Origin and Comparative Anatomy of the Pectoral Limb

Abstract

Scapula During the evolution of the upper extremity, the scapula, more than any other bone of the shoulder girdle, reflects momentous alterations that have been brought about by increased functional demands of a prehensile limb. Changes in posture provided the stimulus which initiated the numerous morphologic changes. In the cervical region but was freed from the skull. Rhachitomous amphibians possessed massive scapulae with the glenoid cavity pointing laterally. The articulating surface was screw-shaped, and the limbs were held in the coronal plane horizontal to the ground. In the Reptilia the scapula with the entire girdle migrated a great distance from the skull in order to permit a more efficient mode of locomotion. The scapula was still broad and massive in the primitive forms. However, later with increased efficiency in locomotion, there was a trend toward reduction of this bone, the glenoid cavity shifting from a position directed laterally to one directed posteriorly and inferiorly. As a result of the change in posture, the coracoid’s function decreased. Hence, a gradual reduction in its size is noted in this group. Up to this stage in evolution of the pectoral girdle no evidence of a spine on the dorsal surface of the scapula is found except in the Therapsida whose posture is not unlike that of the mammals. Posture was responsible for the development of the scapular spine which is found in all mammals except the very primitive forms, the Monotremata. With rearrangement of some and disappearance of other muscles, the need of a procoracoid and coracoid no longer existed. Therefore, the former element disappeared entirely, while the latter was reduced to the coracoid process. The shape of the scapula is dependent upon posture and the functional requirements of the muscles attached to it. It is broad and massive in forms which need large powerful serratus anticus muscles to support heavy bodies in a quadruped position. In mammals which have partially or completely freed the pectoral limbs, the shape of the scapula exhibits a trend towards the pattern found in man. These alterations are brought about by change in posture from the pronograde to the orthograde and highly specialized functional requirements of a prehensile limb. The most significant scapular change is in the relation of length to breadth of a bone. Pronograde forms disclose a long narrow scapula, while in the ascent toward man it becomes broader. This morphologic change is most obvious in the primates. That portion of scapula below the spine demonstrates the most pronounced alterations, those in the region above the spine being insignificant. Morphologic modifications in the scapula can be expressed by a scapular index, a ratio of the breadth (measured along the base of the spine) to the length (measured from the superior from the inferior angle). The scapular index is high in the pronograde in which the scapula is long, narrow and slender. The index progressively decreases in the successive stages of development approaching man (orthograde). This is the result of a gradual increase in the breadth of the scapula and elongation of the bone below the level of the spine, giving rise to a progressive increase in the “infraspinous index” (Fig. 7). Inman, Saunders and Abbott, in their comprehensive study of the function of the shoulder joint, observed that lengthening of the scapula below the spine changed the relation of the axillary border of the scapula to the glenoid fossa, thereby altering the angle of pull of the muscles attached to this region, a feature of great significance in the mechanism of the shoulder.