Appendicular Skeleton

APPENDICULAR SKELETON The bony framework of the upper and lower limbs is built up on the same plan in both. Each consists of a limb girdle connect ing it with the axial skeleton, a proximal single bone segment, a distal double bone segment, the hand and foot segments and the digits (phalanges). It should be understood that in the fol lowing descriptions the terms internal and external are used in relation to the mid-line of the body and not to that of the limb.

Upper Limb.

The upper limb in man consists of a proxi mal part or shoulder, a distal part or hand, and an intermediate shaft, which consists of an upper arm, and a forearm. In each of these subdivisions certain bones are found: in the shoulder, the clavicle and scapula; in the upper arm, the humerus; in the f ore arm, the radius and ulna, the bone of the upper arm in man being longer than the bones of the fore arm ; in the hand, the carpal and metacarpal bones and the pha langes. The scapula and clavicle together form an imperfect bony arch, the Shoulder Girdle; the shaft and hand form a free di vergent Appendage. The shoulder girdle is the direct medium of connection between the axial skeleton and the divergent part of the limb; its anterior segment, the clavicle, articulates with the upper end of the sternum, whilst its posterior segment, the scapula, approaches, but does not reach, the dorsal spines.

Clavicle.—The clavicle or col lar bone (fig. 14) is an elongated bone which extends from the upper end of the sternum horizontally outward, to articulate with the acromion process of the scapula. It presents a strong sigmoidal curve, is slender in the female, but powerful in muscular males; its sternal end is thick and somewhat triangular ; its acrom ial end, flattened from above downward, has an oval articular surface for the acromion. Its shaft has four surfaces for the attachment of muscles; and strong ligaments connecting it with the coracoid and the first rib.

Scapula.—The scapula or shoulder blade (fig. 14) is present

in all mammals. It lies at the upper and back part of the wall of the chest, reaching from the second to the seventh rib. Its form is plate-like and triangular, with three surfaces, three borders and three angles. Its ventral surface is in relation to the ribs, from which it is separated by certain muscles : the dorsum is traversed from behind forward by a prominent spine, which subdivides this aspect of the bone into a supra-spinous and an infra-spinous fossa. The spine arches forward to end in a broad flattened process, the acromion, which has an oval articular surface for the clavicle; both spine and acromion are largely developed in the human scapula in correlation with the great size of the trapezius and del toid muscles, which are concerned in the elevation and abduction of the upper limb. The borders of the scapula give attachment to several muscles. The angles are inferior, antero-superior and postero-superior. The antero-superior is truncated and has a large, shallow, oval, smooth surface, the glenoid fossa, for articulation with the humerus, to form the shoulder joint. Overhanging the glenoid fossa is a curved beak-like process, the coracoid, cor responding with the separate coracoid bone of monotremes, birds and reptiles.

Humerus.—The humerus (fig. 14) is a long bone, and consists of a shaft and two extremities. The upper extremity possesses a convex spheroidal smooth surface, the head, for articulation with the glenoid fossa of the scapula; it is surrounded by a narrow constricted neck, and where the neck and shaft become continuous with each other, two processes or tuberosities are found, to which are attached the rotator muscles arising from the scapular fossae. Between the tuberosities is a groove in which the long tendon of the biceps rests. The shaft is triangular in section above, but flattened and expanded below. A shallow groove winds round the back of the bone, in which the musculo-spiral nerve is lodged.