Station 4: The trunk and arm

Clavicle

The clavicle is a long bone that maintains a constant distance between the scapula and the sternum and thereby adds strength to the shoulder girdle. In generalized quadrupeds that use their forelimbs primarily for support or striding locomotion, that strength is supplied by the musculature (without a clavicle), while animals that use the forelimbs for manipulation or more complicated forms of locomotion the medial end of the s-shaped clavicle articulates with the sternum, and the lateral end is flattened for the articulation with the scapula.

Rib

Each rib has two extremities, a posterior (or vertebral), and an anterior (or sternal), and an intervening portion—the body (or shaft).

Scapula

The scapula is a triangular flat bone that rests on the upper back. It serves as an attachment for some of the muscles and tendons of the arm, neck, chest and back and aids in the movements of the arm and shoulder. It is well padded with muscle so that great force is required to fracture it. On the head of the scapula, between the processes mentioned above, is a depression called the glenoid cavity. It joins with the head of the upper arm bone (humerus).

Sternum

The sternum is composed of three segments and is similar to a broadsword:

• Manubrium, articulates with the clavicle and the first two ribs.
• The sternal body articulates with ribs.
• Xiphoid process often a cartilaginous tab, becomes bony in later years.

The three segments of this bone are sometimes separate but are usually fused in adults.

Humerus

The humerus is the largest bone of the upper limb. The rounded head at the proximal end articulates with the glenoid fossa of the scapula, and the distal end articulates with the radius and ulna. 

The smooth, dome-shaped head of the bone lies at an angle to the shaft and fits into a shallow socket of the scapula (shoulder blade) to form the shoulder joint. At the neck of the humerus, the bone narrows to form a cylindrical shaft. It flattens and widens at the lower end and, at its base, joins with the bones of the lower arm (the ulna and radius) to make up the elbow, at the olecranon fossa.

Radius

The radius is the lateral bone of the forearm and “follows” the thumb in the movements of the forearm. The proximal surface, or head is rounded for articulation with the capitular surface of the humerus, so think of the “radius of a circle” when remembering the radius. At the distal end is the styloid process, which articulates with the wrist, and the ulnar notch which articulates with the ulna.

Ulna

The ulna is the longer of the two bones of the forearm; the other being the radius. When the palm faces forward, the ulna is the inner bone (the one nearest the body) running down the forearm parallel to the radius. The upper end joins with the radius at the radial notch and extends into a rounded projection (the olecranon process) that fits around the lower end of the humerus to form the elbow joint at the semilunar notch. The lower end of the ulna is rounded and forms a joint with the wrist bones and lower end of the radius.

Hand

The bones of the hand provide support and flexibility to the soft tissues. They can be divided into three categories:

• Carpal bones (Proximal) – A set of eight irregularly shaped bones. These are located in the wrist area.
• Metacarpals – There are five metacarpals, each one related to a digit
• Phalanges (Distal) – The bones of the fingers. Each finger has three phalanges, except for the thumb, which has two.