The true vertebra are divided into three sets, named from the regions they occupy, cervical, dorsal, lumbar. They present, 1. certain general characters by which they may at once be distinguished from bones of any other class; 2. those of each region (cervical, dorsal, lumbar,) exhibit peculiar characters by which they are severally distinguished ; 3. certain vertebras present special or individual characters.
1. General charcters of a vertebra
The objects presented by each vertebra are, a ring, a body, articulating processes, transverse and spinous processes, and notches. Of these, the ring, or foramen, merely to suit the purposes of methodical description, may be considered the central part. As the whole series of vertebrae is intended to form a pillar of support, each, with the ex- ception of the first, presents in front a convex mass (the body), which is a section of a cylinder, and which, by being piled one over the other, form the pillar. As each bone must be securely joined with the one above it and that below it, there exist certain prominences (articu- lating processes) for articulation with them. The column being flexible, and partaking in the several movements of the body, it is required that there be levers (transverse and spinous processes) for the attach- ment of the muscles or moving powers. Finally, it is necessary that a free communication should be allowed for the nerves with the nervous centre contained in the canal, and this purpose is served by the notches.
The various parts of a vertebra here named require more detailed notice.
The ring (foramen vertebrale, rachidium,) is formed in front by the body, and posteriorly by what is named the arch of the vertebrae, from which latter the several processes project. As the vertebrae are piled one over the other, the rings are arranged so as to form, with the aid of the interposed ligamentous structures, a flexible canal for the lodgment and protection of the spinal cord.
The body forms the anterior and most considerable part of the bone.
Rounded before, and marked in the middle by a transverse groove, which gives it a constricted appearance, it is slightly hollowed posteriorly, where it contributes to the formation of the vertebral canal, and in most instances is flat on the superior and inferior surfaces, by which, through the medium of fibro-cartilage, it is connected with the contiguous bones. Its outer surface all round presents numerous foramina for the passage of blood-vessels, principally veins. One of the holes situated about the middle of the posterior surface exceeds the others very much in size ; it lodges a large vein.
From the body at its lateral margins two processes pass backwards, called " pedicles'." The pedicles join with the laminae, or plates, and these, by inclining inwards, meet at the median line posteriorly, so as to complete the " arch" of the vertebra. From the point of junction of the pedicles with the laminae at each side, the articulating and transverse processes project ; and from the union of the two laminae the spinous process takes its origin.
1.1. Articulating or oblique processes
For maintaining the connexion between the contiguous vertebrae, there are four processes, — two superior, and two inferior, — which project, one on each side, from the junction of the laminae with the pedicle. Two of these processes project upwards, and two downwards ; the smooth surfaces of the upper pair look backwards, those of the lower, forwards ; they are coated with cartilage, and articulate with corresponding processes of the next vertebrae. Their margins are rough for the attachment of ligaments.
The transverse and spinous processes form a series of levers for the attachment of muscles. The transverse processes, two in number (one on each side), and named from their direction, project laterally from the arch near the articulating processes, between which their bases are interposed. The spinous process is a single projection, situated posteriorly in the median line; this process, or rather the ap- pearance presented by the aggregation of those of the several vertebrae, has given to the entire column one of its designations (spine).
1.2. Laminae, or plates
The parts of the arch which intervene between the bases of the spinous and the transverse processes are thus named.
1.3. Pedicles and notches
Lastly, the processes which extend from the plates to the body of the bone, are called "pedicles," as above stated.
In each pedicle are seen two excavations, or notches (incisurae), one on the upper, the other on the lower border, the latter being deeper than the former. When the vertebrae are placed in their natural position, the notches in the contiguous margins of each pair of them form rounded apertures, which communicate with the vertebral canal, and give transmission to the spinal nerves and to the entering and emerging - vessels. From their position and mode of formation, they are called the intervertebral foramina.
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