# Lecture 15: Clinical Anatomy of the Thorax and Abdomen
## Unit 1.4: Human Gross Anatomy II - Thorax and Abdomen
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## Learning Objectives
By the end of this lecture, students will be able to:
1. Apply anatomical knowledge to physical examination of the thorax and abdomen
2. Correlate surface anatomy with underlying structures
3. Describe the anatomical basis for common clinical procedures
4. Explain referred pain patterns based on innervation
5. Apply anatomical knowledge to surgical approaches
6. Integrate thoracoabdominal anatomy for clinical reasoning
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## Surface Anatomy of the Thorax
Understanding thoracic surface landmarks provides the foundation for accurate physical examination and procedural guidance. The suprasternal (jugular) notch, palpable as a depression at the superior border of the manubrium, corresponds to the T2-T3 vertebral level and serves as the reference point for tracheal palpation to assess midline position. The sternal angle (angle of Louis) marks the junction between the manubrium and body of the sternum at the T4-T5 level, providing the most reliable landmark for identifying the second rib—the first rib being largely obscured by the clavicle. The xiphoid process at the T9-T10 level marks the inferior extent of the sternum and guides hand positioning for cardiopulmonary resuscitation.
Cardiac valve positions and their optimal auscultation sites differ significantly due to the direction of blood flow. The mitral valve lies anatomically at the left fourth costal cartilage but is best heard at the apex in the fifth intercostal space along the midclavicular line, where ventricular contraction directs flow. The tricuspid valve at the lower left sternal border projects sound to the same region. The aortic valve, anatomically positioned at the right third intercostal space, is optimally auscultated at the right second intercostal space where ascending aortic flow carries the sound. Similarly, the pulmonary valve sound projects from its anatomical position at the left third intercostal space to the left second intercostal space.
Lung surface projections guide examination and procedural planning. The lung apex extends 2-3 centimeters above the medial third of the clavicle, making it vulnerable to injury from supraclavicular approaches. The lower lung border crosses the sixth rib at the midclavicular line, the eighth rib at the midaxillary line, and the tenth rib posteriorly. The pleural reflection extends approximately two rib spaces below the lung border at each point (following the "8-10-12 rule"), creating a costophrenic recess where pleural effusions first accumulate. The oblique fissure courses from the T3 spinous process posteriorly to the sixth rib anteriorly at the midclavicular line.
Panel A: Anterior view of the thorax with surface landmarks labeled: suprasternal notch at the superior sternum, sternal angle at the T4-T5 level with second rib attachment, and xiphoid process inferiorly. Panel B: Cardiac silhouette in red with four valve positions marked by colored dots (mitral, tricuspid, aortic, pulmonary) at their anatomical positions with arrows to their auscultation sites. Panel C: Lung fields shaded light blue with lower borders marked at the 6th, 8th, and 10th ribs along the midclavicular, midaxillary, and posterior lines respectively. Panel D: Pleural reflections as dotted lines two rib spaces below the lung borders with the oblique fissure traced as a diagonal line.
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## Physical Examination of the Thorax
Thoracic examination follows the traditional sequence of inspection, palpation, percussion, and auscultation. Inspection assesses respiratory rate and pattern, chest wall symmetry during respiration, recruitment of accessory muscles suggesting respiratory distress, and jugular venous distension that may indicate right heart failure or pericardial disease.
Palpation evaluates several parameters. Tracheal position should be midline at the suprasternal notch; deviation suggests tension pneumothorax (away from the affected side), mediastinal mass, or volume loss. Chest expansion symmetry is assessed by placing hands on the posterolateral chest wall and observing thumb movement during deep inspiration. Tactile fremitus—the palpable vibration of spoken words transmitted through the chest wall—decreases with pleural effusion or pneumothorax (which insulate the chest wall from the lung) and increases with consolidation (which better transmits vibrations). The point of maximal impulse (PMI) of the cardiac apex normally localizes to the fifth intercostal space at the midclavicular line.
Percussion elicits different notes reflecting underlying tissue. Resonance, the normal finding over aerated lung, produces a low-pitched sound with sustained duration. Hyperresonance, the exaggerated resonance found in pneumothorax or emphysema, sounds abnormally loud and sustained. Dullness replaces resonance over consolidated lung or pleural effusion. Flatness, a particularly dense percussion note, indicates massive effusion.
Auscultation characterizes breath sounds and additional sounds. Normal vesicular breath sounds are soft, low-pitched, and heard throughout inspiration and early expiration. Bronchial breath sounds—loud, high-pitched, with equal inspiratory and expiratory duration—indicate consolidation or airway pathology. Absent breath sounds suggest effusion, pneumothorax, or main stem bronchial obstruction. Crackles (rales) suggest fluid in alveoli, wheezes indicate airway narrowing, and pleural friction rubs reflect inflamed pleural surfaces. Cardiac auscultation evaluates S1 and S2 and identifies murmurs suggesting valvular disease.
Panel A: Inspection showing a frontal view of a patient with symmetric chest wall expansion indicated by arrows, respiratory rate counter, and labeled accessory muscles (sternocleidomastoid, scalenes) with notation about respiratory distress. Panel B: Palpation demonstrating tracheal assessment with a hand at the suprasternal notch and hands placed posterolaterally for expansion assessment with thumbs near the midline. Panel C: Percussion displaying a chest diagram with zones colored for resonant lung fields in green, cardiac dullness in blue, and hepatic dullness in gray with percussion technique demonstrated. Panel D: Auscultation showing anterior and posterior chest views with numbered circles indicating systematic auscultation sites and waveform icons representing vesicular, bronchial, crackles, and wheezes.
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## Thoracic Procedures
Central venous catheterization through the internal jugular vein utilizes the anatomical relationship of this vessel to the sternocleidomastoid muscle, where it runs between the two heads of the muscle in the carotid triangle. The approach enters lateral to the carotid pulse, with the needle directed toward the ipsilateral nipple. Risks include carotid artery puncture (medially) and pneumothorax (inferiorly). The subclavian vein approach passes beneath the middle third of the clavicle, with the needle aimed toward the sternal notch. This route carries higher pneumothorax risk but lower infection rates for long-term access.
Thoracentesis drains pleural effusion by inserting a needle into the pleural space. The procedure site is selected one to two intercostal spaces below the percussed or ultrasonographically confirmed fluid level, typically between the seventh and ninth intercostal spaces along the midscapular line posteriorly. The needle passes immediately above a rib to avoid the intercostal neurovascular bundle, which runs along the inferior rib margin.
Chest tube (tube thoracostomy) placement for pneumothorax or hemothorax employs the "safe triangle" bounded by the lateral border of pectoralis major anteriorly, the anterior border of latissimus dorsi posteriorly, and the fifth rib inferiorly. Entry typically occurs at the fourth or fifth intercostal space anterior to the midaxillary line. Blunt dissection carries the tube over the rib and through the parietal pleura into the pleural space.
Pericardiocentesis drains pericardial effusion, typically for cardiac tamponade. The subxiphoid approach inserts the needle just inferior to the xiphoid process, angled 45 degrees posteriorly and aimed toward the left shoulder. The needle traverses the diaphragm and pericardium to reach the pericardial space. Ultrasound or ECG guidance reduces the risk of myocardial injury.
Panel A: Internal jugular and subclavian central line approaches showing the sternocleidomastoid with its two heads, the internal jugular vein in blue between them, the carotid artery in red medially, and needle trajectories with danger zones highlighted. Panel B: Thoracentesis with a posterior chest view showing the fluid level in blue, the entry site marked with an X, and an inset cross-section showing the needle passing above the rib avoiding the neurovascular bundle along the inferior rib margin. Panel C: Chest tube insertion with the safe triangle outlined in green on a lateral chest view bordered by labeled muscles and rib with an inset showing blunt dissection technique. Panel D: Pericardiocentesis with an anterior view showing the subxiphoid approach with the needle angled toward the left shoulder and a cross-sectional inset showing the needle passing through the diaphragm into the pericardial space.
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## Surface Anatomy of the Abdomen
The abdomen may be divided for clinical description into either four quadrants or nine regions. Quadrant division uses vertical and horizontal lines intersecting at the umbilicus. The right upper quadrant contains the liver, gallbladder, right kidney, and hepatic flexure of the colon. The left upper quadrant houses the stomach, spleen, left kidney, and splenic flexure. The right lower quadrant contains the cecum, appendix, and right adnexal structures in females. The left lower quadrant contains the sigmoid colon and left adnexal structures.
The nine-region system uses two vertical lines (midclavicular) and two horizontal lines (subcostal and transtubercular) to create three rows of three regions. The upper row comprises the right hypochondriac, epigastric, and left hypochondriac regions. The middle row consists of the right lumbar (flank), umbilical, and left lumbar regions. The lower row includes the right iliac (inguinal), hypogastric (suprapubic), and left iliac regions.
Critical landmarks guide clinical assessment. The umbilicus lies at the L3-L4 vertebral level and marks the center of the abdomen. The anterior superior iliac spine (ASIS) anchors the lateral end of the inguinal ligament. The pubic tubercle, palpable at the superior margin of the pubic bone approximately 2-3 centimeters from midline, marks the medial attachment of the inguinal ligament and serves as a key landmark in hernia examination. McBurney's point, located one-third of the distance from the ASIS to the umbilicus along a connecting line, overlies the base of the appendix and represents the site of maximum tenderness in appendicitis.
Panel A: Left half showing the four-quadrant system with dashed lines intersecting at the umbilicus with each quadrant labeled (RUQ, LUQ, RLQ, LLQ) and major organs listed within each area. Panel B: Right half showing the nine-region system with two vertical and two horizontal demarcation lines creating the grid with all nine regions labeled. Panel C: Key landmarks highlighted with arrows including the umbilicus centrally, both ASIS points laterally, and the pubic tubercles medially on each side. Panel D: McBurney's point marked with a red dot in the right lower quadrant along a measured line from the ASIS to the umbilicus showing the one-third position.
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## Physical Examination of the Abdomen
Abdominal examination follows a modified sequence: inspection, auscultation, percussion, and palpation—with auscultation performed before palpation to avoid altering bowel sounds. Inspection assesses contour (flat, scaphoid in cachexia, or distended in ascites or obstruction), surgical scars suggesting prior procedures, striae indicating rapid distension, visible peristalsis in obstruction, caput medusae (periumbilical venous dilation in portal hypertension), and spider angiomata in liver disease.
Auscultation evaluates bowel sounds, which normally occur every 2-5 seconds. Hyperactive high-pitched sounds suggest early obstruction or gastroenteritis. Absent sounds after several minutes of listening indicate ileus or late obstruction. Bruits over the aorta, renal arteries, or iliac arteries suggest vascular stenosis or aneurysm.
Percussion determines organ sizes and detects fluid or air. Liver span in the right midclavicular line normally measures 6-12 centimeters, with the upper border at the fifth intercostal space (dullness replacing lung resonance) and the lower border at or just below the costal margin. Traube's space, the left lateral chest wall area bounded by the sixth rib superiorly, the left midaxillary line laterally, and the costal margin inferiorly, is normally tympanic due to the gastric air bubble; dullness suggests splenomegaly. Shifting dullness—dullness that moves with position change—indicates ascites.
Palpation proceeds from light to deep. Light palpation with the fingertips detects tenderness, superficial masses, and guarding (voluntary or involuntary muscle contraction). Deep palpation assesses for organomegaly and deeper masses. The liver edge is sought by palpating below the right costal margin during inspiration. An enlarged spleen becomes palpable below the left costal margin; if the spleen is not immediately felt, examination proceeds with the patient in the right lateral decubitus position. Kidney palpation employs bimanual technique with one hand posteriorly lifting the flank and the anterior hand palpating deeply; alternatively, ballottement detects the kidney between the hands. The aorta is palpated midline above the umbilicus to assess pulsation and width.
Panel A: Inspection showing an anterior abdominal view with labels for normal contour versus distension, surgical scars in common locations, and caput medusae pattern in a separate inset. Panel B: Auscultation showing stethoscope placement with labeled sites for bowel sounds centrally, aortic bruit epigastrically, and renal artery bruits at bilateral flanks. Panel C: Percussion demonstrating liver span measurement at the MCL with the upper border at dullness replacing resonance and the lower border at the costal margin showing 10 cm liver span. Panel D: Palpation showing liver edge detection at the right costal margin during inspiration, splenic palpation at the left costal margin, and special signs including Murphy's sign, psoas sign, and obturator sign.
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## Abdominal Procedures
Paracentesis removes ascitic fluid for diagnostic or therapeutic purposes. The preferred site lies in the left lower quadrant, approximately 2 centimeters medial and superior to the ASIS, to avoid the cecum on the right and the inferior epigastric vessels medially. An alternative infraumbilical midline approach traverses the relatively avascular linea alba. Ultrasound guidance improves safety and success rates.
Nasogastric tube placement estimates the required length by measuring from the nose to the ear to the xiphoid process, typically totaling 50-60 centimeters. The tube passes through the naris into the nasopharynx, then descends through the oropharynx and esophagus into the stomach. Position confirmation before use prevents pulmonary complications.
Peritoneal dialysis catheter insertion for chronic renal failure creates permanent access to the peritoneal cavity through the infraumbilical midline, where the abdominal wall is thinnest. The catheter tip lies in the pelvis to optimize dialysate contact with the well-vascularized peritoneal surface.
Laparoscopic port placement exploits anatomical features to minimize injury. The initial camera port typically enters at the umbilicus, where the abdominal wall is thinnest, using either an open (Hasson) or closed (Veress needle) technique. Working ports are positioned based on the target organ while avoiding the inferior epigastric vessels, which run deep to the rectus abdominis in a line from the femoral artery to a point midway between the umbilicus and pubic symphysis.
Panel A: Paracentesis with an anterior abdominal view marking the left lower quadrant site 2 cm medial and superior to the ASIS with the inferior epigastric vessels drawn as red avoidance lines and an alternative infraumbilical midline approach. Panel B: NG tube length estimation with a side view showing the measurement path from nostril to ear to xiphoid totaling approximately 55 cm. Panel C: Peritoneal dialysis catheter placement with a sagittal view showing infraumbilical entry through the linea alba and the catheter coiled in the pelvis. Panel D: Laparoscopic port positioning for cholecystectomy with four port sites marked at the umbilical, epigastric, and two right subcostal positions with the inferior epigastric vessels drawn in red showing safe zones.
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## Surgical Incisions and Approaches
Surgical incisions balance adequate exposure against wound complications and cosmesis. The midline (median) incision through the linea alba provides rapid, extensible access to the entire abdomen with minimal blood loss, as the linea alba is relatively avascular. It traverses skin, subcutaneous tissue, the fused aponeuroses of the anterior abdominal wall muscles, transversalis fascia, extraperitoneal fat, and peritoneum.
The paramedian incision parallels the midline through the rectus sheath, retracting the rectus muscle laterally. Though more time-consuming, it may offer improved wound strength. The Kocher (subcostal) incision runs parallel to and 2-3 centimeters below the right costal margin, providing excellent exposure for open cholecystectomy and biliary procedures. The McBurney incision uses an oblique approach at McBurney's point for appendectomy; the transverse Lanz incision at the same location offers superior cosmesis. The Pfannenstiel incision is a transverse suprapubic approach that provides access to the pelvis while healing with an inconspicuous scar, commonly used for cesarean section and gynecological procedures. The rooftop (bilateral subcostal or Chevron) incision extends across both upper quadrants for major hepatobiliary and pancreatic procedures.
Laparoscopic approaches have transformed abdominal surgery. Laparoscopic cholecystectomy typically employs ports in the umbilical, epigastric, and right subcostal positions. Laparoscopic appendectomy uses umbilical, suprapubic, and left lower quadrant ports. Port placement for colonic resection varies based on the involved segment.
Panel A: Midline incision in red running vertically through the umbilicus for general abdominal access and paramedian incision in orange slightly lateral to midline. Panel B: Kocher subcostal incision in green running obliquely below the right costal margin for biliary surgery and McBurney incision in blue as an oblique line at McBurney's point for appendectomy. Panel C: Pfannenstiel incision in purple as a curved transverse suprapubic line for cesarean and gynecological procedures and rooftop chevron incision in brown extending across both subcostal regions for major hepatobiliary procedures. Panel D: Inset cross-section through a midline incision with layers labeled: skin, subcutaneous fat, linea alba as fused aponeuroses, transversalis fascia, preperitoneal fat, and peritoneum.
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## Referred Pain Patterns
Visceral pain from abdominal organs is often poorly localized and may be perceived at distant sites due to the convergence of visceral and somatic afferent neurons at the same spinal cord levels. This phenomenon of referred pain follows predictable patterns based on the embryological origin and segmental innervation of each organ.
Diaphragmatic irritation, particularly of the central portion innervated by the phrenic nerve (C3-5), classically refers pain to the shoulder because the supraclavicular nerves derive from the same cervical segments. Cardiac pain distributes across the T1-T4 dermatomes, explaining chest, left arm, and jaw symptoms in myocardial ischemia. Esophageal pain presents as retrosternal discomfort through T4-T6. Gastric pain localizes to the epigastrium via T6-T9.
Gallbladder pain characteristically affects the right upper quadrant with radiation to the right shoulder or infrascapular region, reflecting both T7-T9 segmental innervation and diaphragmatic irritation by an inflamed gallbladder contacting the diaphragm. Pancreatic pain presents in the epigastrium with characteristic radiation to the back, corresponding to its retroperitoneal position and T7-T9 innervation. Small intestinal pain, including early appendicitis, localizes to the periumbilical region through T9-T10. As appendiceal inflammation involves the parietal peritoneum, pain localizes to the right lower quadrant through somatic innervation—explaining the classic migration of appendicitis pain. Colonic pain distributes to the lower abdomen via T11-L1. Renal and ureteral pain radiates from flank to groin, following the T10-L1 dermatomes.
Panel A: Anterior torso with the shoulder region shaded light blue for diaphragmatic pain at C3-5 and the left chest and arm in red for cardiac pain at T1-4. Panel B: The epigastrium in yellow for stomach pain at T6-9 and the right upper quadrant and right shoulder in green for gallbladder pain at T7-9 with the epigastrium in orange with an arrow to the back for pancreatic pain at T7-9. Panel C: The periumbilical region in purple for small intestine pain at T9-10 and the lower abdomen in brown for colonic pain at T11-L1. Panel D: Inset showing a cross-section of the spinal cord demonstrating convergence of visceral and somatic afferents at the same level explaining the neuroanatomical basis for referred pain.
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## Integration: The Acute Abdomen
The acute abdomen requires systematic anatomical reasoning to generate an appropriate differential diagnosis based on pain location. Right upper quadrant pain suggests hepatobiliary pathology (cholecystitis, hepatitis) or even referred pain from right lower lobe pneumonia irritating the diaphragm. Left upper quadrant pain raises concern for splenic pathology, gastric ulcer, or pancreatitis. Right lower quadrant pain classically indicates appendicitis but also includes ovarian pathology in females, Meckel's diverticulitis, and mesenteric adenitis. Left lower quadrant pain in adults most commonly represents diverticulitis, though sigmoid volvulus and ovarian pathology must be considered. Epigastric pain encompasses peptic ulcer disease, pancreatitis, and, importantly, referred pain from myocardial infarction. Periumbilical pain suggests small bowel obstruction, early appendicitis (before parietal peritoneal involvement), or abdominal aortic aneurysm. Suprapubic pain indicates bladder, uterine, or other pelvic pathology. Diffuse abdominal pain with peritoneal signs suggests perforated viscus, generalized peritonitis, or mesenteric ischemia.
The clinical history should establish where the pain began (indicating the visceral source), where it has migrated (indicating somatic involvement), radiation patterns (following referred pain pathways), and associated symptoms that suggest specific organ involvement.
Panel A: Anterior abdomen divided into regions color-coded with differential diagnoses for RUQ (cholecystitis, hepatitis, pneumonia) and LUQ (splenic pathology, gastric ulcer, pancreatitis). Panel B: Differential diagnoses for RLQ (appendicitis, ovarian pathology, Meckel's) and LLQ (diverticulitis, sigmoid volvulus, ovarian pathology) with arrows showing classic appendicitis pain migration from periumbilical to RLQ. Panel C: Differential diagnoses for epigastric (peptic ulcer, pancreatitis, MI), periumbilical (SBO, early appendicitis, AAA), and suprapubic (bladder, uterus, PID) regions. Panel D: Sidebar listing the four key history questions: where did pain start, where is it now, radiation pattern, and associated symptoms.
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## Emergency Procedures
Cricothyrotomy provides emergency surgical airway access when endotracheal intubation fails and the patient cannot be ventilated by mask. The cricothyroid membrane lies between the thyroid and cricoid cartilages in the anterior neck, identifiable as a soft depression below the laryngeal prominence (Adam's apple). This membrane is relatively avascular in the midline, and its superficial position permits rapid access. A horizontal incision through the skin and membrane allows tube placement into the trachea.
Needle decompression treats tension pneumothorax, where accumulated pleural air under pressure compromises venous return and causes cardiovascular collapse. The classic site is the second intercostal space at the midclavicular line, where a large-bore needle penetrates the chest wall and releases the trapped air. An alternative site, often preferred due to thinner chest wall, is the fifth intercostal space at the anterior axillary line. The needle passes immediately above the rib to avoid the intercostal neurovascular bundle.
Emergency thoracotomy provides direct cardiac access for traumatic arrest or cardiac tamponade. The left anterolateral approach enters the fifth intercostal space, extending from the sternum to the midaxillary line. This incision provides access to the heart for direct cardiac massage, control of cardiac hemorrhage, and cross-clamping of the descending thoracic aorta.
Resuscitative aortic occlusion controls hemorrhage from injuries below the occlusion point. Zone 1 occlusion of the descending thoracic aorta (between the left subclavian artery and the celiac trunk) addresses abdominal and junctional hemorrhage. Zone 3 occlusion of the infrarenal aorta addresses pelvic hemorrhage. REBOA (Resuscitative Endovascular Balloon Occlusion of the Aorta) accomplishes this endovascularly through femoral artery access.
Panel A: Cricothyrotomy anatomy in a lateral neck view with labeled thyroid cartilage, cricothyroid membrane highlighted in yellow, cricoid cartilage, and tracheal rings with an anterior view showing the horizontal incision site. Panel B: Needle decompression with an anterior chest view marking the second ICS at MCL and fifth ICS at AAL with an inset cross-section showing the needle passing over the rib avoiding the neurovascular bundle. Panel C: Emergency thoracotomy with a left lateral chest view showing the incision line across the fifth intercostal space from sternum to midaxillary line with the heart and descending aorta accessible through the approach. Panel D: Aortic zones for occlusion showing Zone 1 (descending thoracic, between left subclavian and celiac trunk) in red, Zone 2 (visceral segment) in between, and Zone 3 (infrarenal) in blue.
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## Summary
Surface anatomy provides the foundation for physical examination and procedural guidance, with key thoracic landmarks including the sternal angle at T4-5 and cardiac auscultation sites that differ from anatomical valve positions. Abdominal examination follows the sequence of inspection, auscultation, percussion, and palpation, with special signs such as Murphy's sign for cholecystitis and Rovsing's sign for appendicitis helping localize pathology. Referred pain follows predictable patterns based on shared spinal cord levels between visceral and somatic afferents, explaining classic presentations such as shoulder pain from diaphragmatic irritation and periumbilical-to-right-lower-quadrant migration in appendicitis. Surgical incisions are selected based on target organ accessibility and tissue disruption considerations. Emergency procedures including cricothyrotomy, needle decompression, emergency thoracotomy, and aortic occlusion require precise anatomical knowledge for safe and effective performance.
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## Key Terms
**McBurney's point**: The surface landmark for the base of the appendix, located one-third of the distance from the anterior superior iliac spine to the umbilicus along a connecting line; tenderness here suggests appendicitis.
**Murphy's sign**: Arrest of inspiration when the examiner's fingers palpate deeply at the gallbladder point during inspiration; pain causes the patient to catch their breath, indicating cholecystitis.
**PMI (Point of Maximal Impulse)**: The location where the cardiac apex beat is most readily palpable, normally at the fifth intercostal space along the midclavicular line.
**Safe triangle**: The anatomical zone for chest tube insertion, bounded by the lateral border of pectoralis major anteriorly, the anterior border of latissimus dorsi posteriorly, and the fifth rib inferiorly.
**Referred pain**: Visceral pain perceived at a somatic location distant from the affected organ, caused by convergence of visceral and somatic afferent neurons at the same spinal cord levels.
**Traube's space**: The area of the left lateral chest wall bounded by the sixth rib, left midaxillary line, and costal margin; normally tympanic on percussion due to the underlying gastric air bubble, with dullness suggesting splenomegaly.
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