Serratus Anterior :  origin, insertion,  nerve supply and function 

What is  serratus anterior ?

 

Serratus Anterior also know as  Boxer muscle / punching muscle.  Serratus anterior muscle is not strictly muscle of  the pectoral region, but it is convenient to consider it here. The serratus anterior muscle is a fan-shaped muscle at the lateral wall of the thorax. 

ORIGIN :

Serratus anterior  muscle arises by 8th digitations from the upper 8th  ribs in the midaxillary plane and from the facia covering the intervening intercostal muscles. The first digitation appears in the posterior triangle of neck. It is arises from the  outer border of 1st rib and from a rough impression on the 2nd rib. Also  5th to 8th digitation interdigitate with the costal origin of external oblique muscle of abdomen.

Insertion:

All 8 digitations pass backwards around the chest  wall.

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 2/3 digitations are inserted lower  down on the  medial border.

The lower 5/4 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 and also called long thoracic nerve. The nerve  enters through  the apex of  axilla behind  1st part of axillary artery to reach the medial wall of axilla. It lies on the surface  of the  muscle.

👉 C5 root supplies 1st and 2nd  digitations.

👉 C6 root supplies 3rd and 4th digitations. 

👉 C7  root supplies 5th to 8th digitations .

Actions:

1.  Along with the  pectoralis minor, the muscle pulls the scapula forwards  around the chest wall to protect the upper limb.

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

3.  The muscle steadies the scapula during weight carrying.

4.  It helps in forced inspiration.

Additional Features :

1.  Paralysis of the serratus anterior produces 'winging of scapula' in which the inferior angle and the medial border of the scapula are unduly prominent. The patient is 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 medial border and the inferior angle of the scapula prominent (winging of scapula) if the serratus anterior is paralysed. 

FACTS TO REMEMBER:

1.  Pectoralis major forms part of the bed for the mammary gland. 75% of lymph from mammary gland drains into axillary; 20% into parasternal and 5% into intercostal lymph nodes.

2.  The sternocostal head of pectoralis major causes

extension of the flexed arm against resistance.

3.  Pectoralis minor divides the axillary artery into three parts. 

CLINICOANATOMICAL PROBLEM:

A 45-year-old women complained of a firm painless

mass in the upper lateral quadrant of her left breast. The nipple was also raised. Axillary lymph nodes were palpable and firm. It was diagnosed as cancer breast.

 ⚫Where does the lymph from upper lateral quadrant drain? 

⚫What causes the retraction of the nipple?

 Answer: The lymph from the upper lateral quadrant drains mainly into the pectoral group of axillary lymph nodes. The lymphatic also drain into supraclavicular and infraclavicular lymph nodes. Blockage of some lymph vessels by the cancer cells causes oedema of skin with dimpled appearance. This is called peau d'orange. When cancer cells invade the suspensory ligaments, glandular tissue or the ducts, there is retraction of the nipple.