=Hyoid Bone.= =Os hyoideum= (Fig. 38 and Fig. 104).--The hyoid bone forms the support for the tongue and gives origin to muscles passing to the tongue and larynx. It also supports the thyroid cartilage (Fig. 104, 1).
[Illustration: FIG. 38.--HYOID BONE, DORSAL VIEW.
_a_, body; _b_, _c_, _d_, _e_, cranial cornu; _f_, caudal cornu; _b_, ceratohyal; _c_, epihyal; _d_, stylohyal; _e_, tympanohyal; _f_, thyrohyal.]
It consists of a transverse bony bar, the =body= (Fig. 38, _a_) and of two =cornua= or horns attached to each end of the body.
The cranial cornu (lesser cornu of human anatomy) is the longer (Fig. 38, _b_-_e_). Each arises from the cranial face of the body at its lateral end, curves laterad, and then caudodorsad. It consists of four bony pieces movably united by cartilage.
The terminal piece is the =tympanohyal= (_e_); it is imbedded in the tympanic bulla just ventrad of the stylomastoid foramen. It is not therefore seen attached to the cornu after the latter has been separated from the skull. The other pieces become successively shorter toward the body, and are called =stylohyal= (_d_), =epihyal= (_c_), and =ceratohyal= (_b_).
The caudal cornua (_f_) (greater cornua of human anatomy) arise from the ends of the body. Each consists of a single piece of bone, the =thyrohyal= (_f_), which passes caudolaterad; its free end is united to a process of the thyroid cartilage (Fig. 104, 1).
=The Skull as a Whole.=--In the following description of the skull as a whole the mandible, hyoid, and ear-bones are not included.
The skull forms a bony box which contains the brain and is produced craniad into the =facial= portion which encloses the nasal cavity and forms the framework of the face.
In =dorsal view= (Fig. 39) the skull presents a smooth convex surface, broadest caudad, with the two =zygomatic arches= (_g_) curving out some distance laterally. The following bones are visible in dorsal view: the occipital (1), interparietal (2), parietals (3), temporals (4), frontals (5), malar or zygomatic bones (6), nasals (7), maxillaries (8), premaxillaries (9), and lachrymals (10).
[Illustration: FIG. 39.--SKULL, DORSAL SURFACE.
1, occipital bone; 2, interparietal bone; 3, parietal bones; 4, temporal; 5, frontal; 6, malar; 7, nasal; 8, maxillary; 9, premaxillary; 10, lachrymal, _a_, lambdoidal ridge; _b_, external occipital tubercle; _c_, sagittal crest; _d_, parietal eminence; _e_, line which forms the dorsal boundary of the temporal fossa; _f_, zygomatic process of the frontal; _g_, zygomatic arch; _h_, frontal process of the malar; _i_, supraorbital arch; _j_, nares; _k_, foramen incisivum or anterior palatine foramen; _l_, sphenopalatine foramen; _m_, zygomatic process of the temporal; _n_, infraorbital foramen; _o_, opening of lachrymal duct.]