This consists originally of two half rings of cartilage encircling the sides of the body a short way behind the head. These two halves meet one another in the ventral middle line, and separate the anterior elements of the sternum from the posterior ones.
Each half-ring bears on the middle of its outer and posterior surface a prominent cup, the glenoid cavity, with which the proximal arm-bone articulates. This cup divides the half-arch into a dorsal scapular and a ventral coracoid portion.
The scapular portion consists of two parts, the suprascapula and the scapula.
The suprascapula (fig. 30, A, 2) is a wide, thin plate attached by its ventral and narrowest border to the scapula. Its proximal and anterior half is imperfectly ossified, its whole border or sometimes only its dorsal and posterior borders consist of unaltered hyaline cartilage, while the rest of it is composed of calcified cartilage. The scapula (fig. 30, A, 3) is a fairly stout rod of bone constricted in the middle, and forming the dorsal half of the glenoid cavity.
The coracoid portion consists of three parts, the coracoid, precoracoid and clavicle.
The largest and most posterior of these is the coracoid (fig. 30, A, 4) which like the scapula, is contracted in the middle and expanded at the ends, especially at the ventral end. It forms a large part of the glenoid cavity. The ventral ends of the coracoids which meet one another in the middle line are unossified, and form narrow strips of calcified cartilage, the epicoracoids (fig. 30, A, 5); these are often regarded as sternal elements.
The precoracoid forms a narrow strip of cartilage lying in front of the coracoid, from which it is separated by the wide coracoid foramen (fig. 30, A, 9). The dorsal end is continuous with an area of unossified cartilage which separates the coracoid and scapula and forms part of the glenoid cavity.
The clavicle is a narrow membrane bone closely attached to the anterior surface of the precoracoid, its dorsal end is expanded.
The Sternum.
The sternum consists of four parts arranged in two groups; two parts to each group. The anterior members are the episternum and omosternum.