Short Notes on Muscles of the Pectoral Region

Short Notes on Muscles of the Pectoral Region

Pectoralis Major

1. Structures under Cover of Pectoralis Major

(a) Bones and cartilages: Sternum, ribs, and costal cartilages.

(b) Fascia: Clavipectoral.

(c) Muscles: Subclavius, pectoralis minor, serratus anterior, intercostals and upper parts of the biceps brachii and coracobrachialis.

(d) Vessels: Axillary.

(e) Nerves: Cords of brachial plexus with their branches.

2. Bilaminar Tendon of Pectoralis Major :

The muscle is inserted by a bilaminar tendon into the lateral lip of the intertubercular sulcus of the humerus.

The anterior lamina is thicker and shorter than the posterior. It receives two strata of muscle fibres:

Superficial fibres arising from the clavicle and deep fibres arising from the manubrium.

The posterior lamina is thinner and longer than the anterior lamina. It is formed by fibres from the front of the sternum, 2nd to 6th ribs and their costal cartilages and from the aponeurosis of the external oblique muscle of the abdomen.

Out of these only the fibres from the sternum and aponeurosis are twisted around the lower border of the rest of the muscle. The twisted fibres form the anterior axillaiy fold.

These fibres pass upwards and laterally to get inserted successively higher into the posterior lamina of the tendon. Fibres arising lowest find an opportunity to get inserted the highest and form a crescentic fold which fuses with the capsule of the shoulder joint.

3. Clinical Testing

(i) The clavicular head of the pectoralis major can be tested by attempting to lift a heavy table/ rod. The sternocostal head can be tested by trying to depress a heavy table/rod.

(ii) The clavicular head can also be tested by flexing the arm to a right angle; the sternocostal head by extending the flexed arm against resistance.

(iii) Press the fists against each other.

Clavipectoral Fascia

Clavipectoral fascia is a fibrous sheet situated deep to the clavicular portion of the pectoralis major muscle. It extends from the clavicle above to the axillary fascia below.

Its upper part splits to enclose the subclavius muscle. The posterior lamina is fused to the investing layer of the deep cervical fascia and to the axillary sheath. Inferiorly, the clavipectoral fascia splits to enclose the pectoralis minor muscle.

Below this muscle, it continues as the suspensory ligament which is attached to the dome of the axillary fascia, and helps to keep it pulled up.

The clavipectoral fascia is pierced by the following structures.

(i) Lateral pectoral nerve;

(ii) Cephalic vein;

(iii) Thoracoacromial vessels;

(iv) Lymphatics passing from the breast and pectoral region to the apical group of axillary lymph nodes.

Serratus Anterior

Serratus anterior muscle is not strictly a muscle of the pectoral region, but it is convenient to consider it here.

Origin

Serratus anterior muscle arises by eight digitations from the upper eight ribs, and from the fascia covering the intervening intercostal muscles.

Insertion

The muscle is inserted into the costal surface of the scapula along its medial border.

The first digitation is inserted from the superior angle to the root of the spine.

The next two digitations are inserted lower down on the medial border.

The lower five digitations are inserted into a large triangular area over the inferior angle.

Nerve Supply

The nerve to the serratus anterior is a branch of the brachial plexus. It arises from roots C5, C6 and C7.

Actions

1. Along with the pectoralis minor, the muscle pulls the scapula forwards around the chest wall to protract the upper limb (in pushing and punching movements).

2. The fibres inserted into the inferior angle of the scapula pull it forwards and rotate the scapula so that the glenoid cavity is turned upwards. In this action, the serratus anterior is helped by the trapezius which pulls the acromion upwards and backwards.

When the muscle is paralysed, the medial margin of the scapula gets raised specially when ‘pushing movements’ are attempted. This called Swinging of the scapula’.

3. The muscle steadies the scapula during weight- carrying.

4. It helps in forced inspiration.

Additional Features

1. Paralysis of the serratus anterior produces Svinging of scapula’ in which the inferior angle and the medial border of the scapula are unduly prominent.

The patient is not unable to do any pushing action, nor can he raise his arm above the head. Any attempt to do these movements makes the inferior angle of the scapula still more prominent.

2. Clinical testing: Forward pressure with the hands against a wall, or against resistance offered by the examiner, makes the inferior angle of the scapula prominent (winging of scapula) if the serratus anterior is paralysed.