Thorax: Breast and Pectoral Region Anatomy

Pectoral Region & Breast Anatomy Notes

Objectives

• Describe commonly-used vertical reference lines on the anterior and lateral thoracic wall.

• Describe the structure of the female breast.

• Describe the vascular supply and lymphatic drainage of the breast in relation to the spread of breast cancer.

• Describe the muscles of the pectoral region in terms of attachments, innervations, and major actions.

Thoracic Wall Vertical Reference Lines

• Anterior Axillary Line: A vertical line passing through the anterior axillary fold.

• Posterior Axillary Line: A vertical line passing through the posterior axillary fold.

• Midaxillary Line: A vertical line positioned midway between the anterior and posterior axillary lines.

• Anterior Median Line: A vertical line running down the midline of the anterior body (midsagittal).

• Midclavicular Lines: Parasagittal vertical lines, each passing through the midpoint of its respective clavicle.

• Posterior Median (Midvertebral) Line: A vertical line running along the tips of the spinous processes of the vertebrae.

• Scapular Lines: Lines parallel to the posterior median line that intersect the inferior angles of the scapulae.

Structure of the Female Breast

• Nature: An accessory reproductive organ, fundamentally a modified sweat gland.

• Composition: Consists of glandular tissue embedded within a fatty connective tissue matrix.

• Location: Situated within the superficial fascia, anterior to the deep pectoral fascia.

• Extent: Spans from the anterior axillary line to the sternum, extending vertically from rib $2$ to rib $6$.

• Axillary Process (Tail of Spence): An extension of breast tissue into the axilla.

External Features:

• Areola: A pigmented area of skin surrounding the nipple.

  • Montgomery Tubercles: Sebaceous glands located within the areola.

• Nipple: The central projection of the breast.

  • Lacks hair, fat, and sweat glands.

  • Contains smooth muscle fibers.

  • Fissured by the openings of the lactiferous ducts.

  • Typically located at the $4^{th}$ intercostal space (note: this position is not accurate for adult women due to variability).

Internal Features:

• Retromammary Space (Bursa):

  • A loose connective tissue space located between the superficial and deep pectoral fascia.

  • Allows for movement of the breast on the pectoral fascia.

• Suspensory Ligaments of Cooper (Fibrous Septa):

  • Fibrous tissue cords connecting the breast parenchyma to the overlying dermis.

  • More developed in the superior parts of the breast.

  • Provide support for the gland lobules.

• Glandular Tissue (Parenchyma):

  • Comprises $15-20$ lobes of glandular tissue.

  • Each lobe contains $20-40$ terminal ductal lobular units.

  • Each lobe is drained by a lactiferous duct.

  • Lactiferous Sinus: A dilated portion of a lactiferous duct located just deep to the areola.

  • Supported by the suspensory ligaments (of Cooper), which form fibrous septa anchored to the skin and deep fascia.

• Alveoli/Acini: The functional units within the lobules where milk is produced.

Vascular Supply of the Breast

Arterial Supply (mirrored by venous drainage):

• Medial Breast: Supplied by perforating branches of the internal thoracic artery (known as medial mammary arteries).

• Lateral Breast: Supplied by:

  • Superior thoracic artery (lateral mammary arteries).

  • Lateral thoracic artery (lateral mammary arteries).

  • Pectoral branch of the thoracoacromial artery.

• General Supply: Anterior and posterior intercostal arteries contribute mammary branches.

Venous Drainage

• Mainly drains into the axillary vein.

• Venous drainage generally mirrors the arterial supply pathways.

Cutaneous Innervation of the Breast

• Primary Nerves: The $4^{th}$ to $6^{th}$ intercostal nerves.

  • Supply is primarily via their lateral and anterior cutaneous branches.

• Nipple Innervation: Specifically supplied by the $4^{th}$ intercostal nerve (ventral ramus of T4).

• Nerve Fiber Types: The branches of the intercostal nerves convey:

  • Sensory fibers from the skin of the breast.

  • Sympathetic fibers to the blood vessels in the breasts and the smooth muscle in the overlying skin and nipple.

Lymphatic Drainage of the Breast and Cancer Spread

• Lymph passes from the lobules of the gland, nipple, and areola to the subareolar lymphatic plexus.

Lymph Nodes of the Breast:

• Axillary Lymph Nodes: A primary drainage site, including:

  • Anterior (Pectoral) nodes

  • Posterior (Subscapular) nodes

  • Central nodes

  • Apical nodes

• Parasternal Nodes (Internal Thoracic Nodes)

• Interpectoral Nodes (Rotter’s Nodes)

Lymph Drainage Pathways:

1. Axillary Pathway: Drains approximately $75\%$ of lymph.

   • Lymph flows to axillary nodes (primarily the anterior axillary nodes).

   • Can bypass axillary nodes and drain directly.

2. Internal Thoracic Pathway (Mammary): Drains approximately $20\%$ of lymph to the parasternal lymph nodes.

   • Can communicate with lymph vessels of the opposite breast.

3. Retropectoral and Transpectoral Pathways: Drain approximately $5\%$ of lymph to the posterior intercostal nodes.

4. Subdiaphragmatic Pathway: Drains to inferior phrenic nodes.

Clinical Relevance of Lymph Drainage in Breast Cancer:

• Most breast cancers are found in the upper outer quadrant, which is rich in lymphatic drainage to the axilla.

Signs of Advanced Breast Cancer:

• Peau d’orange: An orange-peel appearance of the skin, caused by obstruction of superficial lymphatics and subsequent edema, as the suspensory ligaments hold the skin down, creating dimpling.

• Skin Dimpling and Nipple Retraction: Occurs when tumors invade through the retromammary space and deep fascia, leading to:

  • Breast Fixation: The breast becomes immobile.

  • Shortened Suspensory Ligaments: Invasion of the ligaments by cancer cells can cause them to shorten, pulling on the skin and nipple.

Fascia of the Pectoral Region

• Pectoral Fascia: Covers the anterior and posterior surfaces of the pectoralis major muscle.

  • Inferiorly, it transitions into the axillary fascia, forming the floor of the axilla.

• Clavipectoral Fascia: A strong fascial layer located deep to the pectoralis major.

  • Attaches superiorly to the clavicle.

  • Envelopes the subclavius and pectoralis minor muscles.

  • Structures Piercing It: Cephalic vein, thoracoacromial artery, and lateral pectoral nerve.

  • Below the pectoralis minor, it is termed the suspensory ligament of the axilla.

  • Connects to and supports the axillary fascia.

Anterior Axio-Appendicular Muscles (Pectoral Region)

• These muscles connect the pectoral girdle and humerus to the axial skeleton.

• Their primary function is to move the pectoral girdle and humerus.

1. Pectoralis Major

• Description: A large, fan-shaped muscle forming the bulk of the chest and the anterior axillary fold.

• Heads: Consists of two heads: a clavicular head and a sternocostal head.

• Proximal Attachment:

  • Clavicular Head: Medial half of the clavicle (anterior surface).

  • Sternocostal Head: Anterior surface of the sternum, superior $6$ costal cartilages, and the aponeurosis of the external oblique muscle.

• Distal Attachment: Lateral lip of the intertubercular sulcus (groove) of the humerus.

• Innervation: Lateral pectoral nerve (from C5, C6) and medial pectoral nerve (from C7, C8, T1).

• Main Actions:

  • Adducts the humerus.

  • Medially rotates the humerus.

  • Clavicular Head: Flexes the humerus.

  • Sternocostal Head: Extends the humerus from a flexed position.

  • Draws the scapula anteriorly and inferiorly.

2. Pectoralis Minor

• Description: A smaller, triangular muscle located deep to the pectoralis major.

• Proximal Attachment: $3^{rd}$ to $5^{th}$ ribs near their costal cartilages.

• Distal Attachment: Medial border and superior surface of the coracoid process of the scapula.

• Innervation: Medial pectoral nerve (C8, T1) and may receive variable contribution from the lateral pectoral nerve.

• Main Actions:

  • Stabilizes the scapula on the thoracic wall.

  • Draws the scapula anteroinferiorly (causing the lateral angle to move inferiorly).

  • Acts as an accessory muscle of respiration.

• Significance: An important anatomical landmark for structures like the axillary artery and brachial plexus. The subcoracoid space (deep to the coracoid process and pectoralis minor) contains parts of the brachial plexus and axillary vessels.

3. Subclavius

• Description: A small, triangular muscle located inferior to the clavicle.

• Proximal Attachment: Junction of the $1^{st}$ rib and its costal cartilage.

• Distal Attachment: Inferior surface of the middle third of the clavicle.

• Innervation: Nerve to subclavius (C5, C6).

• Main Actions:

  • Anchors and depresses the clavicle.

  • Stabilizes the sternoclavicular joint.

  • Affords some protection to the subclavian vessels and the superior trunk of the brachial plexus if the clavicle fractures.

4. Serratus Anterior

• Description: A broad, flat muscle with a serrated appearance, lining the lateral thoracic wall.

• Proximal Attachment: External surfaces of the lateral parts of the $1^{st}$ to $8^{th}$ (or $9^{th}$) ribs.

• Distal Attachment: Anterior surface of the medial border of the scapula, including its superior and inferior angles.

• Innervation: Long thoracic nerve (C5, C6, C7).

• Main Actions:

  • Protracts the scapula (pulls it forward, e.g., during punching, hence called the "boxer's muscle").

  • Holds the scapula against the thoracic wall.

  • Rotates the scapula (upward rotation of the glenoid cavity, essential for arm elevation (abduction) above $90^{\circ}$).

Clinical Correlation: Long Thoracic Nerve Injury

• Description: The long thoracic nerve is a branch of the brachial plexus and has a superficial location, making it vulnerable to injury.

• Causes: Can be injured during:

  • Mastectomy (surgical removal of the breast).

  • Knife injuries to the chest wall.

  • Insertion of a chest tube.

• Results: Leads to weakness or paralysis of the serratus anterior muscle.

• Symptoms:

  • Winging of the scapula: The medial border of the scapula becomes prominent and moves away from the posterior thoracic wall, especially when pushing against a wall.

  • Difficulty with arm elevation (abduction) above $90^{\circ}$, as the serratus anterior is crucial for upward rotation of the scapula.