III. THE STERNUM (Fig. 24 es, hs).

Cuvier, l. c., p. 401, Pl. XXIV, Fig. 31 e f.—Dugès, n. 27, 28.—Parker and Bettany, l. c., sternum.

The bones of the sternum are in very close relationship with the shoulder-girdle, and some difference of opinion exists as to the relations of the several parts.

Fig. 23.

Omosternum of Rana escu­lenta, twice nat. size.

a Anterior,
p Posterior extremity.

The following parts must be regarded as undoubtedly belonging to the sternum:‍—

1. An anterior part, consisting of a bony and a cartilaginous portion.

a. The omosternum (Figs. 23, 24 e.s.) is a bony rod, wider behind, narrower in front, and supports

b. The episternum (Fig. 24 es′), a semicircular plate of calcified cartilage, with a thin free border of unchanged hyaline cartilage.

2. A posterior part, which also consists of a bony and a cartilaginous portion.

a. The sternum proper (Fig. 24 h.s.), a rod of cartilage ensheathed in bone, broader in front and narrower behind. To its posterior extremity is attached

b. The cartilaginous xiphisternum (Fig. 24 hs′), a plate, the shape of which is best understood by referring to Fig. 24, and the structure of which is similar to that of the episternum.

3. The epicoracoids‍34 lie between the episternum and the sternum proper: they are a pair of narrow cartilages, closely applied to each other and placed between the ventral ends of the precoracoids and coracoids, see p. 40.

Fig. 24.

Shoulder-girdle and sternum of Rana esculenta, twice nat. size. The scapula and suprascapula are turned outwards.

c. Connecting cartilage. hs. Sternum proper.
cl. Clavicles and precoracoids.  hs′ Xiphisternum.
co Coracoid. o Bone.
es. Omosternum. o′ Calcified cartilage.
es′ Episternum. sc. Scapula.
h Hyaline cartilage. sc′ Suprascapula.