HBF-II LEC 01 Gross Anatomy Thorax Lungs Pleura Notes 2024 Berger.pdf

Thorax, Lungs, & Pleura Page 1 of 25 Dr. Elizabeth Berger Learning Objectives I. Course Objective: Describe the anatomy of the region under study. Session Objectives:  Describe the major features of bones & bony landma...

Thorax, Lungs, & Pleura Page 1 of 25 Dr. Elizabeth Berger Learning Objectives I. Course Objective: Describe the anatomy of the region under study. Session Objectives:  Describe the major features of bones & bony landmarks that form the skeletal framework of the thoracic wall.  Describe the joints and supportive ligaments of the thoracic wall.  Describe the boundaries and contents of the thorax.  Describe the compartments, spaces, transition areas of the pleural cavity.  Describe the surface anatomy of pleurae and lungs.  Describe the trachea and bronchi.  Describe the major features of the lungs. II. Course Objective: Relate the anatomy of structures to their function(s). Session Objectives:  Relate joint structure to movements of the thoracic wall during inspiration.  Relate muscles of the thoracic wall to movements of the ribs.  Relate nerves to their sensorimotor functions.  Relate the vasculature to structures supplied.  Relate the lymphatic plexuses to the lungs. III. Course Objective: Apply knowledge of anatomy to evaluate clinically relevant problems. Session Objectives:  Apply anatomy to evaluate injury to the skeletal framework of the thoracic cavity.  Apply anatomy to evaluate deficits in lung function.  Apply anatomy to evaluate deficits in vasculature to related structures.  Apply anatomy to evaluate injury to pleurae. Thorax, Lungs, & Pleura Page 2 of 25 Dr. Elizabeth Berger Lecture Content Outline I. Thorax A. Surface Anatomy B. Skeletal Framework C. Muscles of the Thorax D. Vasculature of the Thoracic Wall E. Nerves of the Thoracic Wall F. Movements of the Thoracic Wall II. Pleura A. Compartments, Spaces, Transition Areas B. Innervation C. Injury of Pleural Cavity III. Lungs A. External Features B. Trachea & Bronchial Tree C. Vasculature of the Lung D. Nerve Supply of the Lung E. Lymphatic Drainage F. Movements of Respiration Thorax, Lungs, & Pleura Page 3 of 25 Dr. Elizabeth Berger I. Thorax Summary: The thorax contains the heart, lungs and many other important structures within a skeletal framework that also protects upper abdominal organs (liver on the right, spleen on the left). The thoracic cavity communicates with the front of the neck, superiorly, and the abdomen, inferiorly. The thoracic cage, or rib cage, is formed by 12 pairs of ribs, corresponding thoracic vertebrae, costal cartilages, and the sternum – which encloses the thoracic cavity and supports the shoulder girdle. The thoracic cage also provides attachment points for muscles of the neck, thorax, upper limbs, abdomen, and back. A. Thorax – Surface Anatomy Key Landmarks: - jugular (suprasternal) notch - sternal angle (of Louis) - nipple - xiphoid process Figure 1. Figure 2. Thorax, Lungs, & Pleura Page 4 of 25 Dr. Elizabeth Berger B. Skeletal Framework Three primary components: 1) sternum 2) 12 pairs of ribs and costal cartilages 3) 12 thoracic vertebrae and intervertebral discs Figure 3. 1. Characteristics and landmarks of bone structures Sternum – manubrium, body, and xiphoid process with intervening cartilaginous joints Manubrium: Suprasternal (jugular) notch felt at the base of your throat clavicular notches that articulate with the sternal end of the clavicle, forming the sternoclavicular joint Body: Formed by four-fused sternebrae Costal notches for articulation with ribs 2 – 7 Xiphoid process: Descends into the infrasternal angle Figure 4. Ossifies by age 40 Figure 5. Thorax, Lungs, & Pleura Page 5 of 25 Dr. Elizabeth Berger Clinical Notes: - The line of junction between the manubrium and the body projects forward as the sternal angle (of Louis). It is readily palpable and is a landmark for the second costal cartilage. Ribs and intercostal spaces are counted from here. - The sternum is a site for bone marrow biopsy. A median sternotomy allows access to the heart for surgery. Figure 7. Figure 6. Ribs – Elongated, flattened, curved bones; 12 pairs articulate with 12 thoracic vertebrae Structure of a Typical Rib: twisted along its axis with a sharp bend in the shaft anteriorly – costal angle body – long, curved shaft of the bone; anterior end attaches to costal cartilage which attaches to the sternum posterior portion – articulates with the bodies of TV via a head and 2 facets tubercle – bony posterior prominence at the junction of the neck and shaft for articulation with the transverse process of the TV; attachment point for ligaments subcostal groove on the shaft contains the neurovascular bundle Costal cartilages of ribs 1-7 articulate with the sternum and are true ribs. Costal cartilages of ribs 8-10 articulate anteriorly with the costal cartilages of the adjacent superior rib to form the costal margin on each side; costal cartilages 7 – 10 join to form the costal margin. Ribs 11 & 12 are free ribs or floating ribs that do not attach anteriorly to the sternum; they have no neck or tubercles Thorax, Lungs, & Pleura Page 6 of 25 Dr. Elizabeth Berger 2. Joint structures Costovertebral – synovial joint where the head of the rub articulates with the bodies of 2 adjacent vertebrae and the intervening intervertebral disc, and by the tubercle with the transverse process of the corresponding vertebra Sternocostal – synovial joint between costal cartilages 2-7 and the lateral margin of the sternum Costochondral – hyaline cartilaginous joints between ends of bony rib and costal cartilage Interchondral – synovial joints between cartilages 7-10 at costal margin Manubriosternal – fibrocartilaginous joint Xiphisternal – cartilaginous joint Posteriorly, Clinical Notes: - These joints allow for movements of ribs and sternum that increase the diameters of the thorax during respiration. Disorders such as fractures of the ribs, that limit movement, can hamper respiration. - Costal cartilages add resilience to the thoracic cage, and thus protect the sternum and ribs from more frequent fracture. - With age, the costal cartilages may ossify. They become radio-opaque and may be confusing when examining the chest x-ray. 3. Regional relationships The thoracic cavity communicates with the front of the neck by the thoracic inlet: bounded by TV1, the 1st pair of ribs, and the upper margin of the manubrium. The inlet slopes downward and is occupied on each side by the apices of the lungs and pleura and neurovascular structures for the upper limbs. The inlet serves as a central conduit for vessels for the head and neck, trachea, and esophagus. The thoracic cavity communicates with the abdomen by the thoracic outlet closed by the diaphragm, and bounded by TV12, 12th ribs, edges of the lower 6 ribs and xiphisternal joint. 4. Injury to thoracic cavity Dislocation of a rib is the displacement of a costal cartilage from the sternum or dislocation of a sternocostal joint. While a rib separation refers to dislocation of a costochondral junction – between the rib and its costal cartilage. Flail chest occurs when a part of the chest wall is separated from the rest of the chest wall as a result of multiple rib fractures. That part of the chest wall can no longer contribute to inspiration. Defined as > 2 fractures per rib and > 2 ribs. Impairs ventilation, thereby affecting oxygenation of blood. Thorax, Lungs, & Pleura Page 7 of 25 Dr. Elizabeth Berger C. Muscles of the Thorax Along with intercostal muscles, transversus thoracis and subcostal muscles are also defined as muscles of the thoracic wall and act upon the ribs. 1 & 2. Attachments and Actions on the Thorax Muscle Superior Inferior Innervation Main Actions Attachment Attachment External Lower border of Upper border of Intercostal Elevates ribs in intercostal ribs rib below nerves inspiration Internal Costal cartilage Upper border of Intercostal Lowers ribs in intercostal and edge of costal rib below nerves forced groove of rib expiration Innermost Lower border of Upper border of Intercostal Act similar to intercostal ribs rib below nerves internal intercostals Transversus Internal surface of Posterior surface Intercostal Depress ribs thoracis costal cartilages of lower sternum nerves and costal 2–6 cartilages Subcostal Internal surface of Superior borders Intercostal Depress ribs lower rib near of 2nd or 3rd rib nerves their angles below **Back muscles that participate in respiration: serratus posterior superior/inferior and diaphragm Table 1. Muscles of the thoracic wall Figure 8. Thorax, Lungs, & Pleura Page 8 of 25 Dr. Elizabeth Berger Intercostal muscles are arranged in three layers Figure 9. and innervated by intercostal nerves. External intercostal muscles end anteriorly as the external intercostal membrane. Internal intercostal muscles end posteriorly as the posterior intercostal membrane. The neurovascular bundle is located between the inner and innermost intercostal muscles. Vein, artery and nerve (superior to inferior) are protected from injury in the subcostal groove of the rib. D. Vasculature of the Thoracic Wall 1. Course/Branching of Arteries Anterior intercostal arteries from the internal thoracic artery (see below) Posterior intercostal arteries o First 2 PIAs from supreme IA off the costocervical trunk from subclavian o Remainder from the thoracic aorta Internal thoracic artery (ITA) arises from the 1st part of the subclavian artery and descends behind the upper 6 costal cartilages about 1 cm lateral to the sternal margin; it then divides into the musculophrenic and superior epigastric arteries. Thorax, Lungs, & Pleura Page 9 of 25 Dr. Elizabeth Berger Branches of ITA Anterior intercostal arteries in upper 6 spaces arise directly from the ITA. Those in the lower spaces arise from the musculophrenic artery. These arteries anastomose with posterior intercostal arteries. Pericardiophrenic artery accompanies the phrenic nerve; branches supply the sternum, thymus gland and overlying skin of chest wall. 2. Course/Branching of Veins Posterior intercostal veins drain into the azygous vein on the right and the hemiazygos veins on the left. Anterior intercostal veins drain into the internal thoracic veins. Figure 10. Figure 11. Clinical Notes: - The anastomoses between the anterior and posterior intercostal arteries, and between the superior and inferior epigastric arteries provide important collateral pathways in obstruction of the thoracic aorta as in coarctation of the aorta. Thorax, Lungs, & Pleura Page 10 of 25 Dr. Elizabeth Berger E. Nerves of the Thoracic Wall After passing through the intervertebral foramina, 12 pairs of thoracic spinal nerves divide into anterior (ventral) and posterior (dorsal) primary rami. The anterior rami course around the chest wall in the neurovascular plane (between inner and innermost intercostal muscles) to form the intercostal nerves that run along the intercostal spaces. The anterior primary rami of T12 form the subcostal nerves. Figure 12. Intercostal nerves supply these muscles, the overlying skin by anterior and lateral cutaneous branches, and related costal and diaphragmatic parietal pleura. The ventral rami of thoracic nerves 7-11 leave their intercostal spaces and enter the anterior abdominal wall to supply skin, muscle and parietal peritoneum. The dorsal rami of these nerves supply the skin and muscle of the back. Figure 13. Thorax, Lungs, & Pleura Page 11 of 25 Dr. Elizabeth Berger Clinical Notes: - The dermatomes are arranged in a segmental fashion because the thoracoabdominal nerves arise from segments of the spinal cord. These bandlike skin areas are each supplied by the sensory fibers of a single posterior root through the posterior and anterior rami of its spinal nerve. T10 – at the level of umbilicus T12 – suprapubic Since a single thoracic nerve supplies an ipsilateral segment of skin from the anterior to posterior midline of the body, irritation of lower intercostal nerves may give Figure 14. rise to pain related to the anterior abdominal wall. See dermatome map. - When aspirating the chest (thoracocentesis) the needle is placed at the upper border of the rib to prevent injury to vessels and nerve. - The anastomoses between the anterior and posterior intercostal arteries, and between the superior and inferior epigastric arteries provide important collateral pathways in obstruction of the thoracic aorta as in coarctation of the aorta. Lymph nodes are located along the internal thoracic artery. Figure 15. Thorax, Lungs, & Pleura Page 12 of 25 Dr. Elizabeth Berger F. Movements of the Thoracic Wall Movements of the thoracic wall and diaphragm occur to allow for inspiration and expiration. By altering the volume of the thorax, air is moved in and out of the lungs. The attachment of the ribs to the sternum and spine between T1 and T7 (true ribs) limits mobility of the thoracic spine in this area. This is a little mobility where the rib attaches to the cartilage before it attaches to the sternum. Ribs 8 – 10 (false ribs) have longer costal cartilages, which attach to other cartilage, so this area of the rib cage has more mobility. Ribs 11 – 12 (floating ribs) have no anterior attachment at all so the greatest mobility of the thoracic spine is in this area. Due to these attachments, the upper ribs move differently from the lower ribs. Changes in the anteroposterior and lateral dimensions result from elevation and depression of the ribs. The posterior ends of the ribs articulate with the vertebral column, whereas the anterior ends of most ribs articulate with the sternum or adjacent ribs. Because the anterior ends of the ribs are inferior to the posterior ends, when the ribs are elevated, they move the sternum upward and forward. Also, the angle between the body of the sternum and the manubrium may become slightly less acute. When the ribs are depressed, the sternum moves downward and backward. This “pump handle” movement changes the dimensions of the thorax in the anteroposterior direction (Fig. 16). Figure 16. As well as the anterior ends of the ribs being lower than the posterior ends, the middles of the shafts tend to be lower than the two ends. When the shafts are elevated, the middles of the shafts move laterally. This “bucket handle” movement increases the lateral dimensions of the thorax. Thorax, Lungs, & Pleura Page 13 of 25 Dr. Elizabeth Berger II. Pleura A. Pleural compartments, spaces and transition areas Each lung is invested by and enclosed in a pleural sac, which is thereby enclosed by a visceral layer and surrounded by a parietal layer. Superiorly, they extend above 1st rib into the root of the neck Inferiorly, they extend to just above the costal margin The medial wall of each pleural cavity is in the mediastinum The pleura is divided into two major types, based on location: Parietal pleura is associated with the walls of a pleural cavity Visceral pleura reflects from the medial wall and onto the surface of the lung The pleural cavity is a potential space between visceral and parietal pleura, containing a thin film of fluid that facilitates movement of the lungs. Figure 17. 1. Subdivisions of parietal pleura Correspond to the parts of the wall with which they are associated: Costal pleura covers the inner surface of the thoracic wall and is separated from the innermost intercostal muscles by endothoracic fascia. The fascia provides a plane for stripping the pleura from the chest wall. Diaphragmatic pleura covers the upper surface of the diaphragm. Mediastinal pleura covers the mediastinum from the sternum to thoracic spine. Cervical pleura (cupola) is the apical portion that projects into the neck above the medial third of the clavicle. It is strengthened by the suprapleural membrane of fascia that extends from the transverse process of CV7 to the inner border of the 1st rib. Figure 18. 2. Pleural Recesses & Reflections Thorax, Lungs, & Pleura Page 14 of 25 Dr. Elizabeth Berger Borders of the pleura are formed at the continuity of costal diaphragmatic and mediastinal pleura. Anterior border is formed along the line where the costal and mediastinal pleura meet behind the sternum. Here the costomediastinal recess is located (larger on the left). Inferior border is the line of union of costal and diaphragmatic pleura, here the costodiaphragmatic recess is formed. The recesses are regions where parietal pleura doubles back and contact itself, the lungs expand into the recesses during deep inspiration. The costodiaphragmatic recesses can accumulate large volumes of fluid emanating from the lung. Figure 19. 3. Surface markings of pleural reflections Anteriorly, the lines of pleural reflection (costal to mediastinal pleura) meet behind the sternum and extend down to the 6th costal cartilage. The anterior reflection on the left deviates laterally at the 4-5th costal cartilage. Inferiorly, the costal and diaphragmatic pleura meet and this reflection crosses the 8th rib in the midclavicular line, the 10th rib in the midaxillary line and the 12th rib at the lateral border of the erector spinae. The inferior border of the lung crosses two ribs higher – 6th rib in MCL, 8th in MAL, and 10th rib posteriorly. Figure 20. Thorax, Lungs, & Pleura Page 15 of 25 Dr. Elizabeth Berger B. Innervation Somatic innervation by intercostal and phrenic nerves. Parietal pleura is highly sensitive to pain stimuli. Phrenic nerve is sensory to the entire diaphragmatic pleura except for the periphery near costal margin supplied by intercostal nerves. Visceral pleura is insensitive except to stretching; it is innervated by autonomic nerves. Figure 21. C. Injury of Pleural Cavity - The cupola and the apex of the lung extends 2 – 3 cm above the level of the medial third of the clavicle. Thus a penetrating injury in the supraclavicular region of the neck could puncture the pleural cavity and the lung. Thorax, Lungs, & Pleura Page 16 of 25 Dr. Elizabeth Berger - The sympathetic trunk and the 1st thoracic nerve lie behind the cupola and apex of the lung, on the neck of the 1st rib. Disease of the apex of the lung can extend through the cupola and involve these nerves, presenting as paralysis of the intrinsic muscles of the hand, and Horners Syndrome. Figure 22. - Normally the pleural layers are in close opposition and the space between them is a potential one. When air is introduced, from a defect in parietal or visceral pleura (e.g., stab wound of chest wall, tear in visceral pleura) respiration is compromised due to collapse of lung tissue. This is a pneumothorax Figure 23. Figure 24. Figure 25.  Thorax, Lungs, & Pleura Page 17 of 25 Dr. Elizabeth Berger - Pleural tap (thoracocentesis) involves the withdrawal of fluid from the pleural cavity. The needle is inserted at the upper border of the rib to minimize injury to intercostal nerves and vessels. The site of insertion is determined by localizing the fluid level by x0ray, u/s, percussion of the chest and knowing levels of pleural reflection. It is important to avoid injury to lung and liver. Thus, a needle inserted horizontally at the upper border of the 10th rib in the midaxillary line would be below the level of the lung. Why? - Since the parietal pleura is innervated by somatic nerves (intercostal, phrenic) pain is felt in the Figure 26. cutaneous distribution of these nerves to thoracic or abdominal walls. Irritation of the phrenic nerve may present with shoulder tip pain. Why? Lungs A. External lung features As vital organs of respiration, their main function is to oxygenate the blood by bringing inspired air into close relation with the venous blood in the pulmonary capillaries. Although cadaveric lungs may be shrunken, hard to the touch, and discolored in appearance, healthy lungs in living people are normally light, soft, and spongy. Figure 27. They are also elastic and recoil to about 1/3 their size when the thoracic cavity is opened. 1. Surfaces & borders of the lungs Each lung has the following: An apex that extends into the root of the neck A base (diaphragmatic surface) that is related to the diaphragm A costal surface that is related to the ribs A mediastinal surface that abuts on the vertebral column posteriorly and the mediastinum anteriorly Anterior and posterior borders that separate the costal surface from the mediastinal surface An inferior border that separates the diaphragmatic surface from the costal surface Thorax, Lungs, & Pleura Page 18 of 25 Dr. Elizabeth Berger Figure 28. 2. Lung structures Hilum: area on mediastinal surface where structures in the root of the lung enter and exit the lung. Within the hilum the bronchi are posterior, the pulmonary arteries are anterior and superior and the pulmonary veins most anterior and inferior. Bronchial vessels, autonomic nerves and lymphatics also pass through the hilum. Root of each lung: Tubular collection of structures that attach the lung to structures in the mediastinum. The root is Figure 29. Thorax, Lungs, & Pleura Page 19 of 25 Dr. Elizabeth Berger covered by a sleeve of mediastinal pleura that is continuous with the visceral pleura at the hilum. The pulmonary ligament is a fold of pleura that extends inferiorly from the root of the mediastinum. Allows for expansion of pulmonary veins. Contents of each root: One pulmonary artery Two pulmonary veins One main bronchus Bronchial vessels Nerves Lymphatics Relationships of right root: Anterior – SVC, phrenic Posterior – vagus nerve Superior – azygos vein Relationships of left root: Anterior – phrenic nerve Posterior – vagus nerve Superior – arch of aorta Figure 30. Fissures of the Lungs: Oblique fissure – commences posteriorly and intersects the inferior border. Posteriorly, its course approximates the medial border of the scapula with the upper limb elevated vertically above the head. In the left lung, the oblique fissure separates the superior lobe from the inferior lobe. In the right lung, it separates the superior and middle lobes from the inferior lobe. Horizontal fissure is in the right lung only and follows the 4th intercostal space extending from the anterior border of the lung to the oblique fissure. Thorax, Lungs, & Pleura Page 20 of 25 Dr. Elizabeth Berger Right Lung: 3 lobes: superior, middle, inferior 2 fissures: oblique, horizontal Significant relationships of medial surface: IVC, SVC, azygos vein, esophagus, heart Significant relationships of apex: r. subclavian vessel arches over the cupola and apex The orientation of the fissures determines where breath sounds from each lobe are best heard. o Superior lobe – upper anterolateral chest wall o Middle lobe – lower anterolateral chest wall o Inferior lobe – lower 2/3 of posterior chest wall Figure 31. Thorax, Lungs, & Pleura Page 21 of 25 Dr. Elizabeth Berger Left Lung: 2 lobes: superior, inferior 1 fissure: oblique Smaller than right lobe Lower medial surface of the superior lobe is notched by the projection of the heart to the left – cardiac notch. The lingula is a medially directed tongue-like projection of anterior border of the upper lobe below the cardiac notch. Significant relationships of medial surface: heart, arch of the aorta, thoracic aorta and esophagus. Figure 32. Thorax, Lungs, & Pleura Page 22 of 25 Dr. Elizabeth Berger B. Trachea & Bronchial Tree The trachea descends into the thorax anterior to the esophagus. It bifurcates into R and L main bronchi at the level of the sternal angle. The carina is a posterior projection of the last tracheal cartilage at the bifurcation. The right main bronchus is wider, shorter and more vertical than the left, thus the greater tendency of aspirated material to enter the right lung. The main bronchus divides within the lung into lobar (secondary) bronchi each supplying a lobe (on the right side the upper lobe bronchus originates in the root of the lunt). The lobar bronchi divide into segmental (tertiary) bronchi, which aerate bronchopulmonary segments. Figure 33. Thorax, Lungs, & Pleura Page 23 of 25 Dr. Elizabeth Berger 1. Bronchopulmonary segments 10 in each lung Portion of the lung aerated by a tertiary bronchus and supplied by accompanying branch of the pulmonary artery. Branches of the pulmonary veins travel mainly between the segments. Segments are pyramidal shaped with apex toward lung root and surrounded by connective tissue. Each segment is functionally independent and can be removed without affecting adjacent segments The tertiary bronchus to the superior segment of the inferior lobe arises from the posterior aspect of the lower lobe bronchus, thus the tendency of aspirated material to enter this bronchus in a supine individual. Figure 34. Thorax, Lungs, & Pleura Page 24 of 25 Dr. Elizabeth Berger C. Vasculature of the lung Pulmonary arteries carry deoxygenated blood from the right ventricle of the heart to the lungs. The oxygenated blood is transported by pulmonary veins to the left atrium of the heart. Bronchial arteries from the upper thoracic aorta supply the visceral pleura and the walls of the bronchi. About 1/3 of bronchial venous blood enters the azygos system while 2/3 enters the pulmonary veins and thus the left atrium. This bronchopulmonary circulation is only 2% of the cardiac output and adds a small amount of deoxygenated blood to the left atrium. D. Nerve supply of the lung Branches from the vagus nerves and sympathetic trunks are found in the roots of the lungs and distributed to visceral pleura and the muscular walls of bronchi and blood vessels (see Fig. 19). Figure 35. Vagus – constrictor to bronchial muscle and stimulates glandular secretion. Sympathetics – relax bronchial muscle, thus bronchodilator, reduce secretion. Clinical Notes: - Sympathomimetic drugs and parasympathetic antagonists are used in treatment of asthma. Thorax, Lungs, & Pleura Page 25 of 25 Dr. Elizabeth Berger E. Lymphatic drainage From the lungs to bronchopulmonary lymph nodes in the root of the lung, then to tracheobronchial nodes and finally to thoracic duct on the left side or to right lymphatic duct. Figure 36. F. Movements of respiration During inspiration the movements of the chest wall and diaphragm result in an increase of the vertical, anteroposterior and transverse diameters of the thorax. These movements bring about an increase in negative intrapleural pressure with resultant expansion of lung tissue. Air flows into the lungs. In expiration, relaxation of the respiratory muscles and the elastic recoil of the lungs reduce thoracic capacity forcing air out of the lungs. Forced expiration (sneezing) is aided by contraction of abdominal muscles, which act through the viscera, forcing the diaphragm upward. The vertical diameter is increased by contraction and descent of the diaphragm. The diaphragm accoungs for 2/3 of the expansion of the thoracic cavity. Elevation of the upper ribs pushes the sternum forward, thus increasing the anteroposterior diameter of the thorax (remember the pump-handle movement?). This movement is facilitated by hinge movement at the manubriosternal joint. If this joint is ankylosed thoracic expansion becomes limited. Elevation of the lower ribs increases the transverse diameter (recall bucket-handle?) movement. Accessory muscles of respiration come into play in deep and forced inspiration. (e.g., scalenes, sternocleidomastoid, serratus anterior).