Anatomy of the Thoracic Wall, Axilla and Breast.

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Anatomy of the Thoracic Wall, Axilla and Breast.pdf

The pectoralis major muscle (Figs. 1 and 2), which has a triangular shape and is very evident on the anterior

thoracic wall, has two origins: one clavicular (medial third of the clavicle) and the other sternocostal (external face of

the first seven costal cartilages, lateral margin of the sternum and upper part of the rectus abdominis muscle). The

clavicular and sternocostal fibers converge to be inserted into the lateral lip of the intertubercular groove of the

humerus. The innervation is performed by the pectoralis medialis and lateralis nerves, which are branches of

fasciculus medialis and lateralis of the plexus brachialis, respectively. It forms the anterior axillary fold (Williams et

al.; Drake et al.).

The pectoralis minor muscle (Figs. 1 and 2) is located at a deeper level than the pectoralis major muscle, and also

has a triangular shape, but of smaller dimensions. It originates from the external face of the third, fourth and fifth ribs and

is inserted in the coracoid process of the scapula. It is innervated also by the pectoralis lateralis and medialis nerves

(Williams et al.; Drake et al.).

The serratus anterior muscle (Fig. 1) covers most of the lateral thoracic wall and originates as muscle strips from

the external face of the first nine ribs. Its fibers converge posteriorly to be inserted into the medial margin of the

scapula, thus contributing towards forming the medial wall of the axilla. The more caudal fibers interfinger with those

of the obliquus externus muscle of the abdomen. This muscle stabilizes the scapula by exerting traction anteriorly and

promoting elevation of the glenoid cavity. The innervation of this muscle is performed by the thoracicus longus nerve

(Bell’s nerve) (Williams et al.; SBA, 2001; Drake et al.).

The diminutive subclavius muscle (Fig. 2), which is posterior to the costocoracoid ligament of the clavipectoral

fascia and difficult to see, originates from the first rib, close to the chondrocostal junction, and its fibers run superolaterally to be inserted in the inferior face of the middle third of the clavicle. Its function is to stabilize the clavicle in movements of the shoulder. Its innervation is performed by the nerve of the subclavius muscle, a branch of the truncus superior of the plexus brachialis (Williams et al.; Drake et al.).

The latissimus dorsi muscle (Figs. 2 and 3) is a wide lamina located on the dorsum of the trunk. It originates from

the last six vertebral spinous processes, the iliac crest and the fascia thoracolumbalis. The fibers converge to a single

twisted tendon that is inserted in the intertubercular groove of the humerus. Its innervation is performed by the thoracodorsalis nerve, a branch of the fasciculus posterior of the plexus brachialis (Williams et al.; Drake et al.).

The teres major muscle (Fig. 2) originates at the inferior angle of the scapula and rises parallel to the lateral

scapular margin towards the medial crest of the intertubercular groove of the humerus, where it is inserted. It receives innervation from the subscapularis inferior nerve, a branch of fasciculus posterior of the plexus brachialis. Together with the latissimus dorsi muscle, it forms the posterior axillary fold (Williams et al.; Drake et al.).

2.4. Nerves. 

Most of the nerves found in the axilla come from the plexus brachialis. Only the intercostobrachialis nerve does not come from this plexus. The plexus brachialis (Fig. 6) is formed anatomically by the primary ventral branches (“roots”, radices) of the four inferior cervical nerves and the first thoracic nerve (C5 to T1). There may possibly be contributions from the primary ventral branches of the fourth cervical nerve and the second thoracic nerve (Lockart et al.; Williams et al.). 

After the components of the plexus emerge from the intervertebral foramens, they are positioned between the scalenus anterior and scalenus medius muscles. In the lower part of the neck, the primary ventral branches of the plexus brachialis joint to form three trunks(trunci): superior (C5 and C6), medius (C7) and inferior (C8 and T1).

Each trunk bifurcates into anterior and posterior divisions (divisiones) as the plexus runs posteriorly to the clavicle via

the apex of the axilla. From the mixing of the fibers from the anterior and posterior divisions of the plexus brachialis, the

lateralis, medialis and posterior fascicles are formed. The names of the fascicles refer to their relationships with the

second portion of the axillaris artery, i.e. they are respectively lateral, medial and posterior in relation to the second portion

of the axillaris artery (Snell; Moore & Dalley).

In accordance with the relationship with the clavicle, the plexus brachialis is, for teaching purposes, divided into the supraclavicular and infraclavicular portions. In the axilla, the following nerves are found (Lockhart et al.; Williams et al.; Romrell & Bland; SBA; Drake et al.).

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Intercostobrachialis nerve: This corresponds to the lateral cutaneous branch of the second intercostalis nerve. It

arises from the second intercostal space and runs obliquely towards the arm, where it anastomoses with the cutaneous

brachii medialis nerve, which is a branch of the plexus brachialis. A second intercostobrachialis nerve may also

occasionally be observed emerging from the third intercostal space.

Thoracicus longus nerve (Bell’s nerve): This originates from the posterior face of the primary ventral branches of C5, C6 and C7. It runs downwards and goes posteriorly to the neurovascular bundle, towards the lateral thoracic wall to innervate the serratus anterior muscle. It is covered by fascia of this muscle.

Subclavius nerve (a branch of the upper trunk of the plexus brachialis): The fibers of this nerve derive mainly from C5 nerve, with contributions from C4 and C6 nerves. It runs downwards to the clavicle and supplies the subclavius muscle.

Pectoralis lateralis nerve (a branch of the fasciculus lateralis of the plexus brachialis): This supplies the pectoralis major muscle, after penetrating the clavipectoral fascia together with the thoracoacromialis artery and cephalica vein (it does not penetrate the pectoralis minor muscle). It sends out a communicating branch to the pectoralis medialis nerve, which innervates the pectoralis minor muscle.

Musculocutaneus nerve (a branch of the fasciculus lateralis of the plexus brachialis): Upon leaving the axilla, this nerve penetrates the coracobrachialis muscle and innervates it. Upon leaving this, it runs between the biceps brachii and brachialis muscles and supplies them. It continues superficially and laterally as a cutaneus antebrachii lateralis nerve in the forearm.

Medianus nerve: This is formed by the lateral and medial roots coming from the fasciculus lateralis and fasciculus medialis, respectively. It innervates most of the flexors and pronators muscles of the forearm and five intrinsic muscles of the hand, and it picks up the sensitivity of the skin of part of the hand and fingers.

Pectoralis medialis nerve (a branch of the fasciculus medialis of the plexus brachialis): This nerve penetrates the

pectoralis minor muscle to supply it, and continues to also innervate the pectoralis major muscle.

Cutaneus brachii medialis nerve (a branch of the fasciculus medialis of the plexus brachialis): This is a thin nerve that picks up the sensitivity of the medial face of the arm and the superior medial face of the forearm.

Cutaneus antebrachii medialis nerve (a branch of the fasciculus medialis of the plexus brachialis): This nerve is bigger than the preceding one and is located between the axillaris artery and vein, supplying the skin of the medial face of the forearm.

Ulnaris nerve (a branch of the fasciculus medialis of the plexus brachialis): This runs the whole length of the

arm without innervating anything. In the forearm it innervates two flexors muscles and in the hand it is the principal nerve, since it innervates the majority of its intrinsic muscles.

Thoracodorsalis nerve (a branch of the fasciculus posterior of the plexus brachialis): This innervates the

latissimus dorsi muscle. It accompanies the subscapularis and thoracodorsalis arteries anteriorly to the subscapularis

muscle. 

Subscapularis superioris nerve (a branch of the fasciculus posterior of the plexus brachialis): This is located medially to the thoracodorsalis neurovascular bundle and innervates the subscapularis muscle.

Subscapularis inferioris nerve (a branch of the fasciculus posterior of the plexus brachialis): This innervates the teres major muscle and the lower part of the subscapularis muscle. It is located laterally to the thoracodorsalis  neurovascular bundle.

Axillaris nerve (a branch of the fasciculus posterior of the plexus brachialis): This supplies the teres minor muscle as it leaves the axillary space through the quadrangular space. It innervates the deltoideus muscle from its deep posterior part and continues as a cutaneus brachii lateralis superioris nerve, innervating the skin on the lower half of the deltoideus muscle.

Radialis nerve (a branch of the fasciculus posterior of the plexus brachialis): This is the biggest nerve in the

plexus brachialis. After leaving the axilla, it penetrates the groove of the radialis nerve of the humerus, where it may

be damaged in the event of humeral fractures. It innervates all the extensors muscles of the posterior compartments of

the arm and forearm, and also the supinator and brachioradialis muscles. It originates the cutaneus brachii and antebrachii posteriores nerves, and also the cutaneous brachii lateralis inferior nerve.

Injuries to nerves may occur during axillary lymphonodectomy. By respecting the axillaris vein as the most cranial limit of the field of lymph node dissection, the fascicles and different branches of the plexus brachialis will be protected from inadvertent lesion. Nonetheless, some nerves may suffer injury during lymphonodectomy.

The intercostobrachialis nerve is frequently sectioned because it crosses the axilla obliquely towards the arm and

is located within the product from the lymph node dissection. Because this is a sensitive nerve, the impairment

caused will consist of hyposthesia of the skin that covers the axilla and medial face of the arm. 

The thoracicus longus nerve runs laterally along the thoracic wall and is usually detached from its bed when the fascia of the serratus anterior muscle is removed, such that the nerve adheres to the dissection product. If this situation is not noticed, it may result in sectioning of the nerve and consequent denervation of the serratus anterior muscle, which causes posterior displacement of the scapula (“winged scapula”) (Romrell & Bland). 

The thoracodorsalis nerve may suffer injury if the subscapulares vessels are damaged or ligated. The subscapulares superior and inferior nerves become damaged when the fascia of the subscapularis muscle is extracted and the field of dissection extends posteriorly to the axillaris vein, or over the latissimus dorsi muscle superolaterally (subscapularis inferior nerve). Ligation of the thoracoacromialis artery beyond its emergence from the clavipectoral fascia will cause sectioning of the pectoralis lateralis nerve. Injury to the pectoralis medialis nerve occurs when the pectoralis minor muscle is extracted, or when the space between the pectoral muscles is extensively dissected.