A segment of sternum is included as the "haemal spine" of the vertebra (_hs_); the vertebral rib is the "pleurapophysis" (_pl_); the sternal rib the "haemapophysis" (_h_); the uncinate process of the vertebral rib is known as the "diverging appendage" (_a_). The whole vertebrate skeleton is composed of a series of vertebrae which show these typical parts. We arrive thus at the conception of an "Archetype" of the vertebrate skeleton, such as is represented in Fig. 6. The archetype is only a scheme of what is usually constant in the vertebrate skeleton, and both the number and the arrangement of the bones in any real Vertebrate are subject to variation. "It has been abundantly proved," Owen writes, towards the end of his volume, "that the idea of a natural segment (vertebra) of the endoskeleton does not necessarily involve the presence of a particular number of pieces, or even a determinate and unchangeable arrangement of them. The great object of my present labour has been to deduce ... the relative value and constancy of the different vertebral elements, and to trace the kind and extent of their variations within the limits of a plain and obvious maintenance of a typical character" (p. 146). It goes without saying that Owen considered the skull to be formed of vertebrae--the vertebral theory of the skull was, in his system, a deduction from the vertebral theory of the skeleton. He recognised four cranial vertebrae; the arrangement of them, and the relation of their constituent bones to the parts of the typical vertebra are shown in the table appearing on page 106. So far as their first three elements are concerned, these vertebrae are practically identical with the vertebrae distinguished in the classical vertebral theory of the skull, as enunciated by Oken. A divergence appears with the determination of the other elements of the vertebrae. The upper and lower jaws are associated with the nasal and frontal vertebrae respectively, not however as limbs of the head, but as constituent elements of these vertebrae. In the same way the hyoid apparatus is part and parcel of the parietal vertebra, and the pectoral girdle and fore-limbs part of the occipital vertebra. [Illustration: FIG. 6.--The Archetype of the Vertebrate Skeleton. (After Owen.)] Cranial Vertebrae.[164] (After Owen, 1848, p. 165.)