of the basioccipital, exoccipital, and supraoccipital bones; the middle segment of the basisphenoid, alisphenoid, and parietal bones; and the anterior segment of the presphenoid, orbitosphenoid, 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 eyeball 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 fossæ, or cavities on each side of the median nasal septum, forming the passage for the air to pass between the external and the internal nares, and containing 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 penultimate segment of the brain-case, where also the bony capsule of the organ of hearing is placed), each being composed of a single solid piece of bone, and the two 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. [Illustration: FIG. 6.—Longitudinal and vertical section of the skull of a Dog (_Canis familiaris_), with mandible and hyoid arch. _an_, Anterior narial aperture; _MT_, maxillo-turbinal bone; _ET_, ethmo-turbinal; _Na_, nasal; _ME_, ossified portion of the mesethmoid; _CE_, cribriform plate of the ethmo-turbinal; _Fr_, frontal; _Pa_, parietal; _IP_, interparietal; _SO_, supraoccipital; _ExO_, exoccipital; _BO_, basioccipital; _Per_, periotic; _BS_, basisphenoid; _Pt_, pterygoid; _AS_, alisphenoid; _OS_, orbitosphenoid; _PS_, presphenoid; _PI_, palatine; _VO_, vomer; _Mx_, maxilla; _PMx_, premaxilla; _sh_, stylohyal; _eh_, epihyal; _ch_, ceratohyal; _bh_, basihyal; _th_, thyrohyal; _s_, symphysis of mandible; _cp_, coronoid process; _cd_, condyle; _a_, angle; _id_, inferior dental canal. The mandible is displaced downwards, to show its entire form; the * indicates the part of the cranium to which the condyle is articulated.[9]]