(1638). A vertebra consists, in its typical completeness, of the elements or parts represented in the following diagram: -

Fig. 307.

(1639). The names in the above diagram printed in Roman type signify those parts which, being usually developed from distinct and independent centres, have been named autogenous elements. The italics denote the parts more properly called processes, which shoot out as continuations from some of the preceding elements, and are termed exogenous.

(1640). The autogenous elements generally circumscribe holes about the centrum, which in the chain of vertebrae form canals. The most constant and extensive canal is that formed above the centrum (fig. 307) for the lodgment of the main trunk of the nervous system (neural axis) by the elements thence termed neurapophyses. The second canal, below the centrum (fig. 307), is in its entire extent more irregular and interrupted; it lodges the central organ and large trunks of the vascular system (haemal axis), and is usually formed by the laminae which are therefore called hcemapophyses. At the sides of the centrum, most commonly in the cervical region, a canal is circumscribed by the pleurapo-physis, or costal process, by the parapophysis, or lower transverse process, and by the diapophysis, or upper transverse process, which canal includes a vessel, and often also a nerve.

(1641). Thus, a typical or perfect vertebra, with all its elements, presents four canals or perforations around a common centre; such a vertebra we find in the thorax of Man, and most of the higher classes of vertebrates, also in the neck of many birds. In the tails of most reptiles and mammals the inferior are articulated or anchylosed to the under part of the central elements, space being needed there only for the caudal artery and vein. But where the central organ of the circulation has to be lodged, an expansion of the haemal arch takes place, constituting a thorax. Accordingly, in order to construct the thoracic cavity, the pleurapophyses (fig. 307) are much elongated, and the haemapophyses (fig. 307) are removed from the centrum, and are articulated to the distal ends of the pleurapophyses, the bony hoop being completed by the intercalation of the haemal spine (fig. 307) between the ends of the haemapo-physes. And this spine is here sometimes as widely expanded (in the thorax of Birds and Chelonians for example) as is the neural spine (parietal bone or bones) of the middle cranial vertebra of Mammals. In both cases also it may be developed from two lateral halves; and a bony intermuscular crest may be extended from the mid-line, as in the skull of the Hyena and the breast-bone of Birds.

(1642). The ossified parts of the abdominal vertebrae of osseous Pishes answer to the centrum, the neurapophyses, the neural spine, the parapophyses, the pleurapophyses, and certain appendages to be hereafter noticed.

(1643). In the air-breathing Vertebrata, in which the heart and breathing-organs are transferred backwards to the trunk, the corresponding osseous segments of the skeleton are in most instances developed in their typical completeness, in order to encompass and protect those organs. The thoracic haemapophyses in the Crocodiles are partially ossified, and in Birds completely so, in which class the haemal spines of the thorax coalesce together, become much expanded laterally, and usually develope a median crest downwards, to increase the surface of attachment for the great muscles of flight. This speciality is indicated by the name "sternum," applied to the confluent elements in question.

(1644). The typical thoracic vertebrae in Birds support diverging appendages, either anchylosed, as in most, or articulated, as in the Penguin and Apteryx, to the posterior border of the pleurapophysis. The function of such appendages in this form of typical vertebra is to connect one haemal arch with the next in succession, so as to associate the two in action, and to give firmness and strength to the whole thoracic frame.

(1645). The diverging appendages are, as might be expected, of all the elements of the vertebral segment, the least constant in regard to their existence, and the subjects of the greatest amount and variety of modification. Simple, slender spines or styles in Fishes (fig. 311,13) - simple plates retaining long their cartilaginous condition in Crocodiles - short, flat, slightly-curved pieces in most Birds, - such, with one exception, is the range of the variety of form to which these parts are subject in the segments of the trunk. But that exception is a remarkable one, inasmuch as we are enabled to trace the diverging appendage of that vertebral segment of the body, which from its form and character constitutes the pelvic arch, through various progressive phases of development - from that of a simple, articulated, solitary ray, such as exists in the Lepidosiren, through innumerable modifications, whereby it is adapted for swimming, steering, balancing, and anchoring - for exploration, for burrowing, creeping, walking, and running - for leaping, seizing, climbing, or sustaining erect the entire frame of the animal - under the general appellation of the posterior or pelvic limb.

(1646). Any given appendage, however, as Professor Owen * justly observes, might have been the seat of such developments as convert that of the pelvic arch into a locomotive limb; and the true insight into the general homology of limbs enables us to point out many potential pairs in the typical endoskeleton. The possible and conceivable modifications of the vertebrate archetype are far from having been exhausted in the forms that have hitherto been recognized, from the primaeval fishes of the palaeozoic ocean of this planet up to the present time; or, in other words, it would be by no means contrary to the general laws of osteogenic development, however different from the ordinary course of nature, were vertebrate animals to occur possessed of more than the two pairs of locomotive extremities usually conferred; so that such beings as hippogriffs and other winged quadrupeds, however fabulous, would be by no means monstrous productions.