The ribs increase in length to the ninth (the ninth and tenth are of the same length) and then decrease to the last. They decrease in breadth behind the fifth. The first is nearly in a dorsoventral plane, while the others have their dorsal ends inclined slightly craniad.
[Illustration: FIG. 16.--STERNUM, VENTRAL VIEW.
_a_, manubrium; _b_, the separate pieces forming the body; _c_, bony part of the xiphoid process (the expanded cartilaginous portion not being shown); _d_, facet for attachment of first rib.]
The tubercles become less prominent as we pass caudad and are absent on the last two or three ribs, which do not articulate with the transverse process.
The first nine ribs (=true ribs= or =costæ veræ=) are attached separately to the sternum by their costal cartilages. The last four (false ribs or costæ spuriæ) are not attached separately to the sternum. The costal cartilages of the tenth, eleventh, and twelfth are united to one another at their sternal ends. They may be united also to the ninth costal cartilage or to the sternum by a common cartilage of insertion, or they may be quite free from the sternum. The thirteenth costal cartilages are free (floating ribs).
=Ligaments of the Ribs.=--The articular surfaces between the head of the rib and the centra, and between the tubercle and the transverse process of the vertebra, have each an articular capsule. There are also a number of small ligamentous bands from the tuberosity and the neck of the rib to the transverse process of the vertebra.
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.