Part 5

A =line= (Linea) is a very small ridge.

A =head= (Caput) is a rounded articular enlargement at the end of a bone; it may be joined to the shaft by a constricted part, the =neck= (Collum).

A =condyle= (Condylus) is an articular eminence which is somewhat cylindrical; a non-articular projection in connection with a condyle may be termed an =epicondyle=.

A =trochlea= is a pulley-like articular mass.

A =glenoid cavity= (Cavitas glenoidalis) is a shallow articular depression, and a =cotyloid cavity= or =acetabulum= is a deeper one.

The term =facet= is commonly applied to articular surfaces of small extent, especially when they are not strongly concave or convex.

The terms =fossa=, =fovea=, =groove= or =sulcus=, and =impression= are applied to various forms of depressions.

A =foramen= is a perforation for the transmission of vessels, nerves, etc.

A =sinus= or =antrum= is an air-cavity.

Other terms, such as =canal=, =fissure=, =notch=, etc., require no explanation.

VERTEBRAL COLUMN

The =vertebral column= (Columna vertebralis) is the fundamental part of the skeleton. It consists of a chain of median, unpaired, irregular bones which extends from the skull to the end of the tail. In the adult certain vertebræ have become fused to form a single bony mass with which the pelvic girdle articulates. Vertebræ so fused are termed =fixed= or =“false” vertebræ= (Vertebræ immobiles), as distinguished from the =movable= or =“true” vertebræ= (Vertebræ mobiles).

The column is subdivided for description into =five regions=, which are named according to the part of the body in which they are placed. Thus the vertebræ are designated as =cervical=, =thoracic= (or dorsal), =lumbar=, =sacral=, and =coccygeal= or =caudal= (Vertebræ cervicales, thoracales, lumbales, sacrales, coccygeæ). The number of vertebræ in a given species is fairly constant in each region except the last, so that the =vertebral formula= may be expressed (for the horse, for example) as follows:

C{7}T{18}L{6}S{5}Cy_{15–21}.

FIG. 4.—FIRST THORACIC VERTEBRÆ OF HORSE.

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.