Muscles protracting the pectoral(shoulder) girdle

Serratus anterior

Pectoralis minor

Serratus anterior

Serratus anterior is a large flat muscular sheet covering the side of the thorax, being sandwiched between the ribs and the scapula. Loose fascia exists between the deep surface of the muscle and the ribs or intercostals fascia, and also between its superficial surface and subscapularis in order to facilitate free movement of the scapula. Serratus anterior forms the medial wall of the axilla, and is partly covered by the breast inferolaterally. The upper digitations are behind the clavicle while latissimus dorsi crosses its lower border.

Serratus anterior attaches by fleshy digitations just beyond the midaxillary line to the outer surfaces of the upper eight or nine ribs and the intervening intercostal fascia. The upper digitations arises from ribs one and two, whereas each of the remaining digitations arise from a single rib. The lower four digitations interdigitate with the costal attachment of the external oblique of the abdomen.

From this extensive attachment the muscle fibres run backwards to insert into the costal surface on the medial border of the scapula between the superior and inferior angles. The digitations are not however evenly distributed in their attachment to the scapula. The first passes almost horizontally to the superior angle, while the lower four condense to attach to the inferior angle, with the intervening ones spread along the medial border.

Nerve supply

Serratus anterior is supplied by the long thoracic nerve, root value C5, 6, 7, with the first two digitations supplied by C5, the next two by C6 and the remaining four by C7. The nerve enters the muscle on its superficial aspect. The skin over the accessible parts of the muscle is supplied by nerves with root values of T3 to T7.

Action

Serratus anterior is a major protractor of the pectoral girdle and as such is involved in all thrusting, pushing and punching movements where the scapula is driven forwards carrying the upper limb with it. Note the massive development of this muscle in boxers.

Serratus anterior plays a vital role in stabilizing the scapula during movements of the upper limb and contracts strongly to hold the medial border of the scapula against the chest wall when the arm is flexed or when a weight is carried in front of the body. Failure to perform this action, as when paralysed, results in “winging” of the scapula where the medial border stands away from the chest wall, thus severely affecting the function and mobility of the upper limb.

The lower digitations of the muscle work together with trapezius to rotate the scapula laterally, so that the glenoid fossa looks upwards and forwards. When paralysed, the loss of the rotating action of serratus anterior means that the upper limb cannot be abducted by more than 90°, thereby seriously limiting the functional capacity of the limb. There is some controversy as to whether serratus anterior acts as an accessory muscle of inspiration during respiratory distress. The line of action of the muscle fibres, except perhaps for the first two digitations and maybe the last, are not directed to cause elevation of the ribs. Indeed, they are more likely to cause depression of the ribs.

Palpation

In a muscular subject the digitations of serratus anterior can be felt and often seen running forwards in the region of the midaxillary line, especially when performing “press-ups”.

Pectoralis minor

Pectoralis minor is a thin, flat, triangular muscle situated on the anterior chest wall deep to pectoralis major. Inferiorly it attaches to the outer surfaces of the third, fourth and fifth ribs close to their costal cartilages and the intervening intercostal fascia. There may be additional attachments to the second or sixth rib, or more rarely, to both. The fibres converge to a short, flat tendon as they pass superolaterally to attach to the upper surface and medial border of the coracoid process of the scapula.

Nerve supply

Pectoralis minor is supplied by the medial pectoral nerve which pierces it. However, within the axilla the medial and lateral pectoral nerves communicate, thereby ensuring that pectoralis minor is supplied by both pectoral nerves. The segmental supply is by C6, 7 and 8.

Action

By exerting a strong pull on the coracoid process, the scapula can be pulled forwards and downwards during pushing and punching movements. When leaning on the hands it helps to transfer the weight of the trunk to the upper limb. The nature of its attachment to the coracoid process allows pectoralis minor to help produce medial rotation of the scapula against resistance(figure b). With the scapula and upper limb fixed, pectoralis minor may be used as an accessory muscle of inspiration during respiratory distress.

Palpation

Contraction of pectoralis minor, lying deep to the great bulk of pectoralis major, is difficult to palpate.