The Philosophy of Health; Volume 1 (of 2) / or, an exposition of the physical and mental constitution of man

by Southwood Smith

View of some of the vertebræ, which by their union form
the spinal column.

79. Both the upper and under edges of the arch form a notch (fig. XLVI. 7.), which, together with a corresponding notch in the contiguous vertebra, completes another aperture rounder and smaller than the former, but still of considerable size (fig. XLVI. 7.), the passage of the spinal nerves (fig. XLVII. 3).

80. From both the upper and under sides of the arch proceed two short but strong projections of bone (fig. XLVI. 8.), termed the articulating processes, because it is chiefly by these processes that the vertebræ are connected together. From the beginning to the end of the series, the two upper processes of the one vertebra are united with the two lower processes of the vertebra immediately above it (fig. XLVI. 9), and around the edges of all the articulating processes are visible rough lines, which mark the places to which the articulating ligaments are attached.

81. No vertebra, except the first, rests immediately upon its contiguous vertebra (fig. XLV. 2, 4). Each is separated from its fellow by a substance of a peculiar nature interposed between them, termed the intervertebral substance (figs. XLVII. 2, and L. 2). This substance partakes partly of the nature of cartilage, and partly of that of ligament. It is composed of concentric plates, formed of oblique fibres which intersect each other in every direction. This substance, for about a quarter of an inch from its circumference towards its centre, is tough, strong, and unyielding; then it becomes softer, and is manifestly elastic; and so it continues until it approaches the centre, when it becomes pulpy, and is again inelastic. The exterior tough and unyielding matter is for the firmness of the connexion of the several vertebræ with each other; the interior softer and elastic matter is for the easy play of the vertebræ upon each other; the one for security, the other for pliancy. And the adjustment of the one to the other is such as to combine these properties in a perfect, manner. The quantity of the unyielding substance is not so great as to produce rigidity; the quantity of the elastic substance is not so great as to occasion insecurity. The firm union of its solid matter renders the entire column strong; the aggregate elasticity of its softer substance renders it springy.

1. One of the Lumbar vertebræ. 2. Intervertebral substance. 3. A portion of the spinal cord in its canal.

82. The column is not constructed in such a manner as to admit of an equal degree of motion in every part of it. Every thing is contrived to give to that portion which belongs to the neck freedom of motion, and, on the contrary, to render that portion which belongs to the back comparatively fixed. In the neck the mechanism of the articulating processes is such as to admit of an equal degree of sliding motion forwards, backwards, and from side to side, together with a turning motion of one bone upon another; at the same time, the intervertebral substance between the several vertebræ is thick. In consequence of this mechanism, we can touch the breast with the chin, the back with the hind head, and the shoulders with the ear, while we can make the head describe more than a semicircle. But, in the back, the articulating processes are so connected as to prevent the possibility of any motion, either forwards or backwards, or any turning of one vertebra upon another, while the intervertebral substance is comparatively thin (fig. XLV. 2, 4). That portion of the column which belongs to the back is intended to afford a fixed support for the ribs, a support which is indispensable to their action in the function of respiration. In the loins, the articulating processes are so connected as to admit of a considerable degree of motion in the horizontal direction, and from side to side, and the intervertebral substance here progressively increases in thickness to the point at which the upper portion of the column is united to the sacrum (fig. XLV. 2, 4), where the degree of motion is extensive.

83. The canal for the spinal cord, formed partly by the concavity in the posterior surface of the vertebra, and partly by the arch that springs from it (fig. XLVI. 6.), is lined by a continuation of the dense and strong membrane that constitutes the internal periosteum of the cranium, the dura mater (fig. XLVIII. c), which, passing out of the opening in the occipital bone, called the foramen magnum (figs. XXXIX. 7, and XLIX. 3), affords a smooth covering to the canal throughout its whole extent.