Only in those species, as Man and the smaller kinds of the Primates and some other orders, in which the brain holds a large relative proportion to the rest of the body, 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 number, the ridges and crests on the cranium for the attachment of the muscles necessary to put this apparatus in motion, and 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 assume their distinctive development characteristic of the species. The hyoidean apparatus in mammals (Fig. 6) supports the tongue and larynx, and consists of an inferior median portion termed the basihyal, from which two pairs of half arches, or cornua, extend upwards and outwards. The anterior is the more 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 stylohyal. The posterior cornu (thyrohyal) is united at its extremity with the thyroid cartilage of the larynx, which it suspends in position. The median portion, or basihyal, is sometimes, as in the Howling Monkeys, enormously enlarged and hollowed, admitting into its cavity an air-sac connected with the organ of voice. [Illustration: FIG. 9.—Anterior surface of Human thoracic vertebra (fourth). _c_, Body or centrum; _nc_, neural canal; _p_, pedicle, and _l_, lamina of the arch; _t_, transverse process; _az_, anterior zygapophysis.]