Distally, the heads of metacarpals II to V (i.e., except that of the thumb) are interconnected by strong ligaments. Lack of this ligamentous connection between the metacarpal bones of the thumb and index finger together with the biaxial saddle joint between the metacarpal bone of the thumb and the carpus provide the thumb with greater freedom of movement than the other digits of the hand.
The bones of the digits are the phalanges (Fig. 7.7). The thumb has two phalanges, while each of the other digits has three.
The metacarpophalangeal joints are biaxial condylar joints (ellipsoid joints) that allow abduction, adduction, flexion, extension, and circumduction (Fig. 7.8). Abduction and adduction of the fingers is defined in reference to an axis passing through the center of the middle finger in the anatomical position. The middle finger can therefore abduct both medially and laterally and adduct back to the central axis from either side. The interphalangeal joints are primarily hinge joints that allow only flexion and extension.
Some muscles of the shoulder, such as the trapezius, levator scapulae, and rhomboids, connect the scapula and clavicle to the trunk. Other muscles connect the clavicle, scapula, and body wall to the proximal end of the humerus. These include the pectoralis major, pectoralis minor, latissimus dorsi, teres major, and deltoid (Fig. 7.9A,B). The most important of these muscles are the four rotator cuff muscles—the subscapularis, supraspinatus, infraspinatus, and teres minor muscles—which connect the scapula to the humerus and provide support for the glenohumeral joint (Fig. 7.9C).
Muscles in the arm and forearm are separated into anterior (flexor) and posterior (extensor) compartments by layers of fascia, bones, and ligaments (Fig. 7.10).
The anterior compartment of the arm lies anteriorly in position and is separated from muscles of the posterior compartment by the humerus and by medial and lateral intermuscular septa. These intermuscular septa are continuous with the deep fascia enclosing the arm and attach to the sides of the humerus.
In the forearm, the anterior and posterior compartments are separated by a lateral intermuscular septum, the radius, the ulna, and an interosseous membrane, which joins adjacent sides of the radius and ulna (Fig. 7.10).
Muscles in the arm act mainly to move the forearm at the elbow joint, while those in the forearm function predominantly to move the hand at the wrist joint and the fingers and thumb.
Muscles found entirely in the hand, the intrinsic muscles, generate delicate movements of the digits of the hand and modify the forces produced by tendons coming into the fingers and thumb from the forearm. Included among the intrinsic muscles of the hand are three small thenar muscles, which form a soft tissue mound, called the thenar eminence, over the palmar aspect of metacarpal I. The thenar muscles allow the thumb to move freely relative to the other fingers.
The upper limb is directly related to the neck. Lying on each side of the superior thoracic aperture at the base of the neck is an axillary inlet, which is formed by: the lateral margin of rib I, the posterior surface of the clavicle, the superior margin of the scapula, and the medial surface of the coracoid process of the scapula (Fig. 7.11).
The major artery and vein of the upper limb pass between the thorax and the limb by passing over rib I and through the axillary inlet. Nerves, predominantly derived from the cervical portion of the spinal cord, also pass through the axillary inlet and the axilla to supply the upper limb.
Muscles that attach the bones of the shoulder to the trunk are associated with the back and the thoracic wall and include the trapezius, levator scapulae, rhomboid major, rhomboid minor, and latissimus dorsi (Fig. 7.12).