It is a narrow space between the chest wall and the pectoral limb containing a large amount of loose connective tissue.
This space is crossed by the large vessels and the nerves intended to supply the muscles of the pectoral limb.
It corresponds to the armpit of the man.
In resting condition, this space can scarcely exist, except perhaps to a small extent at its lower part: but it is clearly observed when the limb is abducted.
Pectoral muscles
These muscles form a thick mass, which occupy the space between the lower part of the chest wall and the medial face of the shoulder and arm. They are divisible into two layers – superficial and deep. The superficial layer is again divisible into anterior and posterior muscles.
Superficial Pectoral (Pectoralis superficialis)
Anterior superficial pectoral
This is thicker and smaller of the two layers of the superficial pectoral, extending from the manubrium sterni to the antero-ventral part of the arm.
Orgin: The ventro-lateral aspects of the anterior part of the sternum.
Insertion: In common with the brachiocephalicus, to the anterior edge of the humerus.
Action: To adduct the limb and draw it forwards.
Posterior superficial pectoral
This is closely blended with the preceding muscle and extending from the ventro-lateral aspect of the sternum to the medial side of the forearm.
Origin: The ventral and lateral aspects of the anterior part of the sternum behind the origin of the preceding muscle.
Insertion: To the crest of the humerus and fascia of the forearm.
Action: To adduct the limb and to tense the fascia of the forearm.
Deep Pectoral (Pectoralis profundus)
It is a large, fleshy muscle extending from the level of xiphoid cartilage forwards and upwards to the level of the shoulder joint.
Origin: The ventral surface of the sternum and tunica abdominalis.
