356 MAMMALIA [SKELETON. the spinal cord. It consists of a basal axis, continuous serially with the axes or centra of the vertebrae, and of an arch above, roofing over and enclosing the cavity which contains the cephalic portion of the central nervous sys tem (see fig. 4). The base with its arch is composed of three segments placed one before the other, each of which is comparable to a vertebra with a greatly expanded neural arch. The hinder or occipital segment consists of the basi-occipital, ex-occipital, and supra-occipital bones; the middle segment of the basi-sphenoid, ali-sphenoid, and parietal bones; ani the anterior segment of the pre- sphenoid, orbito-sphenoid, and frontal bones. The axis is continued forwards into the mesethmoid, or septum of the nose, around which the bones of the face are arranged in a manner so extremely modified for their special purposes that anatomists who have attempted to trace their serial homologies with the more simple portions of the axial skeleton have arrived at very diverse interpretations. The characteristic form and structure of the face of mammals is mainly dependent upon the size and shape of (1) the orbits, a pair of cup-shaped cavities for containing the eye ball and its muscles, which may be directed forwards or laterally, placed near together or wide apart, and may be completely or only partially encircled by bone; (2) the nasal fossae, cavities on each side of the median nasal septum, and forming the passage for the air to pass between the external and the internal nares, and contain ing in their upper part the organ of smell; (3) the zygomatic arch, a bridge of bone for the purpose of muscular attachment, which extends from the side of the face to the skull, overarching the temporal fossa; (4) the roof of the mouth, with its alveolar margin for the implantation of the upper teeth. The face is completed by the mandible, or lower jaw, consisting of two lateral rami, articulated by a hinge joint with the squamosal (a cranial bone interposed between the posterior and penul timate segment of the brain case, where also the bony capsule of the organ of hearing is placed), each being com posed of a single solid piece of bone, and united together in the middle line in front, at the symphysis, which union may be permanently ligamentous or become completely ossified. Into the upper border of the mandibular rami the lower teeth are implanted. In only those species in which the brain holds a large relative proportion to the rest of the body, as in Man and the smaller species of the Primates and some other orders, does the external form of the skull receive much impress from the real shape of the cavity containing the brain. The size and form of the mouth, and the modifications of the jaws for the support of teeth of various shape and num ber, the ridges and crests on the cranium for the attach ment of the muscles necessary to put this apparatus in motion, outgrowths of bone for the enlargement of the external surface required for the support of sense organs or of weapons, such as horns or antlers (which outgrowths, to prevent undue increase of weight, are filled with cells containing air), cause the principal variations in the general configuration of the skull. These variations are, however, only characteristically developed in perfectly adult animals, and are in many cases more strongly marked in the male than the female sex. Throughout all the later stages of growth up to maturity the size and form of the brain case remain comparatively stationary, while the accessory parts of the skull rapidly increase and take on their distinctive development characteristic of the species. The hyoidean apparatus in mammals supports the tongue and larynx, and consists of a median portion below, the basi-hyal, from which two pairs of half arches, or cornua, extend upwards and outwards. The anterior is the most important, being connected with the periotic bone of the cranium. It may be almost entirely ligamentous, but more often has several ossifications, the largest of which is usually the stylo-hyal. The posterior cornu (thyro-hyal) is united at its extremity with the thyroid cartilage of the larynx, which it suspends in position. The median portion, or basi-hyal, is sometimes, as in the Howling Monkeys, enormously enlarged and hollowed, admitting into its cavity an air-sac connected with the organ of voice. The vertebral column consists of a series of distinct Ver bones called vertebrae, arranged in close connexion with colv each other along the dorsal side of the neck and trunk, and in the median line. 1 It is generally prolonged posteriorly beyond the trunk, to form the axial support of the appendage called the tail. Anteriorly it is articulated with the occipital region of the skull. The number of distinct bones of which the vertebral column is composed varies greatly among the Mammalia, the main variation being due to the elongation or otherwise of the tail. Apart from this, in most mammals the number is not far from thirty, though it may fall as low as twenty-six (as in some Bats), or rise as high as forty (Hyrax and Choloepus). The different vertebrae, with some exceptions, remain through life quite distinct from each other, though closely connected by means of fibrous structures which allow of a certain, but limited, amount of motion between them. The exceptions are the following : near the posterior part of the trunk, in nearly all mammals which possess completely developed hinder limbs, two or more vertebras become ankylosed together to form the " sacrum," the portion of the vertebral column to which the pelvic girdle is attached; also, in certain species of Whales and of Armadillos, there are constant ossific unions of certain vertebrae of the cer vical region. Although the vertebrae of different regions of the column of the same animal or of different animals present great diversities of form, there is a certain general resemblance among them, or a common plan on which they are con structed, which is more or less modified by alteration of form or proportions, or by the addition or suppression of parts to fit them to fulfil their special purpose in the economy. An ordinary or typical vertebra consists in the first place of a solid piece of bone, the body or centrum (fig. 5, c), of the form of a disk or short cylinder. The bodies of contiguous vertebrae are con nected together by a very dense, tough, and elastic ma terial called the " intervertebral substance," of peculiar and com plex arrangement. This sub stance forms the main, and in some cases the only, union between the vertebrae. Its elasticity provides for the verte brae always returning to their normal relation to each other onrl frt tho riVlnmn won oral I v Flo 5. Anterior Surface of Human ana to the column generally, Thoracic Ver tebra (Fourth). <-, When they have been dis- body or centrum; we, neural i i . i e i_ i canal; p, pedicle, and /, lamina turbed therefrom by muscular O f the areh; <, transverse process; action. A process (p) arises . anterior zygapophysU. on each side from the dorsal surface of the body. These, meeting in the middle line above, form together an arch, surmounting a space or short canal (nc). As in this space lies the posterior prolongation of the great cerebro- spinal nervous axis, or spinal cord, it is called the neural canal, and the arch is called the neural arch, in con tradistinction to another arch on the ventral surface of the 1 For the sake of uniformity, in all the following descriptions of the vertebral column, the long axis of the body is supposed to be in the
horizontal position.