Shoulder Girdle

SHOULDER GIRDLE, or the girdle of the anterior (upper) limbs (see Girdles of the limbs), already receives significant development in fish; it encompasses the body of the fish from the sides, directly behind the gill apparatus. On the ventral side, both of its halves converge and connect with each other along the midline. On the side, the girdle is equipped with a projection for the articulation of the pectoral fin. This location serves as the boundary between the dorsal, or scapular, section of the girdle and the ventral, or coracoid, section. In higher fish, in connection with this primary girdle, a secondary girdle develops from a series of dermal bones. Already in sturgeon, one can see large dermal bones adjacent to the cartilaginous girdle from the outside—the cleithrum on the dorsal section and the clavicle (clavicula) on the ventral section. In addition to these bones, dermal bones also develop on the dorsal side of the girdle, connecting the girdle to the bones of the skull (supracleithra). With the development of such a strongly reinforced secondary girdle in higher fish, the primary girdle partially loses its significance and decreases in size. During the ossification of the latter, two typical bones usually develop in it: scapulare and coracoideum.

In terrestrial vertebrates, the strengthening of the shoulder girdle is achieved in a different way. The development of head mobility and the separation of the neck were possible only with the reduction of the dermal bones connecting the girdle to the skull. The reduction of this connection (cleithrum, supracleithrale) can still be traced in fossil stegocephalians. In all modern terrestrial vertebrates, only one bone from the secondary shoulder girdle of fish is preserved, which enters into a particularly close connection with the primary skeleton—the clavicle. The strengthening of the girdle is achieved, firstly, by increasing the surface area of the primary girdle and, in particular, its scapular section, and secondly, by connecting with the ribs of the thoracic region of the spine through the mediation of elements of the sternum. The distally strongly expanded dorsal section of the girdle always ossifies (os scapulare). The ventral section of the girdle is more or less clearly divided into two parts—the anterior procoracoid and the posterior, proper coracoid, which can ossify independently (os procoracoideum, os coracoideum). In birds, a long and narrow scapula extends backward along the back, massive coracoids are firmly connected to a huge sternum, and thin clavicles fuse at their ventral ends, forming a peculiar "wishbone" (furcula). In mammals, a ridge (spina) develops on the dorsal surface of the scapula, continuing into a special projection (acromion), to which a more or less well-developed clavicle is attached. In the ventral section of the girdle, both the procoracoid and the coracoid are reduced (cloacal mammals still possess both of these bones). The latter retains its own special ossification, but then fuses to the scapula, forming a special "coracoid" process on it. Clavicles, which are well developed in most mammals, are reduced in specialized animals adapted to uniform forms of movement—fast running (ungulates and some carnivores) and swimming (whales and sirenians).