Thoracic Wall Muscles

Muscles of the Thoracic Wall

• These muscles mimic the abdominal arrangement.
• The neurovascular plane is located between the second and third layers.
• Ribs divide muscle segments.
• Volume changes in the thoracic cage are brought about by these muscles, which elevate and depress it during breathing.
• There are 3 distinct layers: external, intermediate, and innermost.

External Layer

• The external layer is composed of 3 muscles.
• Serratus posterior superior originates from the spinous process of C7, 8, T1, 2, as well as the intervening supraspinous ligament.
• It is inserted laterally to the angles of ribs 2-5.
• The serratus posterior superior lies on top of the splenius muscle.
• The dorsal scapular nerve and vessel can be found on it.
• Serratus posterior inferior originates from T11, 12, L1, 2, as well as the intervening supraspinous ligament.
• The serratus posterior inferior is inserted just lateral to the angles of T9, 10, 11, and 12.
• The external layers are weak respiratory muscles.

Levator Costae

• Levator costae runs from the transverse process of C7 to T11, to the superior border of the rib below, lateral to the tubercle.
• These muscles are fan-shaped, and only thoracic muscle supplied by posterior rami.

External Intercostal Muscle

• External intercostal muscle runs from the sharp lower border of the ribs above to the smooth upper border of the rib below.
• The external intercostal muscle stretches from the superior costotransverse ligament to the costochondral junction.
• The anterior intercostal membrane, with fibers that run anterio-inferiorly, completes it.

Intermediate Layer

• Consists of 1 muscle, the intermediate intercostal muscle.
• Fibers run posterio-inferiorly from the costal groove of the rib above to the upper border of the ribs below.
• Goes as far as the sternal border, but the posterior intercostal membrane posteriorly completes it.
• The membrane runs from the rib angle to the superior costotransverse ligaments.

Innermost Layer

• Composed of 3 muscles that span more than 1 intercostal space.
• Innermost intercostal fibers are wider below than above and are located on the thoracic wall's lateral side.
• Subcostalis fibers run in the paravertebral gutter and are better developed below than above.
• Transversus thoracis is found from the lower sternum to the costal cartilages of ribs 2 through 6.

Intercostal Space

• Found between the sharp lower border of the rib below and the smooth upper border of the rib above.
• Contains the intercostal muscles, and the neurovascular plane is located between the intermediate and innermost layers.
• Neurovascular bundle is arranged as VAN (Vein, Artery, Nerve) from superior to inferior in the costal groove's concavity.
• Intercostal space is important for pleural drainage and thoracotomy.

Intercostal Nerves

• These nerves emerge from the intervertebral foramen and have a collateral branch that innervates the space's muscles.
• The collateral branch runs superior to the space's lower rib.
• Lateral and anterior cutaneous branches are other branches.
• The artery is superficial in front of and behind it.
• The neurovascular plane of the abdominal wall houses the lower 5 nerves and collateral branches.
• The first nerve is small and has no cutaneous branch; the subcostal nerve passes behind the lateral arcuate ligament and into the abdomen.

Intercostal Artery

• Has posterior and anterior components.
• Posterior intercostal arteries consist of the superior intercostal artery and branches of the thoracic aorta.
• Anterior intercostal arteries are made up of the internal thoracic artery's branches (1-6) and the musculophrenic artery (7-9).
• 2 anterior branches anastomose with 1 posterior branch.

Intercostal Veins

• 2 anterior intercostal veins per space, which drain into the internal thoracic and musculophrenic veins.
• Posteriorly, they drain into the azygos system of veins.
• The lower 8 right intercostal veins drain into the azygos vein, while the left drain into the hemiazygos and accessory hemiazygos.
• The supreme intercostal vein drains the 1st space; the superior intercostal vein drains the 2nd to 3rd spaces, which drains into the azygos on the right and the brachiocephalic on the left.

Internal Thoracic Artery

• Originates from the first part of the subclavian artery.
• Runs a finger's breadth lateral to the sternum on both sides.
• 2 anterior intercostal arteries per space are given off
• At the costal margin, it divides into superior epigastric and musculophrenic arteries.
• Musculophrenic gives branches to the lower spaces, piercing the diaphragm to supply its abdominal surface.
• Pericardiacophrenic supplies the nerve, pleura, and pericardium.
• In females, perforating branches are larger at the 2nd and 3rd spaces.

Diaphragm

• A derivative of the innermost layer, fibers originate from the costal margin and are completed posteriorly by fibers from the arcuate ligament and the crura.
• The right crus overlaps the upper 3 lumbar vertebrae and intervertebral disc, and the left overlaps the upper 2 lumbar vertebrae.
• The right crus descends left to create the esophageal sling.
• Tendinous arches from the crura form the median arcuate ligament.
• Medial arcuate ligament is a thickening of psoas fascia.
• Lateral arcuate ligament is a thickening of lumbar fascia.
• Diaphragmatic fibers originate from the crura, the medial and lateral arcuate ligaments, the tip of rib 12, costal cartilages, nearby ribs, and the xiphisternum.
• Fibers originate from both sides, forming the domes and descending along the central tendon.
• The right dome is higher than the left.

Diaphragmatic Openings

• Aortic (midline) T12, it transmits the aorta, azygos vein, and thoracic duct.
• Esophageal (left) T10, it transmits the vagal trunk and the left gastric artery branch.
• Vena caval (right) T8, it transmits the IVC and right phrenic nerve.
• Blood is supplied from the lower 5 intercostal and subcostal arteries, and the R + L inferior phrenic arteries on the abdominal surfaces.
• Nerves are supplied from the phrenic and some intercostal nerves.

Anterior Mediastinum

• A potential space is found between the pericardium and sternum.
• The bi-lobed thymus, sternopericardial ligaments, and internal thoracic artery branches are present.
• Thymus is prominent in children extending between the lower pole of the thyroid gland and 4th costal cartilage.
• Blood supply is largely from the inferior thyroid and internal thoracic arteries.

Middle Mediastinum

• Includes the pericardium, great vessels, and lung roots.
• The fibrous pericardium is a single fibrous sac sheet that encloses the heart and fuses with the adventitia of great vessels.
• It adheres to the mediastinal pleura and is inseparable from the central tendon.
• Phrenic nerve runs on its surface, and sternopericardial ligaments link it to the sternum.
• The serous pericardium lines the fibrous pericardium's inner surface.
• The epicardium is formed by reflection at the great vessels that cover the entire heart's surface.
• Transverse and oblique sinuses are shown by the attachment of the serous pericardium.
• Serous pericardium's 2 layers contain the pericardial cavity.
• The phrenic nerve innervates the fibrous and parietal layers of the serous pericardium, while the visceral layer does not.
• The pericardial blood supply comes from the internal thoracic and bronchial arteries, and thoracic aorta.

The Heart

• A 4-chambered muscular pump.
• Atria receive blood while ventricles pump blood.
• The right side is related to pulmonary and the left side to the systemic circulations.
• The right side is anterior, while the atrium is on the right side of their ventricles due to rotation in development.
• Borders and surfaces are demonstrable; each relates to different parts of the heart.
• The right side extends from the 3rd to the 6th costal cartilages (right of SM)
• The inferior border runs from the R 6th costal cartilage to the L 5th intercostal space (MCL).
• The left border runs from the L 5th intercostal space to the L 2nd costal cartilage's lower border (2cm from SM).
• The central fibrous body is the attachment point of the membranous IV septum part of to the fibrous skeleton, providing bases for the tricuspid, mitral, and aortic valves.
• The right atrium is elongated, runs between the SVC and IVC, and the right auricle is on the SVC's left.
• The right auricle overlies the aorta's commencement and the R atrioventricular groove.
• The infundibulum of the R ventricle is clasped by its left counterpart.
• The crista terminalis corresponds with the sulcus terminalis, found between the SVC and the R auricle.
• The smooth and rough parts of the R atrium are demarcated by the crista terminalis.
• The pectinate muscle creates the rough part and is the derivative of the embryonic auricular chamber.
• The coronary sinus is on the left of the IVC opening.
• The interatrial septum separates the R atrium from the left; the fossa ovalis represents the primary septum.
• The right ventricle is found between the R coronary artery in the R atrioventricular groove and the anterior interventricular artery.
• The trabeculae carneae are muscular ridges that project from the wall into the cavity.
• The septomarginal trabeculae is attached to the interventricular septum and the anterior papillary muscle.
• The other papillary muscle is connected to the tricuspid valve's cusps.
• Chordae tendineae prevent cusps from everting during ventricular contraction.
• The cavity narrows superiorly into the pulmonary trunk via the infundibulum (conus).
• Infundibulum is thin-walled, and the semilunar pulmonary valve cusps are attached where it meets the pulmonary trunk.
• The left atrium forms the heart's base behind its right counterpart.
• The left auricle marks the upper border and curves to the front on the left of infundibulum.
• Four pulmonary veins are received symmetrically.
• Except for the trabeculated auricular region, the cavity is smooth-walled.
• The mitral valve with 2 cusps connects it to the left ventricle.
• The L atrioventricular orifice is demarcated from the aortic orifice by the anterior cusp.
• The wall of the left ventricle is 3 times thicker than the RV, and trabeculae carneae are better developed.
• Papillary muscles are attached by chordae tendineae to the mitral valve cusps.
• The interventricular septum bulges into the right ventricle.
• The membranous portion is smooth, thin, and fibrous.
• It joins the anterior cusp of the mitral valve to form the aortic opening's boundary.
• The great vessels are covered by the serous pericardium, and the pulmonary and aortic share the same pericardial covering.
• The IVC and SVC share the same serous pericardial sleeve as the pulmonary veins, but the IVC is shorter than the SVC.
• The ascending aorta bulges to create aortic sinuses on each cusp.
• The R coronary artery rises from the anterior sinus, while the L coronary artery rises from the L posterior sinus.
• Superiorly, the aorta travels to the right behind the infundibulum.
• The pulmonary trunk is slightly higher when it begins than the aorta.
• Pulmonary trunk runs to the left and posterior to the ascending aorta.
• Concavity of the aortic arch, anterior to the left main bronchus; it divides

Blood Supply of the Heart

• Supplied by the right and left coronary arteries.
• The right coronary artery is in the AV groove that turns backward at the inferior border running posteriorly.
• Gives branches along its part, Conus, SA nodal, R marginal, posterior interventricular (dominance), AV nodal, R posterolateral branches.
• Terminates anastomose with the circumflex artery of the left coronary artery.

Left Coronary Artery

• Arises from the left posterior aortic sinus between the left auricle and infundibulum.
• Divided into the circumflex and anterior interventricular branches.
• Anterior interventricular, conus, diagonal arteries
• Coronary arteries freely anastomose
• Coronary sinus drains the heart and empties into the right atrium.
• Drains the following: great cardiac, middle cardiac, small cardiac veins, posterior vein of L ventricle and oblique vein of L atrium.
• Anterior cardiac and R marginal veins drain directly into the R atrium.
• Venae cordis minimae drain all chambers.

Posterior Mediastinum

• The space posterior to the pericardium stretches from the retropharyngeal space to behind the diaphragm.
• Structures traverse from the superior mediastinum into the abdominal cavity.
• The descending aorta originates from T4 to T12 (through the crura).
• The descending aorta gives the posterior intercostal, bronchial, and esophageal arteries.
• The esophagus measures 25 cm in length from the cricoid cartilage to the level of T10, 2.5 cm left of midline.
• It's indented by the arches of the aorta and the azygos.
• Constrictions are present at the cricopharyngeal sphincter, aortic, left principal bronchus, and diaphragmatic passage.
• The blood supply comes from the inferior thyroid, descending aorta, and left gastric arteries.
• The venous return is to the brachiocephalic, azygos system, and the left gastric veins [Porto-systemic anastomosis].
• Nerve supply is the recurrent laryngeal and sympathetic; thoracic sympathetic, greater splanchnic [parasympathetic from vagus].
• Motor supply is upper 1/3rd from nucleus ambiguus cells and lower 2/3rd from the dorsal motor nucleus.
• The thoracic duct runs between the aorta and esophagus from the cisterna chyli (T12).
• It flows into the angle between the left internal jugular and subclavian veins.
• On the right, the azygos system of veins is formed by the union of the ascending lumbar and subcostal veins. These veins arch anteriorly at T4 to empty into the SVC.
• The hemiazygos is formed by the union of the left ascending lumbar and subcostal veins, while the accessory hemiazygos drains the middle 4 spaces.
• Both empty into the azygos at the level of T7,8 behind the esophagus.

Pleura

• The thoracic cavity is divided into 3 compartments, two pulmonary cavities and a central mediastinum.
• The lungs fill the pulmonary cavities and are enveloped in the pleura.
• The pleura has the visceral and parietal layers.
• Visceral pleura covers all lung surfaces, including the surfaces between fissures.
• The visceral pleura is closely attached to the lung tissue and permits the lung's smooth movement within the thoracic cavity.
• It is continuous with the parietal pleura at the hilum of the lungs.
• Parietal pleura lines the inner surface of the thoracic cavity; it has the costal, mediastinal, diaphragmatic, cervical parts, and is less adherent than the visceral pleura because of the endothoracic fascia.
• Costal part lines the thoracic wall's inner surface, with the endothoracic fascia in between.
• The mediastinal portion lines the mediastinum's sides, joining the costal areas in the front and back.
• Continuous to the visceral pleura and is reflects on hilum contents.
• Extends from the hilum inferiorly in the form of a pulmonary ligament.
• The diaphragmatic part lines the diaphragm's thoracic surface up to its perimeter and is connected to the costal part, forming the costodiaphragmatic recess.
• A more elastic modification of the phrenicopleural fascia connects the muscles of the diaphragm
• Cervical part is the superior extension of the costal and mediastinal parts, covers the apex of the lung extending to the root of the neck, it is reinforced by suprapleural membrane.
• Visceral pleura drains into the pulmonary vein, while the parietal drains into the systemic veins.
• Visceral pleura is insensitive to pain, while the parietal pleura is sensitive to pain.

Lungs

• The lung is an elastic organ that fills the pulmonary cavities.
• The horizontal and oblique fissures divide the lungs into lobes.
• The right lung is heavier, larger, and divided into 3 lobes.
• The right lung is shorter in the superio-inferior axis and wider medio-laterally.
• Right lung's anterior border is relatively straight.
• Left lung has a cardiac notch, 2 lobes, and a tongue-like lingula.
• The apex refers to the rounded superior end of the lung which extends beyond the 1st rib into the root of the neck.
• Surfaces: Costal, mediastinal, and diaphragmatic surfaces.
• Costal Surface: Smooth and convex surface related to the ribs and the vertebral column indented by the ribs in a cadaveric lung.
• Mediastinal surface: Concave, related to the middle mediastinum, receives the root of the lung with the pulmonary ligament inferior to it and has esophageal and heart impressions.
• The left lung shows a larger cardiac impression and grooves for the aortic arch, thoracic aorta, and esophagus.
• Diaphragmatic surface concaves on the diaphragmatic domes.
• The blood supply comes from the pulmonary artery, branching alongside with the bronchi to supply the bronchopulmonary segments.
• The pulmonary artery transports blood to the lungs for oxygenation, while the pulmonary vein transports oxygenated blood from the lungs.
• The root of the lung and supporting structures are supplied by bronchial arteries that anastomose distally with branches of pulmonary arteries.
• Superficial and deep lymphatic channels drain to the bronchopulmonary and then tracheobronchial nodes.

Tracheobronchial Tree

• Extends from the neck as a continuation of the larynx (C6) to the level of the upper border of T5.
• Caudally it bifurcates into the main bronchi.
• Measures about 10cm in length but can stretch to 15cm in full inspiration
• About 2cm in diameter.
• Composed of 15-20 C-shaped hyaline cartilage rings.
• Posteriorly, the cartilaginous ring is completed by longitudinal and transverse (trachealis) muscle fibers.
• Lined by a pseudostratified columnar ciliated epithelium with numerous goblet cells and mucous glands.
• The esophagus is its main posterior relation.
• Half of its length is in the neck from the cricoid cartilage to the jugular notch.
• The trachea is midline anterior to the esophagus.
• Between the trachea and esophagus is the recurrent laryngeal nerve.
• The carotid sheath is lateral in relation to trachea.
• The thyroid gland is an anterio-lateral relation, the isthmus been attached to the 2nd-4th rings.
• Anterior relations include the thymus, thyroidea ima artery, inferior thyroid vein, and anterior jugular venous arch in the neck.
• The upper part is more anterior, following the curvature of the cervico-thoracic vertebrae.
• The thoracic part traverses the superior mediastinum.
• Anterior relations: sternohyoid, sternothyroid, manubrium, left brachiocephalic vein, left common carotid artery, and brachiocephalic artery.
• The right side is connected to the right lung by the arch of azygos, pleura, and right vagus. Intervene between the right side and the right lung
• The left common carotid and subclavian arteries separate the left side from the pleura and left vagus.
• The aorta arch arches over the left bronchus, while the arch of azygos arches over the right bronchus.
• Left recurrent laryngeal nerve runs in the groove between the trachea and the esophagus.
• R broncus is about 2.5cm, more vertical than the left, wider, and shorter
• Pulmonary artery lies below and later in front
• Its upper lobe branch is the eparterial branch
• Gives middle and lower lobe branches below the artery.
• The L bronchus is about 5cm long, diameter smaller than right, and enters the left lung at 6th thoracic vertebra
• The aortic arch arches over it
• Posterior relations: the oesophagus, thoracic duct, and descending aorta are and
• Left pulmonary artery is above and later anterior to it.
• Cartilages are C-shaped and present in both the trachea and extrapulmonary.
• There are 15-20 cartilages in trachea, 6-8 in right bronchus, and 9-12 in left.
• Cartilages are flat externally but convex internally and ossify later in life.
• In the cartilaginous rings, the outer is thicker than the inner and enclosed with double-layered fibrous membrane .
• Consist of external longitudinal and inner transverse muscle fibers
• Bronchial segmentation has the hyparterial branches of the right bronchus supply the middle and lower lobes.
• There are 10 branches corresponding to the bronchopulmonary segments on both sides.
• The left superior branch, in the left superior and inferior lobes, is 5cm post bifurcation
• Branches to the lingular region of the superior lobe of the left lung correspond with those of the middle lobe of the right.
• The lobar bronchus supplies each bronchopulmonary segment, which is cone-shaped.
• The arterial supply also supplies Each segment, which is anatomically independent
• Branches of the pulmonary vein and lymphatics run in the plane between segments.
• Lobular bronchi do not have cartilages with diameters of about 0.2mm.
• Respiratory bronchioles divide into alveolar duct alveoli, which are attached around them.
• Alveoli are lined by simple squamous epithelium
• Arterial supply is from the bronchial artery (2 on the left and 1 on the right).
• Venous drainage: right to azygos, left to hemiazygos, also drained to bronchial and pulmonary veins, and supplied by Nerve, a pulmonary plexus, sympathetic and parasympathetic.