SUPERIOR AND POSTERIOR MEDIASTINUM; ANTEROLATERAL ABDOMINAL WALL AND INGUINAL CANAL (Grant's Dissector (16th Ed.) pp. 93-98; 99-112) TODAY’S GOALS (Superior and Posterior Mediastinum): 1. Access the posterior mediastinum. 2. Identify the major structures of the superior and posterior mediastinum. 3. Dissect and identify the components of the sympathetic trunk.

DISSECTION NOTES: Remove the posterior wall of the pericardial sac (may already be gone from removing the heart) and examine the posterior relations of the heart (Dissector p. 96, Fig. 3.26).

In the posterior mediastinum observe the following: Esophagus: collapsed muscular tube posterior to the trachea.

Right and left vagal trunks, and the esophageal plexus (parasympathetics are from CN X and sympathetics from the thoracic sympathetic trunk).

Left recurrent laryngeal nerve (Dissector Fig. 3.24) as it passes around the ligamentum arteriosum (formerly the embryonic ductus arteriosus), which connects the left pulmonary to the aortic arch.

The left vagus nerve contributes parasympathetic fibers to the esophageal plexus and then regroups as the anterior vagal trunk. The right vagus becomes the posterior vagal trunk. The positions of the vagal trunks are due to the rotation of the gut during development.

Q. What structure does the right recurrent laryngeal nerve loop around and pass posterior to on its course to the larynx?

Azygos system of (Dissector p. 97, Fig. 3.27): The azygos courses on the right side of vertebral column, along the posterior body wall. It is formed from the union of the ascending and right subcostal vein. It may also arise as a branch of the IVC. It receives the right posterior and the right superior intercostal vein (to the arch of the azygos). The azygos vein drains into the superior vena cava.

The accessory hemiazygos and course in intercostal spaces on the left side.

Descending aorta: Is the thoracic aorta in the thorax and the abdominal aorta in the . It gives off several branches: • posterior intercostal (usually 9 pairs that are larger than the anterior intercostal arteries) • esophageal branches (minor/branches may be difficult to see) • pericardial and mediastinal branches • bronchial arteries (left usually 2; right usually 1)

8 Thoracic duct (Dissector Fig. 3.27): The largest lymphatic duct in the body; ascends on the anterior surfaces of thoracic vertebral bodies posterior to the esophagus and between the azygos vein and the descending aorta. Its termination in the venous angle of the neck will be studied in the next unit.

Q. What areas of the body does the thoracic duct normally drain?

Sympathetic trunk (Dissector Fig. 3.27): Remove the parietal pleura from the posterior thoracic cage and dissect out the thoracic sympathetic chain/trunk. Note that three pairs of splanchnic nerves arise from each sympathetic trunk. They are listed below. Realistically, only portions of the greater splanchnic nerve will be visible in our dissections. Greater splanchnic nerve (T5-T9) Lesser splanchnic nerve (T10 and T11) Least splanchnic nerve (T12)

At the intersection of the sympathetic trunk with an intercostal nerve, attempt to find rami communicantes (white and gray ramus communicans) connecting the intercostal nerve with a thoracic sympathetic ganglion.

Q. Which rami communicans carry preganglionic fibers? Q. Which rami may be found over the entire length of the sympathetic trunk? Q. Where do the greater, lesser, and least splanchnic nerves synapse?

PEER TEACHING GUIDE: Organize your narrative to minimally include: 1. Demonstration of the structures and relationships of the posterior mediastinum including the esophagus, descending aorta, thoracic duct, and azygos system of veins. 2. Demonstration of the sympathetic chain, rami communicantes, and splanchnic nerves. 3. Demonstration of the recurrent laryngeal nerve and discussion of its clinical significance. 4. Demonstration of the posterior intercostal VAN (vein, artery, nerve) relationship.

9 ANTEROLATERAL ABDOMINAL WALL AND INGUINAL CANAL TODAY’S GOALS (Anterolateral Abdominal Wall and Inguinal Canal): 1. Identify the muscles of the anterior abdominal wall 2. Understand the structure of the inguinal ligament and canal 3. Important surface landmarks for inguinal region are: Iliac crest Anterior superior iliac spine Pubic tubercle Pubic crest

DISSECTION NOTES: Remove the skin and superficial fascia from the abdominal and inguinal regions (Dissector p. 101, Fig. 4.5). Be careful not to enter the inguinal canal. Understand the fascial plane formed by Scarpa’s, Dartos, and Colle’s fasciae (see Grant’s Dissector p. 101, Fig. 4.6). This fascial plane has important clinical implications. Try to demonstrate the boundaries of this plane, especially superiorly and inferiorly. A. Anterolateral Abdominal Wall

1. Fascial Arrangements: Superficial fascia • Camper’s: a superficial fatty layer with variable amounts of fat • Scarpa’s: a membranous layer deep to Camper’s Deep fascia • Fascia surrounding the external oblique

2. Cutaneous Nerves: Ventral rami of thoracoabdominal intercostal nerves and abdominal segmental nerves • Course between the internal oblique and transversus abdominis (similar to the internal and innermost intercostal muscles) to supply the abdominal skin and muscles • T7-9 nerves supply the skin superior to the umbilicus • T10 nerve innervates the skin around the umbilicus • T11-12, L1 nerves supply the skin inferior to the umbilicus

Note: T12 is called the subcostal nerve because it courses below the 12th rib; L1 is the cord level for the iliohypogastric and ilioinguinal nerves. These will be dissected later as they course along the posterior abdominal wall.

Try to observe a few cutaneous nerves as they pierce the rectus sheath a short distance from the median plane. (It would be a good idea to review the distribution of a "typical spinal" nerve.)

10 3. Musculature: Follow the cuts and reflections in Grant’s Dissector Figs. 4.8, 4.9. and 4.10 and identify the: • external abdominal oblique • internal abdominal oblique • transversus abdominis • rectus abdominis

The 3 flat muscles or flank muscles (external oblique, internal oblique, and transversus abdominis) end as aponeuroses, which fuse in the midline to form the linea alba. The flat muscles are best demonstrated on the lateral portion of the abdominal wall before they become aponeurotic.

The rectus abdominis muscle is enclosed by the rectus sheath consisting of the aponeuroses of the 3 flat muscles. Make a series of 3 cuts (see Dissector Fig. 4.13) and open the anterior wall of the sheath. Insert your fingers between the rectus sheath and the anterior surface of the muscle. Note that the muscle is firmly attached to the rectus sheath, especially at the tendinous intersections (Dissector Fig. 4.14; the short muscle bellies between these intersections, when significantly developed, produce the “6-pack abs”). Mobilize the muscle by dividing it at its midpoint and reflect it upward. Observe the posterior wall of the rectus sheath and the arcuate line (Dissector Figs. 4.15, 4.17). Be able to relate the layers of the two levels in Fig. 4.15 with your dissection. Identify the inferior and superior epigastric vessels posterior to the rectus abdominis (Dissector Fig. 4.15; recall that the superior epigastric vessels arise from the internal thoracic vessels). The inferior epigastric arises from the external iliac. (Where do the inferior epigastric vessels enter the rectus sheath?)

Inferior to the arcuate line, observe the transversalis fascia. Deep to the transversalis fascia is a thin layer of extraperitoneal connective tissue to which is attached the parietal peritoneum lining the abdominal cavity.

Note Clinical Correlation: Caput Medusae, Dissector p. 102.

B. Inguinal Region:

Observe the inguinal ligament (=the lower free edge of the external oblique aponeurosis, which attaches from the ASIS to the pubic tubercle) and the course of the inguinal canal (Dissector p. 103, 107, Figs. 4.7, 4.12). The contents of the canal include the spermatic cord in the male and the round ligament in the female. The inguinal canal is an obliquely oriented canal approximately a fingers-breadth superior to and parallel with the inguinal ligament. It is bounded by a pair of rings:

11 • Superficial (external) inguinal ring formed from the lateral and medial crus and intercural fibers, which are all derived from the external oblique aponeurosis. • Deep (internal) inguinal ring (formed from the transversalis fascia; its location can be approximated about midpoint between the ASIS and the pubic tubercle)

Identify the ilioinguinal nerve (L1) exiting the superficial inguinal ring (Fig. 4.8).

Observe the inguinal triangle (or Hesselbach’s triangle) at the medial border of the inguinal ligament. Its boundaries are:

• medial border: rectus abdominis (and sometimes the conjoint tendon/ligament) • lateral border: inferior epigastric vessels • base: inguinal ligament

Note Clinical Correlation: Inguinal Hernias, Dissector p. 106, Fig. 4.12

PEER TEACHING GUIDE: Organize your narrative to minimally include: 1. Review of the layers of the anterior abdominal wall from skin to the parietal peritoneum. 2. Demonstration of the muscles of the anterior abdominal wall (e.g., flank muscles and rectus abdominis) 3. Demonstration of the rectus sheath and layers associated with it. 4. Demonstration of the inguinal ligament, its bony attachments, and the inguinal canal (superficial and deep rings) and its contents. 5. Demonstration of the inguinal triangle (Hesselbach’s triangle), its boundaries, the conjoint tendon, and description of the anatomic differences between direct and indirect inguinal hernias.

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