The major movements at the elbow joint are flexion and extension of the forearm (Fig. 7.5A). At the other end of the forearm, the distal end of the lateral bone, the radius, can be flipped over the adjacent head of the medial bone, the ulna. Because the hand is articulated with the radius, it can be efficiently moved from a palm-anterior position to a palm-posterior position simply by crossing the distal end of the radius over the ulna (Fig. 7.5B). This movement, termed pronation, occurs solely in the forearm. Supination returns the hand to the anatomical position.
At the wrist joint, the hand can be abducted, adducted, flexed, extended, and circumducted (Fig. 7.6). These movements, combined with those of the shoulder, arm, and forearm, enable the hand to be placed in a wide range of positions relative to the body.
The hand as a mechanical tool
One of the major functions of the hand is to grip and manipulate objects. Gripping objects generally involves flexing the fingers against the thumb. Depending on the type of grip, muscles in the hand act to: modify the actions of long tendons that emerge from the forearm and insert into the digits of the hand, and produce combinations of joint movements within each digit that cannot be generated by the long flexor and extensor tendons alone coming from the forearm.
The hand as a sensory tool
The hand is used to discriminate between objects on the basis of touch. The pads on the palmar aspect of the fingers contain a high density of somatic sensory receptors. Also, the sensory cortex of the brain devoted to interpreting information from the hand, particularly from the thumb, is disproportionately large relative to that for many other regions of skin.
The bones of the shoulder consist of the scapula, clavicle, and proximal end of the humerus (Fig. 7.7).
The clavicle articulates medially with the manubrium of the sternum and laterally with the acromion of the scapula, which arches over the joint between the glenoid cavity of the scapula and the head of the humerus (the glenohumeral joint).
The humerus is the bone of the arm (Fig. 7.7). The distal end of the humerus articulates with the bones of the forearm at the elbow joint, which is a hinge joint that allows flexion and extension of the forearm.
The forearm contains two bones:
The lateral bone is the radius.
The medial bone is the ulna (Fig. 7.7).
At the elbow joint, the proximal ends of the radius and ulna articulate with each other as well as with the humerus.
In addition to flexing and extending the forearm, the elbow joint allows the radius to spin on the humerus while sliding against the head of the ulna during pronation and supination of the hand.
The distal portions of the radius and the ulna also articulate with each other. This joint allows the end of the radius to flip from the lateral side to the medial side of the ulna during pronation of the hand.
The wrist joint is formed between the radius and carpal bones of the hand and between an articular disc, distal to the ulna, and carpal bones.
The bones of the hand consist of the carpal bones, the metacarpals, and the phalanges (Fig. 7.7).
The five digits in the hand are the thumb and the index, middle, ring, and little fingers.
Joints between the eight small carpal bones allow only limited amounts of movement; as a result, the bones work together as a unit.
The five metacarpals, one for each digit, are the primary skeletal foundation of the palm (Fig. 7.7).
The joint between the metacarpal of the thumb (metacarpal I) and one of the carpal bones allows greater mobility than the limited sliding movement that occurs at the carpometacarpal joints of the fingers.