The irregular bones, from their peculiar form, cannot be grouped under the preceding heads. They consist of spongy substance enclosed within a thin layer of compact bone.
Surfaces of bones -The surfaces of bones present many and variable features, which call for the use of a number of special descriptive terms. Smooth areas for articulation with other bones are known as articular surfaces and, when small, are frequently termed facets. A condyle is a smooth rounded projection, and a trochlea a pulley-shaped surface; both are covered with articular cartilage in the recent state.
Depressions on bony surfaces are usually termed fossae. They may be large or small, rough or smooth, non-articular or articular.
Any localized elevation or projection on a bony surface constitutes a process. A pointed process is called a spine, but the term is frequently applied to elongated processes with blunt extremities, e.g. the spines of the vertebrae. The terms tubercle and tuberosity are used, without much distinction, for localized, rounded elevations, which may possess smooth or roughened surfaces. An epicondyle is an elevation placed above an articular surface. A hamulus is a hook-like process, and a cornu a horn-like process. A sharp, distinct ridge, whether rough or smooth, is termed a crest, and if it is wide enough to possess borders they are known as lips. A low, narrow ridge is termed a line.
A hole in a bone is known as a foramen, and the term is often applied to the opening of a bony tunnel, which is termed a canal. A groove or furrow is frequently called a sulcus, a notch an incisura, a gap a hiatus, and a thin sheet or plate a lamina.
Many other terms are employed occasionally, but those already defined have the widest use.
THE VERTEBRAL COLUMN [COLUMNA VERTEBRALIS]
In all vertebrate animals the central axis of, the body consists of a vertebral column. As it is essential that provision should be made for a considerable range of movement of the trunk, the column consists, not of a single elongated bone, but of a number of independent, irregular bones, termed the vertebra, which are firmly connected to one another but which are capable of a limited amount of movement on one another. The provision of a central axis is not the only function which the column has to subserve. It is built up so as to surround the spinal cord, to which it affords necessary protection. The human vertebral column must also support the weight of the trunk and transmit it to the lower limbs.
The vertebrae are grouped under the names cervical, thoracic, lumbar, sacral and coccygeal or caudal, according to the region in which they lie, but all the vertebra’, not only of man but also of all vertebrate animals, conform to a general ground-plan, and although, at first sight, there may be little resemblance between a cervical vertebra of a giraffe and a human lumbar Vertebra, the essential features of both will be found to be identical.
THE GENERAL CHARACTERISTICS OF A VERTEBRA
A typical vertebra (fig. 265) consists of two principal parts, an anterior or ventral, termed the body, and a posterior or dorsal, termed the vertebral arch; these enclose a foramen, which is named the vertebral foramen.
In the articulated column the bodies and the intervertebral discs (fibro cartilages) interposed between them form a continuous pillar, which constitutes the central axis of the body and, in man, supports and transmits the weight of the head and trunk. The vertebral foramina constitute a canal in which the spinal cord is lodged and protected. Between contiguous vertebra; two intervertebral foramina, one on each side, open into the canal and serve for the transmission of the spinal nerves and vessels.
The body of a vertebra is more or less cylindrical, but is subject to a wide range of variation in size and shape in different animals and in different regions of the same animal. Its upper and lower surfaces are flattened and roughened to give attachment to the intervertebral discs (fibro cartilages). In front, it is convex from side to side and gently concave from end to end; behind, it is flattened or slightly concave from side to side, and flat froze end to end. On its anterior surface there are a few small apertures for the passage of nutrient vessels; on its posterior surface there is a large irregular aperture (occasionally more than one) for the exit of the basivertebral veins (fig. 266).
The vertebral arch consists of a pair of pedicles and a pair of laminae; it supports seven processes, viz. four articular , two transverse and one pair spinous.
The pedicles (roots of the vertebral arches) are a pair of short, thick processes, which project backwards from the body at the junctions of its lateral and posterior surfaces. The concavities above and below the pedicles are named the vertebral notches; and when the vertebrae are articulated with one another, the notches of contiguous vertebra form the intervertebral foramina, already referred to.
The lamince are broad plates directed backwards and medially from the pedicles. They fuse in the spine posteriorly, and so complete the posterior boundary of the vertebral foramen.
The articular processes, two superior and two inferior, spring from the junctions of the pedicles and lamina. The superior processes project upwards, and their articular surfaces are directed more or less backwards; the inferior project down wards and their articular surfaces look more or less forwards. These processes meet the corresponding processes of the adjoining vertebrae and, while permitting a certain degree of movement, definitely control and restrict its range.
The transverse processes project laterally from the junctions of the pedicles and laminate; they serve for the attachment of muscles and ligaments and are the levers by means of which the rotator and lateral movements of the vertebra can be effected. In addition, in the thoracic region they articulate with and limit the movements of the ribs.
The costal elements develop as essential constituent parts of each vertebral arch. In certain regions (the thoracic region only) they become independent units-the ribs-which articulate with the vertebral column. In other regions they remain stunted and, almost unrecognizable in form, become fused with the vertebra. Originally protective in function, in higher forms they also act as levers which play an important part in the movements of respiration.
Structure of a vertebra (fig. 266).-The body of a vertebra is composed of spongy substance covered by a thin coating of compact bone, which presents numerous orifices for the passage of vessels; the interior of the body is traversed by one or two large canals, for the transmission of veins, which converge towards the large aperture on the posterior surface of the body. In the vertebral arch and the processes projecting from it the compact substance is especially thickened.
In the light of this description of a typical vertebra it is now possible to study the individual vertebra; of the human vertebral column, and observe how the essential features are modified in the different regions. In each region the vertebrae exhibit certain group characters, but, at its upper and lower limits, atypical characters make their appearance for the purpose of adapting the vertebrae concerned to their neighbors.
In man the cervical vertebra: are seven in number; the thoracic, twelve; the lumbar, five; the sacral, five; and the coccygeal, four; making a total of thirty-three. The cervical, thoracic and lumbar vertebrae are separate bones throughout life and are therefore known as the movable vertebrae; the sacral and coccygeal, on the other hand are termed fixed vertebrae, because, owing to the necessity for stability in this part of the column in man, they are united in the adult to form two bones, viz. the sacrum and the coccyx.
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