91. Alike admirable is the mechanism by which the separate pieces of the column are joined together. If but one of the bones were to slip off its corresponding bone, or to be displaced in any degree, incurable paralysis, followed ultimately by death, or instantaneous death, would happen; for pressure on the spinal cord in a certain part of its course is incompatible with the power of voluntary motion, and with the continuance of life for any protracted term; and in another part of its course, with the maintenance of life beyond a few moments. To prevent such consequences, so great is the strength, so perfect the attachment, so unconquerable the resistance of that portion of the intervertebral substance which surrounds the edge of the bodies of the vertebræ, that it will allow the bone itself to give way rather than yield. Yet such is the importance of security to this portion of the frame, that it is not trusted to one expedient alone, adequate as that might seem. Besides the intervertebral substance, there is another distinct provision for the articulation of the bodies of the vertebræ. Commencing at the second cervical vertebra, in its fore part, and extending the whole length of the column to the sacrum, is a powerful ligament, composed of numerous distinct longitudinal fibres (fig. L.), which are particularly expanded over the intervals between the bones occupied by the intervertebral substance (figs. L. 1, and LI. 2, 2). This ligament is termed the _common anterior vertebral_, beneath which, if it be raised from the intervertebral substance, may be seen small _decussating_ fibres, passing from the lower edge of the vertebra above, to the upper edge of the vertebra below (fig. L. 3), from which circumstance these fibres are termed _crucial_. [Illustration: Fig. L. 1. Common anterior ligament; 2. intervertebral substance. The anterior ligament is removed to exhibit (3.) the crucial fibres passing over it.] [Illustration: Fig. LI. 1. Portion of the occipital bone; 2. common anterior ligament.] 92. Corresponding with the ligament on the anterior, is another on the posterior part of the spine (fig. LII. 1), which takes its origin from the foramen magnum (fig. LII. 1); descends from thence, within the vertebral canal, on the posterior surface of the bodies of the vertebra (fig. LII. 1), and extends to the sacrum. This ligament is termed the _common posterior vertebral_, which, besides adding to the strength of the union of the bodies of the vertebræ, prevents the column itself from being bent too much forward. [Illustration: Fig. LII. 1. Posterior vertebral ligament.] 93. Moreover, the bony arches of the vertebræ (fig. LIII. 1) are connected by means of a substance partly ligamentous, and partly cartilaginous (fig. LIII. 2), which, while it is extremely elastic, is capable of resisting an extraordinary degree of force. [Illustration: Fig. LIII. 1. Arches of the vertebræ seen from within; 2. ligaments connecting them.]