Clavicle
The clavicle, also called the collarbone, is the curved ante-rior bone of the shoulder (pectoral) girdle in vertebrates.
It functions as a strut to support the shoulder.
The clavicle is present in mammals with prehensile forelimbs and in bats, and it is absent in sea mammals and those adapted for running. The wishbone, or furcula, of birds is composed of the two fused clavicles, while a crescent-shaped clavicle is present under the pectoral fin of some fish. In humans the two clavicles, on either side of the anterior base of the neck, are horizontal, S-curved rods that connect laterally with the outer end of the shoul-der blade (the acromion) to help form the shoulshoul-der joint;
they connect medially with the breastbone (sternum).
Strong ligaments hold the clavicle in place at either end.
The shaft gives attachment to muscles of the shoulder girdle and neck.
Scapula
The scapula, or shoulder blade, is either of two large bones of the shoulder girdle in vertebrates. In humans they are triangular and lie on the upper back between the levels of the second and eighth ribs. A scapula’s posterior surface is crossed obliquely by a prominent ridge, the spine, which divides the bone into two concave areas, the supraspinous and infraspinous fossae. The spine and fossae give
attach-ends in the acromion, a process that articulates with the clavicle, or collarbone, in front and helps form the upper part of the shoulder socket. The lateral apex of the trian-gle is broadened and presents a shallow cavity, the trian-glenoid cavity, which articulates with the head of the bone of the upper arm, the humerus, to form the shoulder joint.
Overhanging the glenoid cavity is a beaklike projection, the coracoid process, which completes the shoulder socket. To the margins of the scapula are attached muscles
Major bones of the upper body. MedicalRF.com/Getty Images
that aid in moving or fixing the shoulder as demanded by movements of the upper limb.
Sternum
Also known as the breastbone, the sternum is an elon-gated bone in the centre of the chest that articulates with and provides support for the clavicles of the shoulder gir-dle and for the ribs. Its origin in evolution is unclear. A sternum appears in certain salamanders. It is also present in most other tetrapods (four-legged animals) but lacking in legless lizards, snakes, and turtles (in which the shell provides needed support). In birds an enlarged keel devel-ops, to which flight muscles are attached; the sternum of the bat is also keeled as an adaptation for flight.
In mammals the sternum is divided into three parts, from anterior to posterior: (1) the manubrium, which artic-ulates, or connects, with the clavicles and first ribs; (2) the mesosternum, often divided into a series of segments, the sternebrae, to which the remaining true ribs are attached;
and (3) the posterior segment, called the xiphisternum. In humans the sternum is elongated and flat; it may be felt from the base of the neck to the pit of the abdomen. The manubrium is roughly trapezoidal, with depressions where the clavicles and the first pair of ribs join. The mesoster-num, or body of the stermesoster-num, consists of four sternebrae that fuse during childhood or early adulthood. The meso-sternum is narrow and long, with articular facets for ribs along its sides. The xiphisternum is reduced to a small, usu-ally cartilaginous xiphoid (“sword-shaped”) process.
The sternum ossifies from several centres. The xiphoid process may ossify and fuse to the body in middle age; the joint between the manubrium and the mesosternum remains open until old age.
The Ribs
Occurring in pairs of narrow, curved strips of bone (some-times cartilage), the ribs are attached dorsally (in the back) to the vertebrae and, in higher vertebrates, to the breast-bone ventrally (in the front). This arrangement produces the bony skeleton, or rib cage, of the chest. The ribs help to protect the internal organs that they enclose and lend support to the trunk musculature.
The number of pairs of ribs in mammals varies from 9 (whale) to 24 (sloth); of true ribs, from 3 to 10 pairs. In humans there are normally 12 pairs of ribs. The first seven pairs are attached directly to the sternum by costal carti-lages and are called true ribs. The 8th, 9th, and 10th pairs—false ribs—do not join the sternum directly but are connected to the 7th rib by cartilage. The 11th and 12th pairs—floating ribs—are half the size of the others and do not reach to the front of the body. Each true rib has a small head with two articular surfaces—one that articulates on the body of the vertebra and a more anterior tubercle that articulates with the tip of the transverse process of the vertebra. Behind the head of the rib is a narrow area known as the neck; the remainder is called the shaft.
Humerus
The humerus, the long bone of the upper arm, forms the shoulder joint above, where it articulates with a lateral depression of the shoulder blade (glenoid cavity of scap-ula), and the elbow joint below, where it articulates with projections of the ulna and the radius.
In humans the articular surface of the head of the humerus is hemispherical; two rounded projections below and to one side receive, from the scapula, muscles that
rotate the arm. The shaft is triangular in cross section and roughened where muscles attach. The lower end of the humerus includes two smooth articular surfaces (capitu-lum and trochlea), two depressions (fossae) that form part of the elbow joint, and two projections (epicondyles). The capitulum laterally articulates with the radius; the troch-lea, a spool-shaped surface, articulates with the ulna. The two depressions—the olecranon fossa, behind and above the trochlea, and the coronoid fossa, in front and above—
receive projections of the ulna as the elbow is alternately straightened and flexed. The epicondyles, one on either side of the bone, provide attachment for muscles con-cerned with movements of the forearm and fingers.
Radius
The radius forms the outer of the two bones of the fore-arm when viewed with the palm facing forward. All land vertebrates have this bone. In humans it is shorter than the other bone of the forearm, the ulna.
The head of the radius is disk-shaped; its upper con-cave surface articulates with the humerus (upper arm bone) above, and the side surface articulates with the ulna.
On the upper part of the shaft is a rough projection, the radial tuberosity, which receives the biceps tendon. A ridge, the interosseous border, extends the length of the shaft and provides attachment for the interosseous mem-brane connecting the radius and the ulna. The projection on the lower end of the radius, the styloid process, may be felt on the outside of the wrist where it joins the hand.
The inside surface of this process presents the U-shaped ulnar notch in which the ulna articulates. Here the radius moves around and crosses the ulna as the hand is turned to cause the palm to face backward (pronation).
Ulna
The ulna is the inner of the two bones of the forearm when viewed with the palm facing forward. The upper end of the ulna presents a large C-shaped notch—the semilunar, or trochlear, notch—which articulates with the trochlea of the humerus (upper arm bone) to form the elbow joint.
The projection that forms the upper border of this notch is called the olecranon process. It articulates behind the humerus in the olecranon fossa and may be felt as the point of the elbow. The projection that forms the lower border of the trochlear notch, the coronoid process, enters the coronoid fossa of the humerus when the elbow is flexed. On the outer side is the radial notch, which artic-ulates with the head of the radius. The head of the bone is elsewhere roughened for muscle attachment.
The shaft of the ulna is triangular in cross section; an interosseous ridge extends its length and provides attach-ment for the interosseous membrane connecting the ulna and the radius. The lower end of the bone presents a small cylindrical head that articulates with the radius at the side and the wrist bones below. Also at the lower end is a sty-loid process, medially, that articulates with a disk between it and the cuneiform (os triquetrum) wrist bone.
The Hand
The hand is often described as a grasping organ. It is located at the end of the forelimb of certain vertebrates and exhibits great mobility and flexibility in the digits and in the whole organ. It is made up of the wrist joint, the carpal bones, the metacarpal bones, and the phalanges.
The digits include a medial thumb (when viewed with the palm down), containing two phalanges, and four fingers, each containing three phalanges.
The major function of the hand in all vertebrates except human beings is locomotion; bipedal locomotion in humans frees the hands for a largely manipulative func-tion. In primates the tips of the fingers are covered by fingernails—a specialization that improves manipulation.
The palms and undersides of the fingers are marked by creases and covered by ridges called palm prints and finger-prints, which function to improve tactile sensitivity and grip. The friction ridges are arranged in general patterns that are peculiar to each species but that differ in detail.
No two individuals are alike, and in humans the fingerprint patterns are used for identification. The thumb is usually set at an angle distinct from the other digits. In humans and the great apes it rotates at the carpometacarpal joint, and it is therefore opposable to the other fingers and may be used in combination with them to pick up small objects.
Among the apes and some New World monkeys, the hand is specialized for brachiation—hand-over-hand swinging through the trees. Digits two to five are elon-gated and used in clasping tree limbs; the thumb is reduced and little used in swinging. Terrestrial monkeys, such as the baboon, do not have reduced thumbs and can carry out precise movements with fingers and opposing thumb.
The development of dexterity in the hands and increase in brain size are believed to have occurred together in the evolution of humans.
Carpal Bones
The several small angular bones that in humans make up the wrist (carpus) are known as the carpal bones. In horses, cows, and other quadrupeds, these bones consist of the
“knee” of the foreleg. The carpal bones correspond to the tarsal bones of the rear or lower limb.
In humans there are eight carpal bones, arranged in
the scaphoid, lunate, triangular, and pisiform. The row toward the fingers, or distal row, includes the trapezium (greater multangular), trapezoid (lesser multangular), capi-tate, and hamate. The distal row is firmly attached to the metacarpal bones of the hand. The proximal row articu-lates with the radius (of the forearm) and the articular disk (a fibrous structure between the carpals and malleolus of the ulna) to form the wrist joint.
Metacarpal Bones
The tubular bones between the wrist (carpal) bones and each of the forelimb digits in land vertebrates are known as metacarpal bones. These bones correspond to the meta-tarsal bones of the foot. Originally numbering five, metacarpals in many mammals have undergone much change and reduction during evolution. The lower leg of the horse, for example, includes only one strengthened metacarpal; the two splint bones behind and above the hoof are reduced metacarpals, and the remaining two original metacarpals have been lost. In humans the five metacarpals are flat at the back of the hand and bowed on the palmar side; they form a longitudinal arch that accom-modates the muscles, tendons, and nerves of the palm.
The metacarpals also form a transverse arch that allows the fingertips and thumb to be brought together for manipulation.
Fingers
The fingers, which also are called digits, are composed of small bones called phalanges. The tips of the fingers are protected by the keratinous structure of the nails. The fin-gers of the human hand are numbered one through five, beginning with the inside digit (thumb) when the palm is face downward.