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In anatomy, the pectoral girdle is the set of bones that forms the shoulder joint in humans and many animals. This set of bones is sometimes also called the “shoulder girdle,” and is usually made up of the clavicle, or collar bone, and scapula — at least in humans. Some animal species have more bones in this joint, others less. The main function of these joints is to provide support and mobility to the arms, and to allow a full range of motion. In most cases the bones work in conjunction with several major muscle groups in fulfillment of this goal.
General Shape and Location
The word “girdle” generally refers to a complete ring or full circle, and in this respect the pectoral girdle is something of an exception: it does not consist of a complete circle, but rather is more of an arc or semi-circle in shape. The two clavicle bones are divided on the front side by the breastbone, also referred to as the sternum. In addition, there is a space separating the two scapula bones on the backside of the area.
There are no joints that connect the thoracic cage, also called the rib cage, to each scapula. The muscles that connect the two allow the girdle bones to have a substantial range of movement, though. Species that have only scapula bones generally have muscular attachment but no joints connecting the front limb and the thorax, the part of an animal's body positioned between the head and the abdomen.
The clavicle, one of the sets of bones that make up the shoulder girdle, is slim and S-shaped. It connects the bones in the upper arm to the trunk of the body. In addition, the clavicle maintains the girdle’s basic shape and structure by keeping the shoulder in its proper setting and away from the trunk area. This allows more freedom of movement and also promotes more efficiency when it comes to swinging and bending the arms.
The scapula also tends to be somewhat slim and carries a low profile; these bones are flat and triangular in shape, and are positioned on the second to the seventh rib on the backside of the rib cage. A number of muscles are attached to the scapula bones, and one edge has a thin, hollow space called the glenoid cavity. In humans, this is where the top of the humerus, or upper arm bone, is positioned. A thin border that runs alongside the surface of the scapula forms the spine of the bone, which provides strength to the scapula to keep it from breaking or bending.
For most animals, support is the primary goal of this particular set of bones and muscles. The shoulder girdle provides essential structural support and connection to several muscles that are situated primarily in the elbow and shoulder joint area, which, among other things, allows for a wide range of arm movements. Human beings have joints that connect the shoulder girdle and what’s known as the “axial skeletal structure,” the 80 bones within the head and chest of the human body. These joints are called sternoclavicular joints and are positioned on both sides of the ribcage.
The pectoral girdle is often compared to the pelvic girdle, which is a similar series of bones that provides support and facilitates movement of the legs and hips. Unlike the pelvic girdle, though, the bones in the pectoral region aren’t typically designed to bear weight, at least not in animals that walk upright like humans. They provide structure and mobility, but aren’t usually intended to help with balance or absorb weight. This means that they often aren’t as sturdy or substantial as the bones in the pelvis, though the job they do is usually just as important.