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 vertebras) 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 vertebrae 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 (Fovese costales) at the extremities for articula- 

 tion with part of the heads of two pairs of ribs. 



The arch (Arcus vertebrae) 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 arcus vertebrae) 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 laminae 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- 

 brae, and the remaining surface is roughened 

 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 vertebrae. 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 vertebrae have also a ventral spine or a haemal arch. 

 Mammillary processes (Processus mamillares) are found in most animals on 

 the last thoracic and anterior lumbar vertebrae, 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 vertebrae 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 



•he, brae.. 



Transt/- 



Costal- faeef 



FlG. 5. — First Thoracic Vertebra of Horse. 

 To illustrate plan of structure of vertebrse. 



