The pectoral girdle refers to the shoulder region, which attaches the upper arm to the axial skeleton. Hence, it is closely associated with the upper limb. This girdle, which is also called the shoulder girdle, consists of two bones that together make up the shoulder.
It is an important girdle in the body, as it has the responsibility of providing basic structural support. As it has no posterior attachment, it has a big range of motion.
The scapula, also known as the shoulder blade, is the bone which connects the humerus (bone of the upper arm) with the clavicle. The scapula forms the posterior part of the shoulder girdle in humans. It is a flat bone which is roughly triangular in shape. It is placed on the posterolateral aspect of the thoracic cage.
Its ventral surface has a broad concavity, which is known as the subscapular fossa. The scapula is an important bone, as it provides attachment to various muscles of the head, neck, and back regions.
The clavicle, also known as the collarbone, is a small S-shaped bone, which makes up the shoulder girdle along with the scapula. It is a doubly curved short bone that connects the arm to the body, and is located directly above the first rib.
The clavicle aids in the pectoral girdle's function of keeping the arm away from the thorax and the central skeleton, thereby, ensuring that the arm is allowed to perform a wide range of motions with relative ease. It also acts as a rigid support, from which the scapula and upper limb are suspended.
Pectoral Girdle Muscles
This is one of the main muscles involved with the pectoral girdle. It is a fan-shaped muscle that covers the superior part of the thorax. Its origin lies in the clavicular head and the sternocostal head.
When both these parts of the muscle act together, the pectoralis major adducts and medially rotates the humerus at the shoulder joint. This muscle is innervated by the lateral and medial pectoral nerves.
The pectoralis minor is present in the anterior wall of the axilla. It is largely covered by the pectoralis major, and is innervated by the medial pectoral nerve. It is attached to the ribs near the costal cartilages, and it is also attached to the coracoid process of the scapula.
The serratus anterior is a large muscle that lies in the lateral part of the thorax. It is proximally attached to all the ribs, and is distally attached to the medial border of the scapula. It is innervated by the long thoracic nerve.
This is the muscle responsible for protraction of the scapula, and it also holds and fixes it against the thoracic wall. It is activated when punching anyone, therefore, it is also known as the 'boxer's muscle'.
There are a few more muscles which are involved with the pectoral girdle in an indirect manner, that is, they are attached to the scapula or the clavicle, but only partly help in its movement. These include subclavius, levator scapulae, rhomboideus, and trapezius muscle.