26 OSTEOLOGY 
a common plan of structure, which must first be understood. The parts of which 
a vertebra consists are the body, the arch, and the processes. 
The body (Corpus vertebr) 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 vertebra 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 later- 
ally, and is in relation to various muscles and viscera. In the thoracic region the 
body presents two pairs of facets (ove costales) at the extremities for articula- 
tion with part of the heads of two pairs of ribs. 
The arch (Arcus vertebra) 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 pedicle (Radix areus vertebra) forms the lateral 
part of the arch, and is cut into in front and behind by the vertebral notches (In- 
cisura vertebralis cranialis, caudalis). The notches of two adjacent vertebrae form 
intervertebral foramina (Foramina intervertebralia) for the passage of the spinal 
nerves and vessels; in some vertebrae, how- 
ever, there are complete foramina instead 
of notches. The lamin 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 verte- 
bral 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 bor- 
ders of the arch. They present articular 
surfaces adapted to those of adjacent verte- 
Fic. 5.—Firsr Troracrc Vertesra or Horse. bree, and the remaining surface is roughened 
To illustrate plan of structure of vertebrae, for muscular and ligamentous attachment. 
The spinous process or spine (Processus 
spinosus) is single, and projects dorsally from the middle of the arch. It varies 
greatly in form, size, and direction in different vertebra. It furnishes attachment 
to muscles and ligaments. 
The transverse processes (Processus transversi) are two in number and project 
laterally from the sides 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 liga- 
ments. 
Some vertebre have also a ventral spine or a hemal arch. 
Mammillary processes (Processus mamillares) are found in most animals on 
the last thoracic and anterior lumbar vertebree, between the transverse and an- 
terior articular processes or on the latter. 
Accessory processes (Processus accessorii), when present, are situated between 
the transverse and posterior articular processes. 
Development.—The vertebre 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 
