Pectoral region & Axilla Anatomy

Pectoral region definition .

Muscles covering the front of the chest and holding the free upper limb to the torso, and their vessels and nerves constitute the pectoral region. The pyramidal space between the upper part of the thorax and the arm is the axilla.

Adjacency of the thorax, neck, and upper limb  .

The walls of the thorax form a conical structure which is flattened anteroposteriorly. It has an apex superiorly that is cut obliquely to form the superior aperture of the thorax. This is continuous above with the root of the neck and has, as its margins, the first thoracic vertebra, the first ribs, and the upper part of the sternum (manubrium). The upper limb is attached to the trunk by muscles and bones which spread out from the proximal part of the limb to the anterior and posterior surfaces of the thorax.

 Introduction to Axilla .

The axilla is a four-sided pyramidal space between :

(1) the upper limb;

(2) the muscles connecting the upper limb to the front of the thorax;

(3) the muscles connecting the upper limb to the back of the thorax; and

(4) the lateral wall of the thorax. When the arm is by the side, the axilla is a narrow space.

When the arm is abducted, the volume of the axilla increases, and its floor (base) rises, forming a definite ‘armpit’. Also when the arm is abducted, the muscular inferior margins of its anterior wall stands out as the anterior axillary fold, and the inferior margin of the posterior wall stands out as the posterior axillary fold .

The superior part of the axilla—the apex—lies lateral to the first rib and is continuous over its superior surface, with the superior aperture of the thorax below and the root of the neck above. This continuity permits blood vessels from the thorax and nerves from the neck to enter the axilla on their way to the upper limb. (These vessels and nerves pass over the superior surface of the first rib behind the clavicle.

Bones of the pectoral region and axilla .

The clavicle extends laterally from its articulation with the sternum (sternoclavicular joint) to its articulation with the scapula (acromioclavicular joint) on the superior surface of the shoulder. The sternoclavicular joint is the only articulation of an upper limb bone with a bone of the trunk. Thus, the clavicle acts as a support which transmits forces from the upper limb to the trunk and prevents the scapula, and hence the shoulder from sagging downwards and medially under the weight of the limb .

Sagging down of the upper limb is seen when the clavicle is fractured. The scapula lies posterior to the axilla and is almost entirely covered by muscles. Movements of the scapula are limited only by its articulation with the clavicle and, through it, with the sternoclavicular joint around which these movements are forced to take place. The scapula slides freely on the thoracic wall in the absence of bony articulations between it and the wall. The muscles of the scapula either attach the scapula to the humerus or hold it against the thorax.

Surface anatomy of the pectoral region and axilla .

The clavicle (collar bone) is palpable throughout its length. It follows a slight curve which is convex forwards in its medial two-thirds and concave forwards in its lateral one-third . Draw a finger along your clavicle, and note that its ends project above the acromion of the scapula laterally and the manubrium of the sternum medially. Thus, the positions of these joints are easily identified, though the medial end of the clavicle is somewhat obscured by the attachment of the sternocleidomastoid muscle. Between the medial ends of the clavicles, feel the jugular notch on the superior margin of the manubrium .

Draw a finger downwards from this notch in the median plane till a blunt transverse ridge is felt on the sternum. This bony landmark is the sternal angle, a joint between the manubrium and the body of the sternum. At this level, the cartilage of the second rib articulates with the side of the sternum. The second rib may be identified in this way, even in obese subjects, for the sternal angle is always readily palpable. The other ribs are identified by counting down from the second rib. The anterior part of the first rib is hidden by the medial part of the clavicle.

Immediately inferior to the lower end of the body of the sternum is a small median depression, the epigastric fossa which overlies the xiphoid process—the lowest piece of the sternum. The cartilages of the seventh ribs lie on either side of this fossa. The nipple is very variable in position, even in the male, but usually lies over the fourth intercostal space, near the junction of the ribs with their cartilages. It is just medial to a vertical line passing through the middle of the clavicle (the midclavicular line). The infraclavicular fossa is a depression inferior to the junction of the lateral and middle thirds of the clavicle.

The pectoralis major muscle on the front of the chest lies medial to the fossa, and the deltoid muscle, which clasps the shoulder, is lateral to it. The coracoid process of the scapula can be felt just lateral to the fossa and under cover of the deltoid muscle, 2–3 cm below the clavicle. Follow the clavicle laterally to its articulation with the acromion—a subcutaneous, flattened piece of the bone about 2.5 cm wide, on the top of the shoulder. The acromioclavicular joint can be felt as a slight dip, for the clavicle projects slightly above the level of the acromion (acron = summit; omos = shoulder).

Raise the arm from the side, i.e. abduct it, and identify the hollow of the axilla, the anterior axillary fold (containing the pectoralis major muscle), and the posterior axillary fold (containing the latissimus dorsi and teres major muscles) . The teres major is a thick, rounded muscle which connects the inferior angle of the scapula to the humerus and can be felt in the posterior axillary fold when the arm is raised above the head. The latissimus dorsi muscle extends from the lower part of the back to the humerus. It can be made to stand out by depressing the horizontal arm against resistance. With the arm by the side, push your fingers into the axilla.

The anterior and posterior walls are soft and fleshy, but the lateral margin of the scapula can be felt in the posterior wall. The medial wall is formed by the ribs covered by a sheet-like muscle— the serratus anterior. In the lateral angle, the biceps brachii and coracobrachialis muscles lie parallel to the humerus. Some of the large nerves in the axilla can be rolled between the fingers and the humerus, and the axillary artery can be felt pulsating. By pushing the fingers up into the axilla, the head of the humerus can be felt laterally and the lateral border of the first rib medially.

Cutaneous nerves .

The skin on the anterior and lateral surfaces of the thorax is supplied by:

1. The supraclavicular nerves from the cervical plexus—principally the fourth cervical ventral ramus. .

2. The anterior and lateral cutaneous branches of the ventral rami of the second to eleventh thoracic nerves (intercostal nerves) .

The supraclavicular nerves arise in the neck from the third and fourth cervical nerves (C3, C4). Diverging as they descend, the nerves . pierce the deep fascia in the neck. They cross the clavicle to supply the skin on the front of the chest and shoulder down to a horizontal line at the level of the second costal cartilage. They are named, according to their positions: medial, intermediate, and lateral.

The anterior cutaneous branches of the intercostal nerves (except the first and occasionally the second) emerge from the intercostal spaces near the lateral border of the sternum, pierce the pectoralis major, and supply the skin from the anterior median line almost to a vertical line through the middle of the clavicle (mid-clavicular line) . They are accompanied by perforating branches of the internal thoracic artery, an artery which lies immediately deep to the costal cartilages.

In the female, these arterial branches are enlarged in the second to fourth spaces to supply the mammary gland. The arteries have lymph vessels running with them from the skin of the anterior thoracic wall and the medial part of the mammary gland (breast) to parasternal nodes which lie beside the internal thoracic artery. The lateral cutaneous branches of the intercostal nerves pierce the deep fascia along the mid-axillary line.

Each nerve divides and enters the superficial fascia as anterior and posterior branches. The nerves pierce, or pass between, the digitations of the serratus anterior but play no part in supplying this muscle, the pectoral muscles, or the latissimus dorsi over which they run. They supply the part of the skin between the parts supplied by the anterior cutaneous branches (midline in front to the mid-clavicular line) and the dorsal ramus (midline of the back to approximately 10 cm from the midline).

There are usually no lateral or anterior cutaneous branches from the first intercostal nerve. The lateral cutaneous branch of the second intercostal nerve is the intercostobrachial nerve. It emerges as a large single branch and communicates with the medial cutaneous nerve of the arm and the lateral cutaneous branch of the third intercostal nerve. Together, these three nerves supply the skin of the medial side of the arm and the floor of the axilla.

The breast . 

The mamma or breast is made up of:

(1) the mammary gland;

(2) the fatty superficial fascia in which it is embedded; and

(3) the overlying skin with the nipple and the surrounding pigmented skin—the areola .

In the male, the mammary gland is rudimentary; the nipple is small, and the areola is commonly surrounded by fine hairs. In the non-lactating female, the breast consists mainly of the fatty tissue of the superficial fascia, in which are enclosed 15 to 20 lobes of rudimentary glandular tissue. These glands radiate outwards from the  nipple, giving the gland the shape of a flattened cone. Each lobe has a main lactiferous duct which passes to open separately on the nipple.

At the base of the nipple, the duct is dilated to form a lactiferous sinus. The gland has no capsule, but its lobes are separated by fibrous strands of the superficial fascia which pass from the skin to the deep fascia. These fibrous strands are attached to the gland and anchor it both to the skin and to the underlying deep fascia. The base of the mammary gland extends from the margin of the sternum to almost the mid-axillary line, and from the second to sixth ribs. It lies largely on the pectoralis major muscle. Inferolaterally, it extends on to the costal origins of the serratus anterior and the external oblique muscle . of the abdomen.

The ‘axillary tail’ arises from the superolateral quadrant of the breast and passes into the axilla, up to the level of the third rib . The apex of the gland—the nipple— lies a little below the midpoint of the gland, approximately at the fourth intercostal space in the nulliparous woman. The nipple is free of fat but contains circular and longitudinal smooth muscle fibres which can erect or flatten it. The skin of the nipple and areola contains modified sweat and sebaceous glands, particularly at the outer margin of the areola.

These sebaceous glands tend to enlarge in the early stages of pregnancy, and shortly thereafter there is an increase in pigmentation in both the nipple and areola which never return to their original colour. In the later stages of pregnancy, the greater part of the fat in the gland is replaced by the proliferation of its ducts and the growth of many secretory alveoli from their branching ends. The gland receives its blood supply from perforating branches of the intercostal and internal thoracic arteries medially and from the lateral thoracic artery laterally.

Lymph vessels drain principally:

(a) to the axilla—

(i) along the axillary tail to the pectoral lymph nodes, and

(ii) through the pectoralis major and clavipectoral fascia to the apical axillary nodes via the infraclavicular nodes .

(b) to the parasternal nodes along the internal thoracic artery by passing along the branches of that artery which supply the gland; and

(c) some lymph also drains to the posterior intercostal nodes.

Since there is communication of lymph vessels across the median plane, there may be drainage to the opposite side, especially when some of the pathways are blocked by disease. This is not usually very successful in the elderly female and should not be attempted in the male.

Deep fascia .

The deep fascia covering the pectoralis major is continuous with the periosteum of the clavicle and sternum, and passes over the infraclavicular fossa and deltopectoral groove (between the pectoralis major and the deltoid) to become continuous with the fascia covering the deltoid. It curves over the inferolateral border of the pectoralis major to become continuous with the fascia of the axillary floor (axillary fascia).

The axillary fascia stretches between the pectoralis major and the latissimus dorsi. When the arm is abducted, the axillary fascia rises into the axilla to form the armpit. The clavipectoral fascia lies in the anterior wall of the axilla, deep to the pectoralis major. It extends from the clavicle to the axillary fascia and encloses the pectoralis minor and subclavius muscles .

Pectoralis major Muscle .

This powerful, fan-shaped muscle takes origin from the medial half of the front of the clavicle, the anterior surfaces of the sternum and upper six costal cartilages, and the aponeurosis of the external oblique muscle of the abdomen . It is inserted into the lateral lip of the intertubercular sulcus or crest on the humerus .

At the insertion, the abdominal part twists under the sternocostal part to form a U-shaped tendon with it. The lowest abdominal fibres are inserted deep to the upper sternocostal fibres, while the intermediate fibres form the base of the U in the anterior axillary fold. The clavicular part passes Inferolaterally, fuses with the anterior layer of the U-shaped tendon and extends further inferiorly on the humerus. The clavicular part lies at right angles to the abdominal and lower sternocostal parts and has different actions.

Nerve supply: medial and lateral pectoral nerves.

Actions: The pectoralis major adducts and medially rotates the humerus. With the arm above the head, the lowest fibres act with the latissimus dorsi to pull down the arm or raise the body, as in climbing a rope. The muscle can also return the extended humerus to the anatomical position, then continue to flex the shoulder joint with its clavicular part which passes in front of the shoulder.

Pectoralis minor Muscle .

This triangular muscle originates from the third to fifth ribs, near their cartilages and passes superolaterally to the tip of the coracoid process . Nerve supply: medial pectoral nerve. Actions: it pulls the scapula (and hence the shoulder) downwards and forwards. It raises the ribs in inspiration when the scapula is fixed.

Subclavius Muscle .

This small muscle arises from the adjacent parts of the upper surfaces of the first costal cartilage and rib. It passes parallel to the clavicle and is inserted into the groove on the inferior surface of the clavicle . Nerve supply: nerve to the subclavius from the upper trunk of the brachial plexus. Actions: it holds the medial end of the clavicle against the articular disc of the sternoclavicular joint during movements of the shoulder girdle.

Sternoclavicular joint .

The sternoclavicular joint is a synovial joint between the shallow notch at the superolateral angle of the manubrium of the sternum and the larger medial end of the clavicle. A complete articular disc intervenes between these two articular surfaces. The joint also extends on to the superior surface of the first costal cartilage. This is the only articulation of the upper limb bones with the axial skeleton. Thus, the clavicle forms a support which maintains the scapula in position and transmits forces from the upper limb to the trunk, e.g. forces generated in falling on the outstretched hand.

Functionally, the joint behaves like a ball-and-socket joint with a wide range of movements, since it has to move with each change in scapular position. It carries heavy loadings, but the bony surfaces give little intrinsic stability. For this reason, it is strengthened by powerful ligaments which are designed to prevent dislocation of the medial end of the clavicle from the shallow fossa on the sternum. The articular capsule is attached close to the articular margins of the bones. It is thickened anteriorly and posteriorly to form the anterior and posterior sternoclavicular ligaments .

The articular disc is a nearly circular plate of fibrocartilage attached at its margins to the articular capsule. It divides the joint into two separate synovial cavities. Its strongest attachments are to the upper surface of the medial end of the clavicle and to the junction of the sternum and first costal cartilage. It assists the costoclavicular ligament in preventing the upward displacement of the medial end of the clavicle and acts as a shock absorber of compression forces applied from the upper limb .

The costoclavicular ligament is a powerful band which passes upwards and laterally from the junction of the first rib and its cartilage to a rough area on the inferior surface of the clavicle near its medial end. The interclavicular ligament passes between the medial ends of the two clavicles and is fused with the articular capsules and the jugular notch of the sternum .

Boundaries and contents of Axilla .

The anterior wall of the axilla extends from the clavicle to the anterior axillary fold. It consists of the pectoralis major, the pectoralis minor, the subclavius, and the fascia enclosing them . The posterior wall consists of the lateral part of the costal surface of the scapula, covered by the subscapularis superiorly, and the teres major muscle with the latissimus dorsi winding round its lower border inferiorly. The anterior and posterior axillary folds are formed by the lower borders of the pectoralis major and the latissimus dorsi respectively.

The convex medial wall is formed by the lateral wall of the thorax (the first five ribs and intercostal spaces) covered by the serratus anterior. The narrow lateral boundary is formed by the humerus covered by the upper parts of the biceps and  coracobrachialis muscles. The apex of the axilla is bounded by the clavicle, first rib, and upper border of the scapula.

It is continuous medially with the superior aperture of the thorax and the root of the neck. Through the apex, vessels from the thorax and the nerves of the brachial plexus from the neck enter the axilla . These vessels and nerves descend through the axilla to the arm and form the contents of the axilla, together with the axillary lymph nodes and loose fatty tissue.

Serratus anterior Muscle .

The serratus anterior arises from the outer surface of the upper eight ribs. Its fibres pass posteriorly around the lateral surface of the chest wall forming the medial wall of the axilla. On the back, the fibres run deep to the scapula and are inserted into the costal surface of the scapula along the medial border. Nerve supply: long thoracic nerve . Actions: it holds the scapula against the ribs and protracts the scapula. The lower fibres are powerful lateral rotators of the scapula.

Axillary artery .

This is the main artery of the upper limb. It is a continuation of the subclavian artery at the outer border of the first rib. It passes through the apex and lateral part of the axilla to become the brachial artery at the lower border of the teres major, close to the humerus. For the purpose of description, it is divided into three parts which lie superior, posterior, and inferior to the pectoralis minor. The cords of the brachial plexus lie posterior to the first part and are arranged around the second part according to their names.

The main nerves arising from the cords surround the third part. The axillary artery supplies the structures in and surrounding the axilla :

(a) The thoracoacromial artery arises from the second part of the axillary artery and supplies the anterior axillary wall of the axilla, including the clavicle, acromion, and anterior part of the deltoid;

(b) The superior and lateral thoracic arteries supply the medial axillary wall and the lateral part of the mammary gland and surrounding structures;

(c) The subscapular artery supplies the posterior axillary wall, including the scapula and muscles covering its posterior aspect; it gives off two branches—circumflex scapular and thoracodorsal arteries—which anastomose with branches from the subclavian artery; and

(d) The anterior and posterior circumflex humeral arteries supply the proximal part of the humerus, the muscles covering it, and the shoulder joint.

Axillary vein .

The axillary vein lies on the anteromedial aspect of the axillary artery and has the same extent. It is the continuation of the basilic vein and receives tributaries corresponding to the branches of the axillary artery. In addition, the axillary vein also receives the brachial veins inferiorly, and the cephalic vein superiorly. It continuous as the subclavian vein at the outer border of the first rib .

Axillary lymph nodes .

These lymph nodes drain the lymph vessels of the upper limb and the superficial vessels of the trunk above the level of the umbilicus and iliac crest. The nodes are scattered throughout the fascia of the axilla and, for the most part, transfer lymph towards the nodes at its apex (apical nodes). For descriptive purposes, the axillary lymph nodes are divided into five groups, four of which lie in one angle of the axillary pyramid and drain a specific territory. The lateral nodes lie along the axillary vessels and drain the greater part of the upper limb.

The pectoral or anterior group lies in the anteromedial angle, deep to the pectoralis major, and drains the superficial tissues of the anterior and lateral parts of the thoracic and upper abdominal walls. The subscapular or posterior group lies along the subscapular vessels and drains lymph from the corresponding region on the back.

All these nodes communicate with the more centrally placed central nodes. The efferents of all these nodes pass to the apical group which also receives vessels from nodes on the cephalic vein and in the infraclavicular fossa. The efferents of the apical nodes form the subclavian lymph trunk which usually drains into the subclavian vein.

Brachial plexus .

The brachial plexus is an important nerve plexus that supplies sensory and motor innervation to the upper limb. The plexus begins in the lower part of the neck (supraclavicular part: roots and trunks) and passes as divisions behind the middle third of the clavicle into the apex of the axilla.

Roots of the brachial plexus .

The roots of the brachial plexus are formed by the ventral rami of the lower four cervical nerves, the greater part of the ventral ramus of the first thoracic nerve (C5 to T1). Small twigs from the ventral rami of the fourth cervical and second thoracic nerves may join the plexus.

Trunks and divisions of the brachial plexus .

Three trunks—the superior, middle, and inferior trunks—arise from the roots. The ventral rami of the fifth and sixth cervical nerves unite to form the superior trunk; the seventh cervical ventral ramus remains single and continues as the middle trunk. The eighth cervical and first thoracic ventral rami unite to form the inferior trunk. A short distance above the clavicle, each of these trunks splits into an anterior and posterior division.

Cords of the brachial plexus .

The three posterior divisions unite to form the posterior cord of the plexus. The posterior cord supplies the extensor muscles and the skin on the back of the limb. The three anterior divisions supply the flexor muscles and the skin on the front of the limb. The anterior divisions of the upper and middle trunks unite to form the lateral cord of the plexus, and the anterior division of the lower trunk forms the medial cord.

In the axilla (infraclavicular part), the cords first lie posterior to the first part of the axillary artery, but lower down posterior to the pectoralis minor, they surround the second part of the axillary artery in positions which correspond to their names. The plexus ends at the lower border of the pectoralis minor by dividing into a number of branches. The plexus is so arranged that each cord and the nerves which arise from it contain nerve fibres from more than one spinal (segmental) nerve.

Thus, the lateral cord contains nerve fibres from the cervical (C.) nerves 5 to 7 , the medial cord from C. 8 and thoracic (T.) 1 (and 2), and the posterior cord from C. 5 to C. 8 (and T. 1). A knowledge of these ‘segmental values’, or root values is of importance in the diagnosis of injuries to the spinal nerves or to the spinal medulla from which they arise.

Branches of the brachial plexus .

1. Branches arising in the neck but distributed to the upper limb

1. The dorsal scapular nerve (C. 5) supplies the rhomboid major and minor and levator scapulae. It will be seen later on the deep surface of the rhomboid muscles.
2. The suprascapular nerve (C. 5, 6) supplies the supraspinatus and infraspinatus muscles. It runs Inferolaterally behind the clavicle and crosses the superior border of the scapula to its posterior surface .
3. The nerve to the subclavius (C. 5, 6) descends in front of the plexus to supply the subclavius.
4. The long thoracic nerve (C. 5, 6, 7) arises from the posterior aspect of these ventral rami. It descends behind the brachial plexus and axillary artery and then on the lateral surface of the serratus anterior muscle which it supplies.

2. Branches arising in the axilla .

1. The lateral and medial pectoral nerves pass forwards from the corresponding cords of the brachial plexus. They communicate in front of the axillary artery and pass to supply the pectoral muscles in the anterior axillary wall. The lateral pectoral nerve (C. 5, 6, 7) pierces the clavipectoral fascia to enter the deep surface of the pectoralis major superior to the pectoralis minor. The medial pectoral nerve (C. 8, T. 1) supplies and pierces the pectoralis minor to enter the pectoralis major .
2. The upper and lower subscapular nerves (C. 5, 6) arise from the posterior cord of the brachial plexus with the thoracodorsal nerve. They supply the muscles of the posterior axillary wall. The upper supplies the subscapularis muscle; the lower supplies the lower fibres of the subscapularis and teres major.
3. The thoracodorsal nerve passes posteroinferiorly to supply the latissimus dorsi muscle. It runs with the thoracodorsal artery on the deep surface of the muscle.
4. The axillary nerve is a terminal branch of the posterior cord of the brachial plexus and is formed near the lower border of the subscapularis. It leaves the axilla by passing back under the subscapularis.
5. The musculocutaneous nerve arises in the axilla from the lateral cord of the brachial plexus and passes Inferolaterally to supply, and then pierce, the coracobrachialis.
6. The median nerve is formed lateral to the axillary artery by one root each from the medial and lateral cords of the brachial plexus. It crosses anterior to the axillary artery and comes to lie on its medial side.
7. The ulnar nerve arises from the medial cord of the brachial plexus and runs down between the axillary artery and vein.
8. The radial nerve is the other terminal branch of the posterior cord of the brachial plexus in the axilla. In the axilla, it gives off the nerve to the long head of the triceps—a muscle of the arm .

 

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