BIO475 Human Anatomy Lecture 10 Thorax: Overview, Pectoral Region, and Thoracic Wall (Skeleton)

Thorax Overview
Basic Structure of the Thorax

  • Shape: The thorax is an irregularly shaped cylinder or truncated cone, allowing for flexibility and expansion during respiration.

  • Apertures:

    • Superior Aperture: Narrow and serves as the entry point for the neck structures to the thorax.

    • Inferior Aperture: Large and allows for the passage of diaphragm, major blood vessels, and nerves.

  • Contents:

    • Thoracic Wall: Composed of a bony framework of ribs and vertebrae, along with intercostal muscles that aid in breathing.

    • Pleural Cavities: Two pleural cavities surround each lung, containing pleural fluid to reduce friction during respiration.

    • Lungs: Two organs responsible for gas exchange between oxygen and carbon dioxide.

    • Mediastinum: A central compartment that houses the heart, large blood vessels, and other vital structures such as the trachea and esophagus.

  • Functions:

    • Protective Casing: The thorax houses and safeguards the heart, lungs, and important blood vessels from injury.

    • Conduit for Structures: Acts as a passageway for nerves and blood vessels moving in and out of the neck and abdomen.

    • Facilitates Breathing: Changes in thoracic volume during respiration enable inhalation and exhalation.

    • Upper Limb Anchor: Provides attachment for muscles that move and stabilize the upper limb.

Pectoral Region Functions

  • Anchoring:

    • Serves as an anchor point for the upper limb, allowing for a wide range of movement.

  • Compartments:

    • Superficial Compartment: Includes skin, superficial fascia, and mammary glands, which are important for lactation and in females, also play a role in secondary sexual characteristics.

    • Deep Compartment: Composed of muscles (pectoralis major and minor) and associated connective tissue structures that facilitate movement and nerve transmission.

Breast Anatomy

  • Composition:

    • Composed of skin, dense connective tissue, and glandular tissue (mammary glands) responsible for milk production.

  • Origin:

    • Developed from modified sweat glands, demonstrating evolutionary adaptations for nursing offspring.

  • Structure:

    • Contains 15-20 lobules of glandular tissue, each with its lactiferous duct that opens onto the nipple, facilitating milk delivery during lactation.

    • Contains a pigmented areola, which has sensory nerves and may aid in latching during breastfeeding.

    • In non-lactating women, glandular tissue is largely replaced by adipose tissue.

    • Surrounds lobules with dense connective tissue that forms suspensory ligaments, maintaining breast structure.

  • Retromammary Space:

    • A potential space that separates the breast from the underlying pectoralis major muscle, allowing some movement during physical activities.

  • Axillary Process:

    • Lateral extension of the mammary gland which sometimes can extend into the axilla, indicating variations in breast anatomy.

Blood, Nervous, and Lymphatic Supply to the Breast

  • Arterial Supply:

    • Supplied by branches from the axillary artery, internal thoracic artery, and the 2nd to 4th intercostal arteries, ensuring adequate blood flow for metabolic needs.

  • Venous Supply:

    • Venous drainage typically parallels arterial supply, directing blood back to the heart.

  • Nervous Supply:

    • Innervated by anterior and lateral cutaneous branches of the 2nd to 6th intercostal nerves, which provide sensory innervation to the breast skin and deeper structures.

  • Lymphatic Flow:

    • Approximately 75% of lymphatic drainage moves toward axillary nodes, highlighting the importance of early detection practices for breast pathology.

    • Remaining lymphatic flow is directed towards parasternal nodes, establishing a route for disseminating infections or malignancies.

Breast in Men

  • Development:

    • Male mammary glands are not developed but have rudimentary ducts which do not function in lactation.

  • Breast Cancer Statistics:

    • Less than 1% of breast cancer cases occur in men, translating to about 2,700 cases annually, underscoring the need for awareness and research in this demographic.

Breast Cancer in Women

  • Prevalence:

    • Breast cancer ranks among the most common cancers in women, with approximately 264,000 new cases reported each year in the U.S.

  • Tumor Origin:

    • Tumors typically arise from epithelial cells of the lactiferous ducts but can also originate from glandular acini in the lobules, indicating the complex nature of breast tissue.

  • Importance of Early Detection:

    • Early identification through regular screenings significantly improves prognosis and treatment outcomes.

  • Symptoms:

    • Symptoms may not be immediately apparent; changes may include a peau d’orange texture, lump formation, or nipple discharge. Malignant cells can spread via lymphatic vessels, thus highlighting the importance of monitoring lymph node involvement.

Muscles in the Pectoral Region

  • Superficial Muscle:

    • Pectoralis major, responsible for major movements of the shoulder and arm.

  • Deep Muscles:

    • Pectoralis minor and subclavius, which assist in stabilizing the shoulder and facilitating respiratory mechanics.

  • Fascia:

    • The clavipectoral fascia surrounds the deep muscles and provides support and structural integrity.

Components of the Thoracic Wall

  • Posterior Elements:

    • Composed of 12 thoracic vertebrae along with intervertebral discs, which provide structural support and flexibility to the spine.

  • Lateral Elements:

    • Comprised of 12 ribs on each side, contributing to the protective cage around the thoracic cavity.

  • Muscle Layers:

    • Contains three layers of muscles (external, internal, and innermost intercostal muscles) in the intercostal spaces aiding in respiration by expanding and contracting the thoracic cavity.

  • Anterior Element:

    • The sternum (which includes the manubrium, body, and xiphoid process) serves as a central attachment point for ribs and muscles.

Superior Thoracic Aperture

  • Borders:

    • Formed by the body of vertebra TI, rib I on each side, and the manubrium of the sternum.

  • Orientation:

    • The aperture slopes downward because of the incline of rib I from vertebra TI to the manubrium.

  • Consequence:

    • The pleural cavities (lungs) project superiorly above this aperture, influencing clinical approaches to thoracic procedures.

  • Structures Pass:

    • Vessels (like the subclavian artery) and nerves traverse over rib I to reach the upper limb or navigate between the thorax and neck.

Inferior Thoracic Aperture

  • Borders:

    • Bound by the body of vertebra TXII, ribs XI and XII, the cartilaginous ends of ribs VII to X, and the xiphoid process.

    • Floor:

    • The diaphragm forms the floor of this aperture, crucial for respiration.

  • Structure Passage:

    • Structures moving between the thorax and abdomen pass through or behind the diaphragm, impacting both thoracic and abdominal functionality.

Thoracic Skeleton

  • Thoracic Vertebrae:

    • Twelve thoracic vertebrae exhibit distinctive features including a heart-shaped body, elongated spinous process, and contribute to the rib articulations crucial for biomechanics of breathing.

  • Rib Articulations:

    • Articulations occur at:

      • Superior costal demi-facet on the body indicator of rib placement.

      • Inferior costal demi-facet on the body aiding rib stability.

      • Transverse costal facet on the transverse process assisting in rib movement.

Exceptions to Rib Articulations

  • Rib I:

    • Unique features include a complete facet allowing direct articulation and a lack of contact with CVII, contributing to its anatomical significance.

  • Ribs XI and XII:

    • Characterized by having a single facet on their bodies only and lacking transverse costal facets, indicating their reduced significance in rib connections.

  • Other Specific Ribs:

    • Variations occur in ribs II and others, emphasizing anatomical diversity.

Ribs Overview

  • General Characteristics:

    • Comprise twelve pairs of ribs, all bony posteriorly and cartilaginous anteriorly, facilitating flexibility and strength within the thoracic cage.

  • Types of Ribs:

    • True Ribs (I to VII): Directly articulate with the sternum increasing stability.

    • False Ribs (VIII, IX, and X): Their cartilage fuses with the rib above, affecting articulation mechanics.

    • Floating Ribs (XI and XII): Lack any anterior articulation, reducing structural support.

Parts of a Typical Rib

  • Structure:

    • Shaft: curved and flat, enabling smooth transitions during respiration.

    • Anterior end: cartilaginous to allow flexibility.

    • Posterior end: bony, engaging in articulation with vertebrae for stability.

    • Head: features two articular surfaces facilitating connections to vertebrae.

    • Crest: aids in muscle attachment between the head and neck.

    • Neck, Tubercle: connects with vertebra to enhance rib functionality.

    • Angle and Costal groove: essential for anatomical orientation and neurovascular passage.

Typical Rib Exceptions

  • Rib I:

    • Noted for its flat and broad structure, featuring one articular surface and distinct grooves for passing neurovascular bundles.

  • Rib II:

    • Long and flat compared to rib I, indicating its unique role in thoracic integrity.

  • Ribs X, XI, and XII:

    • Exhibit single articular surfaces and atypical shapes, reducing functionality and changing how they articulate.

Cervical Ribs

  • Occurrence:

    • Found in about 1% of the population, representing a congenital anomaly.

  • Articulations:

    • Connects with CVII and anterior rib I, creating potential clinical scenarios.

  • Clinical Implication:

    • Cervical ribs can compress the subclavian artery/vein and brachial plexus, known as thoracic outlet syndrome, leading to significant vascular and neurological implications.

Sternum (Breastbone)

  • Components:

    • Comprised of three elements:

    • Manubrium:

      • Features the jugular notch and articular sites for clavicles and ribs I and II.

    • Body:

      • Contains transverse ridges indicating embryonic development and fusions, articulating with ribs II to VII.

    • Xiphoid Process:

      • Cartilaginous in children but ossified in adults, varies in shape and articulates with rib VII, serving as an anatomical landmark.