III. STERNUM. ([Fig. 16].)
The sternum consists of three portions, a cranial piece or manubrium (a), a caudal piece or xiphoid process (c), and a middle portion or body (corpus), which is divided into a number of segments (b).
To the sternum are united the ventral ends of the first nine ribs. It thus forms the median ventral boundary of the thorax. Since the thorax decreases in dorsoventral measurement craniad, the long axis of the sternum is inclined from its caudal end dorsocraniad, and if continued would strike the vertebral column in the region of the first cervical vertebra.
The manubrium (a) makes up about one-fifth the whole length of the sternum and projects craniad of the first rib. It has the form of a dagger and presents a dorsal surface and two lateral surfaces, the latter uniting ventrad to form a sharp angle. In the middle of the lateral surface near the dorsal margin is an oval articular surface (d) borne on a triangular projection. It looks caudodorsad and is for the first costal cartilage.
The caudal end articulates with the body by a synchondrosis and presents a slightly marked oval facet on each side for the second costal cartilage.
The body consists of six cylindrical pieces (b) enlarged at their ends and movably united by synchondroses. They increase in breadth from the first, and decrease slightly in length and thickness. At the caudal end of each near its ventral border there is a pair of facets looking caudolaterad. They are for the costal cartilages.
The xiphoid process (c) is a broad thin plate of cartilage at its caudal end; bony and cylindrical at its cranial end. It is attached by its base to the last segment of the body by a considerable cartilaginous interval, while the opposite end is free and directed caudoventrad.
The cartilage of the ninth rib is attached to the lateral face of the cartilage between the xiphoid and the body, and just caudad of this the common cartilage of insertion of the tenth, eleventh, and twelfth costal cartilages is attached, if present.
IV. THE SKULL.
The bones of the head consist of the skull proper together with a number of separate bones forming part of the visceral skeleton; these are the lower jaw, the hyoid, and the ear-bones.