Axilla

Question 1

Outline the boundaries of the axilla.

Answer 1

• The axilla is the space between the upper arm and the side of the thorax, bounded in front and behind by the axillary folds, communicating above with the posterior triangle of the neck and containing neurovascular structures and lymph nodes for the upper limb and the side wall of the thorax.
• Its floor is the axillary fascia extending from the anterior to the posterior axillary folds and from the fascia over the serratus anterior to the deep fascia of the arm.
• The suspensory ligament from the lower border of pectoralis minor is attached to the fascial floor from above.
• Its anterior wall is formed by pectoralis major/minor, subclavius and the clavipectoral fascia.
• The posterior wall extends lower. It is formed by subscapularis and teres major with the tendon of latissimus dorsi winding around the latter muscle.
• The medial wall is formed by the upper part of serratus anterior, the lower limit being defined as the level of the 4th rib.
• The anterior and posterior walls converge laterally to the lips of the intertubercular groove of the humerus in which lies the tendon of the long head of biceps, overlapped medially by coracobrachialis and the tendon of the short head of biceps.
• The apex is bounded by the clavicle, upper border of the scapula and the outer border of the 1st rib. It is the channel of communication between axilla and posterior triangle.

Question 2

Outline the brachial plexus.

Answer 2

• Five roots contribute to the formation of the plexus for the upper limb. They are the fibres that remain in the anterior rami of C5–8 and T1 after these have given their segmental supply to the prevertebral and scalene muscles.
• They are to divide into anterior and posterior divisions to supply the flexor and extensor compartments respectively, but before doing so they unite to form three trunks in the following manner.
• Of the five roots of the plexus the upper two unite to form the upper trunk, the lower two unite to form the lower trunk, and the central root runs on as the middle trunk.
• The five roots lie behind the scalenus anterior muscle and emerge from between it and scalenus medius to form the trunks which cross the lower part of the posterior triangle of the neck.
• Each of the three trunks divides into an anterior and a posterior division behind the clavicle.
• Here, at the outer border of the first rib, the upper two anterior divisions unite to form the lateral cord, the anterior division of the lower trunk runs on as the medial cord, while all three posterior divisions unite to form the posterior cord.
• These three cords enter the axilla above the first part of the artery, approach and embrace its second part, and give off their branches around its third part.
• Thus the roots are between the scalene muscles, trunks in the (posterior) triangle, divisions behind the clavicle, and cords in the axilla.
• There are three branches from the roots: one from the upper trunk and 3, 5 and 5 from the lateral, medial and posterior cords, respectively. There are no branches from the divisions.

Question 3

Describe the branches from the roots.

Answer 3

• The three branches from the roots are the dorsal scapular nerve, the nerve to subclavius, and the long thoracic nerve.
• They arise successively from C5, C5, 6 and C5–7, and pass downwards behind, in front of, and behind the roots in that order.
• The dorsal scapular nerve (nerve to the rhomboids) arises from the posterior aspect of C5, pierces scalenus medius and courses downwards in front of levator scapulae, lying on serratus posterior superior. It is accompanied by the dorsal scapular vessels. It supplies both rhomboids and usually gives a branch to levator scapulae.
• The nerve to subclavius arises from the roots of C5 and 6 where they join to form the upper trunk. It passes down in front of the trunks and the subclavian vessels to enter the posterior surface of subclavius. It frequently has a branch (accessory phrenic nerve) which connects with the phrenic nerve, providing an alternate pathway for some fibres from the fifth cervical anterior ramus to reach the diaphragm.
• The long thoracic nerve (nerve to serratus anterior) arises from the posterior aspects of C5, 6 and 7. Branches of C5 and 6 enter scalenus medius, unite in the muscle, emerge from it as a single trunk and pass down into the axilla. On the surface of serratus anterior (the medial wall of the axilla) this is joined by the branch from C7 which has descended in front of scalenus medius. The nerve passes down posterior to the midaxillary line, deep to the fascia on serratus anterior, and supplies the muscle segmentally.

Question 4

Describe the branch from the trunks.

Answer 4

• The suprascapular nerve arises from the upper trunk in the lower part of the posterior triangle and passes backwards and laterally deep to the border of trapezius.
• It passes through the suprascapular foramen (beneath the transverse scapular ligament) and supplies supraspinatus, descends lateral to the scapular spine with the suprascapular vessels and supplies infraspinatus. It supplies the shoulder and acromioclavicular joints.

Question 5

Describe the branches from the lateral cord.

Answer 5

• The three branches from the lateral cord are the lateral pectoral, musculocutaneous and lateral root of the median nerve.
• The lateral pectoral nerve pierces the clavipectoral fascia to supply pectoralis major with fibres from C5, 6 and 7. It communicates across the front of the first part of the axillary artery with the medial pectoral nerve and through this communication supplies pectoralis minor. It has no cutaneous branch.
• The musculocutaneous nerve (C5–7) leaves the lateral cord quite high in the axilla, runs obliquely downwards and enters coracobrachialis, giving a twig of supply to it, before passing through the muscle. Lower down in the arm it supplies biceps and brachialis and becomes the lateral cutaneous nerve of the forearm. An anaesthetic solution injected through the floor of the axilla to effect a brachial plexus nerve block may not affect the musculocutaneous nerve owing to its high take-off from the lateral cord.
• The lateral root of the median nerve is the continuation of the lateral cord (C5–7). It is joined by the medial root of the median nerve (from the medial cord, C8 and T1); the two roots embrace the artery and, when the arm is pulled down to depress the shoulder may, in some cases, compress the vessel.

Question 6

Describe the branches from the medial cord.

Answer 6

• The five branches from the medial cord are the medial pectoral, medial head of the median nerve, ulnar nerve, and the two cutaneous nerves, to the arm and forearm respectively.
• The medial pectoral nerve arises from the medial cord (C8, T1) behind the first part of the axillary artery and is joined by a communication from the lateral pectoral nerve. It enters the deep surface of pectoralis minor, giving a branch of supply before doing so, perforates the muscle and enters the pectoralis major, in which it ends by supplying the lower costal fibres. It may give a direct branch to pectoralis major, which passes around the lower margin of pectoralis minor. The medial pectoral nerve has no cutaneous branch. The medial and lateral pectoral nerves are named in accordance with their origins from the medial and lateral cords of the brachial plexus.
• The medial root of the median nerve is the continuation of the medial cord, with fibres from C8 and T1, and it crosses the axillary artery to join the lateral root.
• The medial cutaneous nerve of the arm (C8, T1) is the smallest and most medial of all the branches. It runs down on the medial side of the axillary vein and supplies skin over the front and medial side of the arm.
• The medial cutaneous nerve of the forearm (C8, T1) is a larger nerve that runs down between artery and vein in front of the ulnar nerve and supplies skin over the lower part of the arm and the medial side of the forearm.
• The ulnar nerve is the largest branch of the medial cord (C7, 8, T1). It runs down between artery and vein as the most posterior of the structures which run down the medial side of the flexor compartment of the arm. It may receive its C7 fibres as a branch from the lateral cord, if these have not already passed to the medial cord from the anterior ramus of C7.

Question 7

Describe the branches from the posterior cord.

Answer 7

• The five branches from the posterior cord are the upper subscapular, thoracodorsal nerve (nerve to latissimus dorsi), lower subscapular, axillary (circumflex) and radial nerves.
• The upper subscapular nerve is a small nerve (C5, 6) which enters the upper part of subscapularis.
• The thoracodorsal nerve (nerve to latissimus dorsi C6–8) is a large nerve which runs down the posterior axillary wall, crosses the lower border of teres major and enters the deep surface of latissimus dorsi, well forward near the border of the muscle. It comes from high up behind the subscapular artery, but as it descends to enter the muscle it lies in front of the artery, at this level called the thoracodorsal artery. It is thrown into prominence in the position of lateral rotation and abduction of the humerus and is thus in danger in operations on the lower axilla.
• The lower subscapular nerve (C5, 6) is larger than the upper subscapular and supplies the lower part of the subscapularis and ends in teres major.
• The axillary nerve (formerly the circumflex nerve) is one of the two large terminal branches of the posterior cord (the other is the radial nerve). The axillary nerve (C5, 6) supplies nothing in the axilla despite its name having been changed from circumflex to axillary. From its origin, it runs backwards through the quadrangular space bounded by subscapularis above, teres major below, long head of triceps medially and the surgical neck of humerus laterally. It then passes just below the capsule of the shoulder joint, with the posterior circumflex humeral vessels below it, and emerges at the back of the axilla below teres minor. Having given a branch to the shoulder joint, it divides into anterior and posterior branches. The anterior branch winds round behind the humerus in contact with the periosteum and enters the deep surface of the deltoid to supply it; a few terminal twigs pierce the muscle and reach the skin. The posterior branch supplies teres minor and deltoid, then winds around the posterior border of deltoid to become the upper lateral cutaneous nerve of the arm.
• The radial nerve (C5–8, T1) is the continuation of the posterior cord, and is the largest branch of the whole plexus. It crosses the lower border of the posterior axillary wall, lying on the glistening tendon of latissimus dorsi. It passes out of sight through the triangular space below the lower border of this tendon as it lies in front of teres major, between the long head of triceps and the humerus. Before disappearing it gives nerves of supply to the long head of triceps and the medial head (a nerve which accompanies the ulnar nerve along the medial side of the arm) and a cutaneous branch which supplies the skin along the posterior surface of the arm (posterior cutaneous nerve of the arm).