The dorsal vertebrae are twelve in number: the neural canal, formed as in the cervical region, contains the corresponding segments of the myelon; the haemal canal is completed by the ribs (pleura-pophyses), the costal cartilages (haemapophyses), and the sternum (the coalesced and expanded haemal spines): it contains that great varicose enlargement of the vascular system, the heart, with its offshoots, the lungs, and the main arterial and venous trunks. There are no lateral canals, as were seen in the neck, for the vertebral arteries; the parapophyses, or the anterior transverse processes are not developed, and the pleurapophyses, no longer confluent with other parts of the vertebrae, are movable bones, somewhat displaced, so as to articulate by a round head with two contiguous vertebrae, and with their intervening fibro-cartilage; and by a tubercle with the extremity of the diapophysis of the superior of the.two vertebrae. Instances of this displacement of the vertebral elements are not uncommon. They may be seen in the connection of the haemal arches with the caudal vertebrae in the kangaroo, the ferret, etc.; and in the disposition of the vertebral segments of the cranium, where the elements undergo much greater change and variety of form and relation than in other parts of the body.
The centra of the dorsal vertebrae, constricted in the middle, are more convex and prominent anteriorly than those of the cervical or of the lumbar regions. And this prominence is the more marked in consequence of the absence of the parapophyses, which, springing from the sides of the centra, give to the cervical vertebrae their flat appearance when viewed from the front in the articulated spinal column.
The diapophyses incline outwards and backwards from between the zygapophyses; they are longer than those of the cervical vertebrae, but they decrease in length from above downwards. About the ninth dorsal vertebra the diapophyses effloresce into three points, which become more strongly marked in the succeeding vertebras, as far down as the third lumbar, where the diapophyses usually resume their single rounded extremity. The extreme point of the three is attached by ligament to the tubercle of the pleurapophysis, and is to be regarded as the extremity of the diapophysis: that nearest the neural spine, is an exogenous process, the epi-diapophysis. In many quadrupeds this is so far elongated as to reach the level of the neural spine, when the vertebral column (viewed posteriorly) presents three rows of spines instead of one. In the. armadillo these lateral spines afford great support to the heavy armour which rests upon the back.
The process of bone between the two preceding, projects downwards, and terminates in a point. Traces of it may be seen, in well marked skeletons, as far as the fourth or even the fifth lumbar vertebra, but it gradually becomes fainter. It is called the metapophysis,* and its relations may be better studied in the skeleton of the monkey than of man. We there find it a long depending bony process, which passes down to the outer side of the superior articulating process of the vertebra below, which is thus wedged in between two bony plates, viz.: the articulating process, or zygapophysis, to which it is properly attached, internally, and the long depending metapophysis, externally. The articulations of the vertebras are rendered more secure by these means; but though they readily allow the spine to curve backwards and forwards, as in the spring of the lion or cat, they do not permit with equal readiness the lateral swaying movement of the
trunk, so necessary to maintain the balance in progression in the erect posture. The long metapo-physis of the monkey serves to limit, and interfere with this movement. In man, on the other hand, the process is short and stunted; but in both it affords attachment to that assemblage of muscular and tendinous slips, known as the longissimus dorsi muscle, part of. the common erector spinas, which extends along the whole length of the vertebral column.
* Metapophysis, with, along with.