ANA 202: Gross Anatomy of the Thorax

Course Overview and Introduction to the Thorax

• Course Code: $ANA\,202$

• Course Title: Gross Anatomy of the Thorax

• Course Credit: $2\,Units$

Boundaries and Regions of the Thorax

• Superior Boundaries:

  • Jugular notch.

  • Sternoclavicular joint.

  • Superior border of the clavicle.

  • Acromion.

  • Spinous processes of $C7$.

• Inferior Boundaries:

  • Xiphoid process.

  • Costal arch.

  • $11th$ and $12th$ ribs.

  • Vertebra $T12$.

• Anatomical Regions:

  • Thoracic wall: The muscular and bony structure protecting the cavity.

  • Thoracic cavity: The internal space housing the heart, lungs, and mediastinum.

Structures of the Thoracic Wall

• Superficial Structures:

  • Skin: The outermost layer.

  • Superficial Fascia: Contains superficial arteries and veins.

    • Superficial Veins: Includes the Thoracoepigastric vein and the Lateral thoracic vein.

  • Superficial Nerves: Includes the Supraclavicular nerve and the anterior and lateral cutaneous branches of the intercostal nerves.

• Deep Structures:

  • Deep Fascia: Divided into a superficial layer and a deep layer (specifically the clavipectoral fascia).

  • Muscles of the Thorax:

    • Subclavius.

    • Pectoralis major.

    • Pectoralis minor.

    • Serratus anterior.

    • Intercostales externi (External intercostals).

    • Intercostales interni (Internal intercostals).

    • Intercostales intimi (Innermost intercostals).

  • Endothoracic Fascia: The innermost lining of the thoracic wall.

Osteology of the Thorax: The Sternum and Ribs

• The Sternum (Breastbone):

  • Manubrium: The superior portion.

    • Contains the Jugular (Sternal) notch and Clavicular notches (articular facets).

    • Articulates with the $1st$ and $2nd$ ribs.

  • Sternal Angle (Angle of Louis): Junction of manubrium and body; marks the level of the $2nd$ rib.

  • Body (Gladiolus): The central, largest part.

    • Contains transverse ridges.

    • Articulates directly with ribs $2-7$.

  • Xiphoid Process: The inferior-most portion; cartilaginous in youth and calcifies with age. It forms the xiphosternal joint and serves as an attachment point for various muscles.

• The Ribs (12 Pairs):

  • True Ribs (1-7): Have direct attachment to the sternum via their own costal cartilages.

  • False Ribs (8-12): Indirect attachment or no attachment to the sternum.

    • Floating Ribs (11-12): A subset of false ribs with no ventral (anterior) attachment.

  • Typical Ribs (2-9): Consist of a head, neck, tubercle, angle, shaft, and subcostal groove.

    • Head: Contains superior and inferior articular facets for articulation with vertebral bodies and the intervertebral disc.

    • Tubercle: Articulates with the transverse process of the vertebra of the same number.

    • Shaft: Contains the costal groove for neurovascular protection.

  • Atypical Ribs:

    • Rib 1: Short, flat (superior-inferior orientation), wide, and supports the subclavian vessels.

    • Ribs 1, 10, 11, and 12: Articulate with only their corresponding numbered vertebra.

    • Ribs 11 and 12: Lack articulation with transverse processes and have no anterior attachment.

Articulations and Joints of the Thorax

• Posterior Joints:

  • Costovertebral Joints: The inferior costal facet of a rib articulates with the superior facet of the vertebra of the same number. The superior costal facet articulates with the inferior articular facet of the vertebra above it.

  • Costotransverse Joint: Between the facet on the rib tubercle and the transverse process of the thoracic vertebra of the same number.

  • Ligaments: Radiate ligaments (securing the head) and costotransverse ligaments (securing the tubercle).

• Anterior Joints:

  • Manubriosternal Joint: A symphysis between the manubrium and body.

  • Xiphisternal Joint: A symphysis between the body and xiphoid process.

  • Interchondral Joints: Synovial joints between the costal cartilages of certain ribs.

Musculature and Neurovascular Bundle

Primary Intercostal Muscles

• External Intercostals:

  • Origin: Inferior border of the rib above.

  • Insertion: Superior border of the rib below.

  • Fiber Direction: Oblique (down and forward).

  • Action: Elevates the rib cage; aids in inspiration.

• Internal Intercostals:

  • Origin: Superior border of the rib below.

  • Insertion: Inferior border of the rib above.

  • Fiber Direction: Right angles to external intercostals.

  • Action: Depresses the rib cage; aids in forced expiration.

• Innermost Intercostals:

  • Origin/Insertion: Similar to internal intercostals but separated by the neurovascular bundle.

  • Action: Function is variable and unclear.

Additional Thoracic Muscles

• Transversus Thoracis:

  • Origin: Costal cartilages of the last $3-4$ true ribs, body of the sternum, and xiphoid process.

  • Insertion: Ribs and costal cartilages $2-6$.

  • Action: Depresses the ribs.

• Levatores Costarum: Associated with rib elevation.

Thoracic Muscle Table (Origin, Insertion, Action, Innervation)

• Pectoralis Major: Sternum, Ribs $2-6$ \u2192 Intertubercular groove of humerus; Adducts, flexes, medially rotates arm; Innervated by Medial & Lateral pectoral nerves.

• Pectoralis Minor: Ribs $3-5$ \u2192 Coracoid process of scapula; Depresses and rotates scapula; Innervated by Medial & Lateral pectoral nerves.

• Serratus Anterior: Ribs $1-9$ \u2192 Scapula; Protracts and rotates scapula; Innervated by Long Thoracic nerve.

• Trapezius: Ligamentum nuchae, $C7-T12$ \u2192 Clavicle, spine & acromion of scapula; Elevates, adducts, rotates, and depresses; Innervated by Accessory nerve.

• Levator Scapulae: $C1-C4$ \u2192 Medial border of scapula; Elevates scapula; Innervated by Dorsal Scapular nerve.

• Rhomboids: $C7-T5$ \u2192 Medial border of scapula; Adducts, elevates, and rotates; Innervated by Dorsal Scapular nerve.

• Deltoids: Clavicle, spine & acromion of scapula \u2192 Deltoid tuberosity of humerus; Flex, abduct, extend, and rotate arm; Innervated by Axillary nerve.

• Latissimus Dorsi: Iliac crest, Sacrum, $T7-T12$, Lumbar fascia \u2192 Intertubercular groove of humerus; Extends, adducts, and medially rotates arm; Innervated by Thoracodorsal nerve.

Neurovascular Bundle

• Organization: Arranged as VAN (Vein, Artery, Nerve) from superior to inferior.

• Location: Situated in the subcostal groove, between the internal intercostal and innermost intercostal muscle layers.

The Diaphragm and Respiration

• The Diaphragm:

  • Function: Primary muscle of inspiration; domes descend to increase the longitudinal dimension of the thoracic cavity.

  • Innervation: Phrenic nerve ($C3, C4, C5$ bilaterally).

  • Anatomy: Contains a central tendon. Major openings include the Aortic hiatus, Esophageal hiatus, and Caval opening (for the inferior vena cava).

• Mechanics of Respiration:

  • Inspiration (Active):

    • Principal Muscles: Diaphragm, External intercostals, Interchondral part of internal intercostals.

    • Accessory Muscles: Sternocleidomastoid (elevates sternum), Scalenes (elevate/fix upper ribs), Levatores costarum.

  • Expiration:

    • Quiet Breathing: Mostly passive result of lung elastic recoil.

    • Active Breathing (Forced): Internal intercostals (depress ribs), Abdominal muscles (Rectus abdominis, External/Internal oblique, Transversus abdominis) which compress abdominal contents to push up the diaphragm.

The Mediastinum

• Concept: The space between the right and left mediastinal pleurae, extending from the sternum to the vertebral column and from the thoracic inlet to the diaphragm.

• Subdivisions:

  1. Superior Mediastinum: From the thoracic inlet to the plane of the sternal angle ($T4$ level).

     • Superficial Layer: Thymus, Left/Right Brachiocephalic veins, Superior Vena Cava.

     • Middle Layer: Aortic arch and its 3 branches, Phrenic nerve, Vagus nerve.

     • Posterior Layer: Trachea, Esophagus, Thoracic duct.

  2. Inferior Mediastinum (Below T4):

     • Anterior: Posterior to sternum; contains fat, thymus remnants, and lymph nodes.

     • Middle: Contains the heart, pericardium, great vessel roots, phrenic nerves, and pericardiacophrenic vessels.

     • Posterior: Posterior to heart, anterior to $T5-T12$; contains esophagus, vagus nerves, thoracic aorta, azygos system, thoracic duct, and sympathetic trunk.

Anatomy of the Heart

• Position: Size of a fist; sits on the superior surface of the diaphragm, left of the midline, anterior to the vertebral column.

• Coverings (Pericardium):

  1. Fibrous Pericardium: Superficial, protects and anchors the heart.

  2. Serous Pericardium: Deep two-layer sac (Parietal layer lines fibrous pericardium; Visceral layer/Epicardium lines heart surface).

  • Pericardial Cavity: Fluid-filled space between serous layers to reduce friction.

• Heart Wall Layers:

  • Epicardium: Visceral serous layer.

  • Myocardium: Thick cardiac muscle layer; thickness varies by chamber (Atria are thin; Left ventricle is thickest for systemic circulation).

  • Endocardium: Endothelial inner lining.

• Chambers and Flow:

  • Atria: Receiving chambers. Right atrium receives from SVC, IVC, and coronary sinus. Left atrium receives from pulmonary veins.

  • Ventricles: Discharging chambers. Right ventricle pumps to pulmonary trunk; Left ventricle pumps to aorta.

• Valves:

  • Atrioventricular (AV): Mitral (Bicuspid) on left; Tricuspid on right. Anchored by chordae tendineae to papillary muscles.

  • Semilunar (SL): Aortic and Pulmonary valves. Prevent backflow into relaxated ventricles.

• Coronary Circulation:

  • Arteries: Right and Left Coronary, Marginal, Circumflex, Anterior and Posterior Interventricular arteries.

  • Veins: Great, Middle, and Small cardiac veins; Coronary sinus.

• Conduction System:

  1. Sinoatrial (SA) node: Pacemaker in the right atrium.

  2. Atrioventricular (AV) node: Impulse delay occurs here.

  3. Bundle of His (AV bundle) and bundle branches.

  4. Purkinje fibers: Carry impulse to ventricular apex.

• Heart Sounds:

  • 1st Sound (Lub): AV valves closing; start of systole.

  • 2nd Sound (Dup): SL valves closing; start of ventricular diastole.

Lungs and Pleura

• Pleura: Double-walled serous membrane consisting of Parietal pleura (outer) and Visceral pleura (inner, covering lung surface).

• Right Lung: Contains $3$ lobes (Superior, Middle, Inferior) and $2$ fissures (Oblique and Horizontal).

• Left Lung: Contains $2$ lobes (Superior, Inferior) and $1$ fissure (Oblique). Notable features include the Cardiac notch and Lingula.

• Lung Surfaces: Apex, Base (diaphragmatic), Costal, and Mediastinal (Hilum).

• The Hilum: The entry/exit point for the bronchus, pulmonary artery, and pulmonary veins.

The Tracheobronchial Tree

• Trachea: $10\,cm$ long; starts at $C6$; supported by $20$ U-shaped hyaline cartilage rings; lined with ciliated epithelium.

• Bronchial Tree:

  • Primary (Main) Bronchi: Right is wider, shorter, and more vertical.

  • Secondary (Lobar) Bronchi: $3$ on right, $2$ on left.

  • Tertiary (Segmental) Bronchi: Supply specific bronchopulmonary segments ($10$ per lung).

• Bronchopulmonary Segments (Right Lung):

  • Superior Lobe: Apical ($B1$), Posterior ($B2$), Anterior ($B3$).

  • Middle Lobe: Lateral ($B4$), Medial ($B5$).

  • Inferior Lobe: Superior ($B6$), Medial ($B7$), Anterior ($B8$), Lateral ($B9$), Posterior ($B10$).

• Bronchopulmonary Segments (Left Lung):

  • Superior Lobe: Apicoposterior ($B1/2$), Anterior ($B3$), Superior Lingular ($B4$), Inferior Lingular ($B5$).

  • Inferior Lobe: Superior ($B6$), Anteromedial ($B8$), Lateral ($B9$), Posterior ($B10$).

• Bronchioles: Lack cartilage; surrounded by smooth muscle for bronchoconstriction/dilation. Subdivided into Terminal and Respiratory genotypes ($3-8$ orders of alveoli).

• Innervation:

  • Parasympathetic (CN X): Bronchoconstriction and cough reflex.

  • Sympathetic ($T1-T5$): Bronchodilation.

• Blood Supply: Bronchial arteries (branches from the aorta/intercostals) provide oxygenated blood to the lung tissue itself (not for gas exchange).