All birds are not, however, equally adapted to flight, for in the domestic cock, which flies but a short distance, and badly, the sternum is less developed (Fig. 7); it is also diminished by slots, which diminish its surface. These slots, two on each side, are called from their position the internal and external slots. They are bounded by narrow, elongated, bony processes, an internal and an external; the expanded lower extremity of the latter overlaps the last inferior ribs (see p. 19). The part of the external border which surmounts this external process is hollowed out into grooves, which receive the inferior ribs, and terminates superiorly in an osseous projection known as _the costal prominence_. In the ostrich, the cassowary, and the apteryx, which run, but do not fly, the sternum has the form of a plate of bone slightly convex, but without a keel. [Illustration: FIG. 7.--STERNUM OF A BIRD (THE COCK): LEFT SIDE, EXTERNAL SURFACE. 1, Keel; 2, internal slot; 3, external slot; 4, internal process; 5, external process; 6, inferior ribs; 7, costal process; 8, surface for articulation with the coracoid bone.] The shape of the sternum, correlated to the faculty of flight (or of swimming; apropos of which we may cite the penguin, of which the rudimentary wings resemble fins, and perform their functions only), or the absence of this faculty, has furnished the division of birds into two groups. In one are included, under the name _Carinates_ (_carina_, keel), those in which the sternum is provided with a keel; in the other division are those in which the sternum is not furnished with one. These latter, on account of their unique mode of progression, are more nearly allied to the mammals. The keel is developed in flying mammals (bats). =Ribs and Costal Cartilages.=--There are on each side of the thorax as many ribs as there are dorsal vertebræ. In animals, as in man, the ribs which articulate with the sternum by their cartilages are called _true_, or _sternal_ ribs; those whose cartilages do not articulate with the sternum are called _false_, or _asternal_. The longer ribs are those situated in the middle region of the thorax. The ribs are directed obliquely downwards and backwards, and this obliquity is more marked in the posterior ones than in the anterior. They are, however, less oblique than in the human being; what proves this is that the first rib in man is oblique, while in quadrupeds it is vertical. The curvature of the ribs is less pronounced in quadrupeds than in the human being, but this is not equal in all animals. The ribs of the bear are more curved than those of the dog; the latter has ribs more curved than those of the horse. Each rib, at its vertebral extremity, presents, from within outwards, a wedge-shaped head for articulation with two dorsal vertebræ, a neck, and a tuberosity. External to the tubercle are found some rough impressions, for muscular attachments, which correspond to the angle of the human rib.