The hyoid consists of a median unpaired portion, formed of two pieces of bone, the basi-hyal (fig. 59, C, 16) in front, and the uro-hyal (fig. 59, C, 17) behind, the two being placed end to end and terminated anteriorly by an unpaired cartilaginous plate, the os entoglossum. At the posterior end there come off a pair of long posterior cornua, each of which consists of two pieces, a longer basibranchial (fig. 59, C, 18), and a shorter cerato-branchial. For the homology of these parts see p. 336.
The Ribs and Sternum.
The last two cervical vertebrae bear long movable ribs which articulate by distinct capitular and tubercular processes, but do not meet the sternum. The thoracic ribs are eight in number, and each is divisible into a vertebral and a sternal portion. The first five thoracic ribs are flattened curved bars of bone, which articulate by a prominent capitulum with the centrum of the corresponding vertebra, and by a tuberculum with the transverse process. Projecting backwards from each is a large hooked uncinate process. The last three ribs which are without uncinate processes, become progressively more slender, and in the eighth the tubercular processes are lost.
The sternal portions of the ribs are imperfectly ossified pieces, short and comparatively thick in the case of the anterior ribs, longer and more slender in the case of the posterior ribs.
The Sternum[1].
The sternum or breast bone is exceedingly large in the Duck, as in all birds, and projects back far beyond the thorax over much of the anterior part of the abdomen. It is an irregularly oblong plate of bone, abruptly truncated behind, somewhat concave dorsally, and drawn out ventrally into a prominent keel, the carina, which projects for some distance forwards beyond the body of the sternum, and tapers off gradually behind. The point where the carina joins the body of the sternum is at the anterior end drawn out into a small process, the rostrum[102]. Just dorsolateral to this are a pair of deep grooves, the coracoid grooves, with which the coracoids articulate.
The sides of the sternum are drawn out in front into a pair of short blunt costal processes; and just behind these are a series of seven surfaces with which the ends of the sternal ribs articulate. Immediately behind these surfaces the sides are produced into a pair of long backwardly-projecting xiphoid processes which nearly meet processes from the posterior end of the sternum.
2. The Appendicular Skeleton.
This consists of the skeleton of the anterior and posterior limbs and of their respective girdles.
A. The Pectoral Girdle[103].