Thorax Anatomy and Cavities

• The thorax is part of the axial body, located between the neck and abdomen.
• It is superiorly bordered by the first rib and inferiorly by the diaphragm.
• The thoracic inlet is an oval opening bordered by the 1st ribs and the superior border of the sternum, allowing passage of trachea, esophagus, and great vessels.
• Thoracic outlet syndrome is a misnomer referring to compression of vessels or nerves at the level of the thoracic inlet.
• The thoracic outlet is the plane defined by the diaphragm.
• The thorax is cone-shaped, widest inferiorly, and comprises the thoracic wall and cavity.
• The thoracic wall provides protection and facilitates respiratory mechanics.
• The thoracic cavity houses organs primarily involved with the cardiorespiratory system and is divided into pleural and mediastinal cavities.
• Pleural cavities are paired, bilateral, containing the lungs.
• The mediastinal cavity is the central cavity with the heart and other thoracic organs.

Thoracic Wall Objectives

• The objectives are to identify anatomical landmarks, name parts of the thoracic cage, explain rib articulation and respiratory function, name intercostal muscle layers and their respiratory function, describe the position of intercostal vessels/nerves, identify structures on radiographs, and explain pathophysiology of clinical conditions like rib fractures.

Thoracic Wall Overview

• The thoracic wall is a barrel-shaped cage that encloses and protects thoracic organs, composed of the bony framework, intercostal muscles, and neurovascular bundles.

Anatomical Lines

• Anatomical lines are important reference points for medical procedures like thoracentesis and ECG lead placement.
• The anterior median (midsternal) line (AML) indicates the intersection of the median plane with the anterior thoracic wall.
• The midclavicular line (MCL) passes through the midpoint of the clavicle, parallel to the AML.
• The anterior axillary line (AAL) runs vertically along the anterior axillary fold, formed by the inferolateral border of pectoralis major.
• The midaxillary line (MAL) runs from the apex of the axillary fossa (armpit), parallel to the AAL.
• The posterior axillary line (PAL) runs vertically along the posterior axillary fold, formed by latissimus dorsi and teres major muscles.
• The posterior median (midvertebral) line (PML) is a vertical line along the spinous processes.
• The scapular lines (SLs) intersect the inferior angles of the scapula.

Bony Framework: Vertebrae

• The posterior midline of the thorax consists of 12 thoracic vertebrae.

Bony Framework: Ribs

• There are 12 pairs of ribs.
• True ribs (upper 7 pairs) attach directly to the sternum via costal cartilage.
• False ribs (8-10) have costal cartilages that fuse with the cartilages above for indirect sternal attachment.
• Floating ribs (11-12) have no sternal attachment.
• A typical rib (3-9) is broad, flat, and has a hollow center containing bone marrow.
• The head of a rib articulates with the bodies of 2 adjacent vertebrae, containing 2 facets.
• The superior facet articulates with inferior demifacet on the superior vertebra.
• The inferior facet articulates with the superior demifacet on the inferior vertebra.
• The tubercle articulates with the transverse process of the inferior vertebra and serves as a site of muscle attachment.
• The rib body curves anteromedially toward the sternum, with the greatest curvature at the costal angle.
• The costal groove, on the inferior internal surface, protects the neurovascular bundle.
• Chest tubes are inserted above the rib to avoid these structures.
• Rib notching is groove erosion due to intercostal artery expansion from aortic coarctation.
• The 1st rib is short, with a long axis in the transverse plane, and has a single facet for articulation with superior facet of rib 1.
• The superior surface of the 1st rib contains a tubercle for the anterior scalene attachment and grooves for the subclavian vessels.
• Ribs 10-12 have single facets for articulation with vertebral bodies.
• Costal cartilages extend the costal bodies toward the sternum.
• The costal margin is the fusion of costal cartilage of ribs 8-10 with 7.
• Costal cartilages provide flexibility to the rib cage.
• Rib fractures are hard to view on radiographs and can be inward or outward.
• Inward fractures can lacerate the lung.
• The weakest part of a structure is at the region of maximum curve.

Sternum

• The sternum is on the anterior midline of the thorax.
• It is a broad, flat bone segment resembling an inverted sword.
• The manubrium is the superior wedge shape bone of the sternum at the manubriosternal joint.
• The sternal angle is at the manubriosternal joint.
• The manubrium articulates with clavicles at clavicular notches and with the 1st rib.
• The body of the sternum articulates with ribs 2-10.
• The xiphoid process is the inferior tip of the sternum, cartilaginous through middle age and ossifies/fuses with age.

Muscles of the Thoracic Wall

• The thoracic walls anchor muscles acting on upper/lower limbs and the vertebral column.
• True muscles of the thoracic wall stay within and act upon the thoracic cage and are primarily involved in breathing.
• External intercostals are the outermost muscle layer between ribs, fibers run inferomedially from the tubercle of the rib and elevate the ribs.
• Internal intercostals lie superficial to the neurovascular bundle, fibers run superomedially, and contract to depress ribs.
• Innermost intercostals lie under the neurovascular bundle and are structurally/functionally similar to internal intercostals.
• Subcostals lie deep to intercostals near angle of lower ribs and thought to be similar to internal intercostals.
• Transversus thoracics muscles radiate from the internal surface of the sternum to the ribs that likely depress the ribs.

Neurovascular Bundle

• There are 11 paired sets of intercostal arteries, veins, and nerves that course along the thoracic wall between intercostal muscle layers.
• They run along the inferior margin of each rib, within the costal groove (VAN orientation - vein, artery, nerve from superior to inferior).
• Posterior intercostal arteries branch from posterolateral surface of the thoracic aorta.
• The 1st 2 intercostal spaces receive branches from the supreme intercostal artery of costocervical trunk.
• Right intercostal arteries arch over vertebral bodies to reach the costal groove, aorta lies to the left of the midline.
• Prior to piercing the innermost intercostal membrane, smaller collateral branches run inferiorly along superior border of inferior rib.
• Lateral thoracic branches come off just past the costal angle.
• Anterior intercostal arteries branch from the internal thoracic arteries, which pass inferiorly along the sternocostal junction.
• They anastomose with the posterior intercostal arteries around costochondral junctions.
• Anterior thoracic branches come off just lateral to the sternum.
• Anterior and posterior intercostal veins follow a similar course to arteries.
• Right posterior intercostal arteries drain into azygous vein.
• Left posterior intercostal arteries drain into the hemiazygous vein inferiorly, accessory hemizaygous vein superiorly, and cross the midline to join the azygous vein.
• Azygous vein drains into superior vena cava just prior to return to heart.
• Intercostal nerves are ventral primary rami of spinal nerves T1 through T11.
• They enter the costal groove with arteries/veins, project along to costosternal joints.
• They provide collateral branches similar to arteries and give off lateral/anterior branches, which travel with respective arteries/veins.
• They are responsible for muscular/cutaneous innervation of the thoracic wall.
• A single pair of subcostal arteries/veins/nerves travel under the 12th rib to abdominal wall.

Movements of Thoracic Joints

• At rest, the diaphragm is almost exclusively responsible for inspiration/expiration ("Parachute motion").
• During inspiration, radiating fibers contract, pulling the central tendon down to increase thoracic volume.
• During expiration, fibers relax, viscoelastic properties of lungs, abdominal organs passively elevate the central tendon, decreasing thoracic volume.
• There is gliding/rotation between the head of the rib and IV disc/vertebral bodies.
• There is superior/inferior gliding between the tubercle of the rib and transverse process of vertebrae.
• During forced inspiration, the anterior segment of ribs elevate the sternum like raising handle on a water pump.
• Ribs also elevate laterally.
• During forced expiration, the anterior segment of ribs depress.

Abdominal Wall Objectives

• Name the surface landmarks/reference planes that define the abdominal quadrants, and recognize the position of abdominal viscera relative to abdominal quadrants.
• Describe the organization of the abdominal wall, specifically the fascial and muscle layers and the rectus sheath.
• Describe the blood supply and venous/lymphatic drainage of the abdominal wall.
• Name the palpable landmarks used to identify the inguinal ligament.
• Describe the organization of the inguinal canal, including the superficial (external) and deep (internal) inguinal rings, and list the major contents of the inguinal canal in females and males.
• Describe the components of the spermatic cord and explain the derivation of fascial layers from the abdominal wall.
• Describe the gross structure of the testis, epididymis and vas (ductus) deferens.
• Explain the gross anatomical pathophysiology of a variety of clinical conditions including, but not limited to indirect inguinal hernia, direct inguinal hernia

Abdominal Wall Overview

• The abdominopelvic cavity is made up of both abdominal and pelvic cavities (continuous with one another); lies between the thoracic diaphragm and pelvic diaphragm.
• The abdominal cavity lies between the thoracic diaphragm and pelvic inlet, bordered anteriorly and laterally by the anterolateral abdominal wall, while the posterior margin is the posterior abdominal wall
• The superior portion is partially protected by the lower thoracic ribs while the inferior portion is partially protected by the greater pelvis.
• The entire cavity is lined with a serous parietal membrane called the peritoneum; some organs are lined with a visceral layer of peritoneum, while others lie outside the parietal peritoneum and are called retroperitoneal.
• Anterior wall contains five peritoneal folds with 3 paired recesses in-between.
• It contains organs of digestion and micturition and is divided into 4 quadrants (right/left upper/lower) or into 9 regions (epigastric, right/left hypochondriac, umbilical, right/left lumbar, hypogastric, right/left inguinal).

Anterolateral Abdominal Wall

• The anterolateral border of the abdominal cavity extends from the xiphoid process and thoracic cage to the inguinal ligament and anterior pelvic brim.
• There are two layers of superficial connective tissue under the skin.
• Camper’s fascia is fatty connective tissue and provides mechanical protection, thermal insulation, and a storage place for fat.
• Scarpa’s fascia is a thin fibrous layer that binds deep muscular layers.
• The lateral abdominal wall is made up of musculotendinous layers.
• External obliques are the most superficial of muscular layers
• Fibers run inferomedially towards the mid-line (hands in pockets) and arise laterally from middle/lower ribs.
• Internal obliques are an intermediate muscle arising from thoracolumbar fascia laterally, as well as parts of the iliac crest and inguinal ligament.
• Fibers start horizontal, splay out medially.
• Transversus abdominus is a deep layer of muscle tissue that originates from the thoracolumbar fascia, as well as ribs and iliac crest.
• Innervated by intercostal nerves 7-11 (thoracoabdominal nerves)
• It compresses and reinforces the anterior abdominal cavity, while obliques also generate bilateral flexion and unilateral trunk rotation (external contralateral, internal ipsilateral).

Anterior Facial Sheaths and Longitudinal Muscles

• The muscular layers of the lateral abdominal wall thin anteriorly into flat tendons called aponeuroses.
• Aponeuroses merge along the midline and are all responsible for secondary structures in the anterior abdominal wall.
• The aponeurosis for the external oblique thickens between the anterior superior iliac spine and pelvic tubercle.
• It folds over upon itself to form the inguinal ligament and curves toward the pelvic inlet to create the inguinal canal.
• Inferiorly, aponeurotic fibers from internal obliques interdigitate with those from transversus abdominus to form a conjoint tendon.
• Running together towards the midline protective layer the Rectus Sheath is formed.
• The anterior and posterior layers of the rectal sheath envelop the rectus abdominus.
• The rectus abdominus is a paired longitudinal muscle from the xiphoid process and inferior costal cartilages to the pubic crest and symphysis pubis, and is the primary flexor of trunk.
• Superior to the rectal sheath is pyramidalis, a small triangular muscle which runs from the pubic crest to the inferior portion of linea alba.
• The belly of rectus abdominus contain three or more tendinous intersections, which anchor the muscle to the rectus sheath.
• The path of the rectus sheath varies from superior to inferior segments of abdominal wall.
• Superiorly, the aponeurosis for the internal oblique separates at the lateral border of rectus abdominus to envelop muscle; consequently, aponeuroses for the external and intenral oblique make up the anterior rectus sheath, while aponeuroses for the internal oblique and transversus abdominus make up the posterior rectus sheath.
• Inferiorly, all three aponeuroses pass anterior to the rectus abdominus; only deep fascial layer lies posterior to the rectus abdominus in this region.
• The region of crossover occurs approximately 2 cm below the umbilicus, visible as the arcuate line.
• Beyond the rectus abdominus, common rectus sheaths from left and right merge along the mid-line to form linea alba (white line).

Deep Facial Layers

• Deep to the musculotendinous layer of the anterolateral abdominal wall is the transversalis fascia.
• The innermost portion of anterolateral wall is parietal peritoneum.
• Anterior wall contains five peritoneal folds and differentiate 3 paired recesses in-between.
• The median umbilical fold extends from the apex of the urinary bladder to the umbilicus and is a remnant of the urachus.
• Medial umbilical folds contain medial umbilical ligaments, obliterated umbilical arteries, and are intermediate of umbilical folds.
• Paired supravesicular fossae lie to either side of the medial umbilical fold between the medial umbilical folds; above the urinary bladder.
• Lateral umbilical folds cover inferior epigastric vessels.
• Paired medial inguinal fossae lie between medial and lateral umbilical folds; weakening in anterior abdominal wall may result in direct inguinal hernias
• Paired lateral inguinal fossa lie to either side of lateral umbilical folds; weakening of anterior abdominal wall may result in more common indirect inguinal hernia
• The median umbilical fold runs superiorly to the umbilicus, the former attachment site for the umbilical cord, and superior to the Umbilicus is the ligamentum teres, formed from the obliterated umbilical vein, attached through the falciform ligament.

Vasculature

• All arteries are accompanied by corresponding veins.
• The inferior portion of abdomen supplied by superficial epigastric and superficial circumflex iliac arteries; both branch from femoral artery.
• Both inferior epigastric and deep circumflex iliac arteries arise from external iliac artery to supply blood to rectus sheath.
• The inferior epigastric artery anastomoses with superior epigastric artery, which comes off the internal thoracic artery.
• The internal thoracic artery also supplies musculophrenic branches along neurovascular plane between internal oblique and transverses abdominus.

Inguinal Canal and Scrotum

• Scrotum and spermatic cord are modified layers of the anterior abdominal wall
• Spermatozoa require a temperature of ~30C lower than normal body temperature and so descend of testicular tissue external to abdominopelvic region.
• Superficial region of skin continues into scrotal sac.
• Camper’s fascia becomes extremely thin in the region of the scrotum, which allows for improved heat dissipation.
• Scarpa’s fascia is continuous with the scrotal sac as dartos fascia and contains a meshwork of smooth muscle fibers (dartos muscle).
• Contraction of dartos muscle occurs in cold external environments and is responsible for wrinkled appearance of scrotal sac, and increases fascial thickness and decreases surface area to prevent excessive loss of heat
• The aponeurosis of external oblique thickens to form superficial inguinal ring (medial termination point of inguinal canal).
• Fibres from the external oblique aponeurosis continue into the scrotal sac as the external spermatic fascia, outer covering of spermatic cord, which runs through inguinal canal from abdominopelvic cavity and emerges into scrotum.
• Fibers from the internal oblique aponeurosis form intermediate inguinal ring; additional fibers continue into scrotum as intermediate fascial layer of spermatic cord.
• Fibers differentiate into cremasteric muscle, smooth muscle tissue innervated by genital branch of genitofemoral nerve, which reflexively draws testicles towards perineum.
• The transversus abdominus terminates within the anterior abdominopelvic wall, forming the internal inguinal ring, which opens into abdominal cavity, no fibers contribute to scrotum/spermatic cord.
• The transversalis fascia continues into the scrotum as the internal spermatic fascia, innermost layer of wall for spermatic cord
• The parietal layer of peritoneum invaginates, fuses as it enters the spermatic cord, forming processus vaginalis; improper fusion results in a patent processus, often cause of inguinal hernias in infant males
• The spermatic cord terminates at the ovoid testicle.
• At the termination point of the testicle, processus vaginalis expands outward to form tunica vaginalis; parietal layer lies in contact with internal spermatic fascia, while visceral is in contact with testicle; testicle is anchored to scrotum by gubernaculum (scrotal ligament).
• Deep to the visceral layer of tunica vaginalis is tunica albuginea, which is fibrous and gives testicle rigidity.
• Segments of tunica albuginea invaginates, forming septa which divide testicle into 13-15 segments, each containing seminiferous tubule (convoluted tubule which is site of spermatogenesis).
• Seminiferous tubules straighten superoposteriorly and merge in a network called rete testes; from rete testes, contents empty into epididymis through efferent ductules.
• The epididymis lies on the posterior surface of the testicle, covered by visceral tunica vaginalis.
• The head of the epididymis lies superiorly and contains convoluted lobule, which collects spermatozoa from efferent ducts
• Tubules of epididymis straighten as they continue posteriorly through narrower body and tail of epididymis
• Upon leaving the inferior tail of the epididymis, tubule is referred to as the ductus (vas) deferens.

Interior of Spermatic Cord

• The ductus (vas) deferens is a straight, thick, rigid tube exiting posterior aspect of epididymis to travel through inguinal canal to abdominopelvic cavity.
• The testicular artery arises from the abdominal aorta and travels through the inguinal canal to appear in the spermatic cord.
• The testicular venules envelop the testicular artery in a complex called pampiniform plexus, for absorption of heat.
• Sympathetic nerve fibers trigger ejaculation.
• In females, the inguinal canal contains the round ligament of the uterus, which terminates in labia majora.