels; in some vertebræ, however,
these are complete foramina instead of notches. The laminæ are plates
which complete the arch dorsally, uniting with each other medially at
the root of the spinous process.
The body and the arch form a bony ring which incloses the =vertebral
foramen= (Foramen vertebrale); the series of vertebral rings, together
with the ligaments which unite them, inclose the =vertebral canal=
(Canalis vertebralis), which contains the spinal cord and its coverings
and vessels.
The =articular processes=, two =anterior= and two =posterior= (Processus
articulares craniales, caudales), project from the borders of the arch
on either side. They present joint surfaces adapted to those of adjacent
vertebræ, and the remaining surface is roughened for muscular and
ligamentous attachment.
The =spinous process= (Processus spinosus) is single, and projects
dorsally from the middle of the arch. It varies greatly in form, size,
and direction in different vertebræ. It furnishes attachment to muscles
and ligaments.
The =transverse processes= (Processus transversi) are two in number and
project laterally from the side of the arch, or from the junction of the
arch and body. In the thoracic region each has a facet for articulation
with the tubercle of a rib (Fovea costalis transversalis). They also
give attachment to muscles and ligaments.
Some vertebræ have also a =ventral= or =hæmal spine=.
=Mammillary processes= (Processus mammillares) are found in most animals
on the last thoracic and anterior lumbar vertebræ between the transverse
and anterior articular processes or on the latter.
=Accessory processes= (Processus accessorii), when present, are situated
between the transverse and posterior articular processes.
=Development.=—The vertebræ are developed by ossification in the
cartilage which surrounds the notochord and forms the sides of the
neural canal. There are =three primary centers of ossification=, one for
the body and one for each side of the arch. =Secondary centers= appear
later for the summit of the spinous process (except in the cervical
region), the extremities of the transverse processes, and the thin
epiphyseal plates at the extremities of the body.
Sometimes there are at first two centers for the body which soon fuse.
The process of ossification extends from the lateral centers to form
not only the corresponding part of the arch, but also the processes
and a part of the body next to the root of the arch (Radix arcus). In
the horse and ox the body and arch are usually fused at birth, but the
epiphyses do not fuse till growth is complete. In the pig, sheep, and
dog the body and arch are united at birth by cartilage (neurocentral
synchondrosis), but fuse in the first few months.