The spinal column is composed of vertebrae whose bodies are separated by elastic fibro-cartilages. The vertebrae articulate with each other at four other points, two on the upper and two on the lower surface of the arch. They are also bound together by firm ligaments in front of and behind their bodies. The effect of these various connections is that, even when the spine is separated from all its attachments except the ligaments, it retains its general form, and its natural conformation may be studied after its removal from the body.
When seen in profile the spine presents the well-known antero-posterior curves, the convexity being forwards in the cervical and lumbar regions, and backwards in the dorsal. These curves are capable of considerable variation in the movements of the body. The whole of the curves may be obliterated and converted into a general convexity backwards by stooping forwards, as when, with the arms extended and the legs straight, an attempt is made to touch the toes with the tips of the fingers. By arching the body backwards the dorsal curve may be partially obliterated, and a general convexity forwards produced. It appears, therefore, that the spine is capable of considerable antero-posterior movement. These anteroposterior movements imply a considerable degree of compressibility of the intervertebral cartilages. • The combined cartilages occupy about a fifth of the entire length of the spinal column, and their compressibility may be inferred from the fact that during the retention of the erect posture the entire length of the column gradually diminishes, so that the stature is usually half an inch to three-quarters less at night than in the morning. This is believed to be due chiefly to the compression of the cartilages, which recover at night when the recumbent posture is assumed. The antero-posterior movement of the spine is freest in the cervical and lumbar regions, and most limited in the dorsal.
The spine is capable to a much more limited extent of lateral movement. The articulating processes, being situated on either side of the arches, prevent any considerable lateral deviation, as they become locked against each other when that occurs. If the surfaces of these processes were horizontal, facing one another above and below, then they might allow of freer lateral movement, but the more they assume the perpendicular position, and the more they face inwards and outwards, the greater is the impediment to lateral movement. It will be found that on passing from above downwards the articulating surfaces assume more and more of a perpendicular position. In the cervical region they are oblique, and face slightly inwards and outwards; in the dorsal they are more perpendicular and face nearly forwards and backwards, while in the lumbar region they are nearly perpendicular, and face each other nearly inwards and outwards. In this way it occurs that while lateral movement is limited in all regions it is almost impossible in the lumbar region. For a similar reason, twisting of the spine on its axis is possible to a very limited extent.
The question of the existence of a natural lateral curvature has been matter of dispute. It is generally stated that there is a slight lateral deviation to the right in the upper dorsal region, and this is usually ascribed to the more frequent and forcible exertion made with the right arm; but the existence of this curve has been seriously questioned (Adams). The late Dr. Foulis in 110 post-mortem examinations found lateral deviation in no less than 58 cases. He did not observe it specially in the upper dorsal region or towards the right, and concluded that it was due to the positions habitually assumed by the persons at their various trades. We may perhaps conclude that normally there is no lateral curvature, but that a very slight permanent deviation is often assumed when a frequently repeated position of the body predisposes to it.
The function of the spine is to support the structures attached to it, and to hold the head erect, the latter function being in man the more prominent one. Any single curvature of the spine will have the tendency to remove the head from the erect position, and tilt it backwards or forwards or to one side, and in order to preserve the erect position there is required a curvature in the opposite direction. The natural antero-posterior curves are in this sense mutually compensatory, the lumbar restores the position lost by the sacral curve, and the cervical that of the dorsal. When abnormal curvatures occur there is a tendency to a similar compensation, so that these curvatures may be divided into Primary and Secondary. It will not be necessary to consider in detail the secondary curvatures; their amount and direction may be inferred from those of the primary ones. It may be stated, however, that there are, frequently, several secondary or compensating curves, the spine presenting several sinuosities in order to reach the stable position for the head.