Lab 1 - Module 2: Axilla - Page 1 of 8

READINGS:

Gray's Anatomy for Students (Fourth Edition): Pages: 710-738

CASE EXERCISE: Axilla & Sentinel Node Context

Sentinel lymph node biopsy samples the first node or nodes most likely to receive metastatic spread. Most breast lymph drains to the anterior (pectoral) axillary nodes, then centrally to central and apical nodes. Tumors in medial quadrants may also drain to parasternal (internal thoracic) nodes. For axillary lymph node dissection, precise knowledge of axillary walls and neurovascular contents is essential to preserve function and reduce morbidity.

Axilla

The Axilla (or arm pit) is a pyramid-shaped space that provides a passageway for nerves and vessels traveling between the trunk and upper limb.

The Medial Wall of the Axilla is formed by the upper ribs, intercostal muscles, and serratus anterior.

Rotate to a right lateral view of the thoracic cage. Identify the apex of the axilla, formed by the convergence of: the clavicle (in the anterior axillary wall), scapula (in the posterior axillary wall), and first rib (in the medial axillary wall). This convergence creates an opening at the apex called the cervicoaxillary canal through which the major nerves and vessels pass.

Identify the following bones:

Clavicle

Scapula

First rib

Add the intercostal muscles. Recall the muscle fiber orientation of the external, internal, and innermost intercostal muscles.

Add the serratus anterior and rotate the model to better visualize this muscle. Please note: do not spend too much time on the muscle tables or you won't have time to complete this lab! You can always come back later on your personal device.

Proximal Attachment (Origin) - lateral surfaces of ribs 1-8 (sometimes 9) and the deep fascia overlying the related intercostal spaces.

Distal Attachment (Insertion) - costal surface of the medial border of the scapula.

Innervation - long thoracic nerve (C5, C6, C7, "raise your arms to heaven").

Action - protracts scapula, holds scapula fixed against thoracic wall, and superiorly rotates glenoid for abduction >90°.

Blood supply - mainly lateral thoracic artery (± contributions from thoracodorsal artery)

The Posterior Wall of the Axilla is formed by the scapula and the subscapularis, latissimus dorsi and teres major muscles.

Add the subscapularis muscles. Rotate the model to visualize this muscle.

Proximal Attachment (Origin) - medial two-thirds of the subscapular fossa.

Distal Attachment (Insertion) - lesser tubercle of the humerus.

Innervation - upper and lower subscapular nerves (C5-C6).

Action - medially rotates and adducts humereus and stabilizes the glenohumeral joint.

Blood supply - primarily subscapular artery, with contributions from the circumflex scapular artery.

Add the teres major muscles. Rotate the 3D model to visualize this muscle better.

Origin - elongated oval area on the posterior surface of the inferior angle of the scapula.

Insertion - medial lip of the intertubecular sulcus (bicipital groove) on the anterior surface of the humerus.

Innervation - lower subscapular nerves (C5-C6).

Action - adducts and medially rotates humerus at the shoulder; assists extension from a flexed position.

Blood supply - mainly circumflex scapular artery

Add the latissimus dorsi muscles and rotate the model to better visualize them.

Proximal Attachment (Origin) - spinous processes of T7-T12, thoracolumbar fascia, iliac crest, and ribs 10-12.

Distal Attachment (Insertion) - floor of the intertubercular sulcus.

Innervation - thoracodorsal nerve (C6-C8).

Action - extends, adducts, and medially rotates the humereus at the shoulder; elevates the trunk with fixed arms

Blood supply - thoracodorsal artery

Add the humerus and see how these posterior wall muscles distally attach to the humerus.

Note the mnemonic at the bicipital groove: to recall the lateral lip (pectoralis major), floor (latissimus dorsi), medial lip (teres major) remember the “lady between two Majors.”

The Lateral Wall of the Axilla is formed by the humerus and the coracobrachialis muscles.

Add the coracobrachialis muscles and rotate the model to better visualize this muscle.

Proximal Attachment (Origin) - apex of the coracoid process.

Distal Attachment (Insertion) - linear roughing on the medial midshaft of the humerus.

Innervation - musculocutaneous nerve (C5-C7) and note that the nerve pierces the muscle.

Action - flexes and adducts at the shoulder and helps resist inferior dislocation.

Blood supply - branches of the brachial artery.

The Anterior Wall of the Axilla is formed by the clavicle, pectoralis minor, and pectoralis major.

Add the pectoralis minor muscles and rotate the model to better examine them.

Proximal Attachment (Origin) - anterior surfaces and superior borders of ribs III-V.

Distal Attachment (Insertion) - coracoid process of the scapula.

Innervation - medial pectoral nerve (C8-T1).

Action - stabilizes the scapula by drawing it anteroinferiorly and assists forced inspiration when the scapula is fixed.

Blood supply - pectoral branch of thoracoacromial trunk and lateral thoracic artery.

Add the pectoralis major muscles and rotate the model to better explore this muscle.

Proximal Attachment (Origin) - clavicular head - anterior surface of the medial half of the clavicle, sternocostal head - anterior surface of the sternum, costal cartilages1-6(7), aponeurosis of the external oblique, and the sternal end of rib 6.

Distal Attachment (Insertion) - lateral lip of the intertubercular sulcus of the humerus.

Innervation - medial pectoral nerve (C8-T1) and lateral pectoral nerves (C5-C7).

Action - adducts and medially rotates humerus at shoulder; clavicular head flexes at shoulder and sternocostal head extends from flexion.

Blood supply - pectoral branch of thoracoacromial trunk and lateral thoracic artery.

Identify the opening at the axillary apex called the cervicoaxillary canal, through which the axillary artery and cords of the brachial plexus pass. Major structures that pass from the neck into the axilla via this passageway include the subclavian → axillary artery, the cords and terminal branches of the brachial plexus, lymphatic vessels, and associated fat. In short: it’s the neck to arm highway for neurovascular and lymphatic traffic. This knowledge is important for surgery in this region and to understand clinically relevant thoracic outlet relationships. Rotate the 3D model to better appreciate the area's spatial relationships.

Now, add the cross-sections panel and transverse anatomic section to better understand sectional anatomy of the major muscles and neurovasculature of the pectoral and axillary regions.