Photo 2. Procedural: subcutaneous tissue removed

Clean and examine the external oblique mm., and unilaterally, examine the external features of the inguinal canal

Find these structures:

• • External oblique m.

    • Inguinal ligament

  • Superficial inguinal ring

    • Medial crus

    • Lateral crus

  • Round ligament of uterus, or spermatic cord

  • Ilio-inguinal n.

    • Anterior labial, or anterior scrotal brs.

  • Rectus sheath

  • Linea alba

3.) Clear the muscular fascia from the external oblique mm. bilaterally.

Note: The external oblique mm. are the most superficial of the anterolateral abdominal muscles. The muscle fibers are oriented inferomedially (think ‘hands in your pockets’).

External oblique m.:

Note: There are three anterolateral abdominal muscles: external oblique, internal oblique, and transversus abdominis. These muscles play a role in lateral flexion of the trunk and maintenance of tone of the abdomen.

Photo 3. External oblique mm.

4.) Unilaterally, clean the external oblique aponeurosis to its inferior-most margin, the inguinal ligament.

Note: The inguinal ligament extends from the ASIS to the pubic tubercle. It forms the inferior portion (‘floor’) of the inguinal canal.

Photo 4. Inguinal ligament

5.) On the ipsilateral side as step 4, locate specializations of the aponeurosis of the external oblique m.: superficial inguinal ring, and its medial and lateral crura. This will require careful blunt dissection in order to not damage the neurovasculature in this region.

Note: The superficial inguinal ring is the medial aperture of the inguinal canal that transmits either the round ligament of the uterus, or spermatic cord. The margins of the superficial inguinal ring are the medial crus (attaching to the pubic symphysis) and the stronger, lateral crus (attaching to the pubic tubercle).

Photo 5. Superficial inguinal ring, medial & lateral crura

6.) Identify the round ligament of uterus or spermatic cord as it exits the superficial inguinal ring. In this area, locate the small anterior labial or anterior scrotal brs. of the ilio-inguinal n. Do NOT follow these structures medially.

Note: The round ligament of uterus is a remnant of the gubernaculum, connecting the uterus to the labium majus. The round ligament presents as a tubular collection of adipose tissue.

Note: The spermatic cord will be dissected in more detail in a later lab. It contains the cremaster m., cremasteric a., testicular a., ductus deferens, artery of ductus deferens, pampiniform venous plexus, genital br. of the genitofemoral n., sympathetic nerve fibers, and lymphatic vessels.

Note: The ilio-inguinal n. (branch of L1 VPR) is located superficial to the round ligament of uterus or spermatic cord in the inguinal canal, and will exit the superficial inguinal ring as either the anterior labial/scrotal brs. These are sensory branches that innervate skin of the medial thigh, and either the mons pubis & labium majus, or root of the penis.

Photo 6. Round ligament of uterus & anterior labial br. of ilio-inguinal n.

Photo 7. Spermatic cord and anterior scrotal br. of ilio-inguinal n.

7.) Locate the rectus sheath medial to the external oblique mm. In the midline, identify the linea alba.

Note: The rectus sheath is the aponeuroses of the three anterolateral abdominal muscles, and encloses the rectus abdominis m. The linea alba spans between the xiphoid process and the pubic symphysis, and is a medial attachment point of the three abdominal oblique muscles.

Photo 8. Rectus sheath, linea alba, external oblique mm.

Unilaterally, open the rectus sheath, and examine and reflect the rectus abdominis m.

Find these structures:

• • Rectus abdominis m.

  • Superior epigastric a. & v.

  • Inferior epigastric a. & v.

  • Transversalis fascia

  • Arcuate line

8.) Open the rectus sheath on one side with a vertical incision lateral to the linea alba. Reflect laterally.

Photo 9. Procedural: incising the rectus sheath

Photo 10. Rectus sheath, reflected

9.) Unilaterally, relieve the rectus abdominis m. from its superior attachment above the costal margin. Reflect the entire muscle inferiorly, taking care to preserve the deep vasculature.

Note: The superior and inferior epigastric a. and v. may be adhered to the deep part of the rectus abdominis m., and will need to be carefully extracted through blunt dissection.

Rectus abdominis m.:

Photo 11. Procedural: reflecting rectus abdominis m.

10.) Locate and clean the superior and inferior epigastric vasculature. Take note of their anastomosis.

Note: The superior epigastric a. is the continuation of the internal thoracic a. The superior epigastric v. drains into the subclavian v. via the internal thoracic v.

Note: The inferior epigastric a. is a branch of the external iliac a. The inferior epigastric v. drains into the external iliac v.

Photo 12. Superior and inferior epigastric vasculature

11.) Locate the posterior wall of the rectus sheath. Follow the sheath inferiorly until it diminishes to only the thin transversalis fascia. This transition is the arcuate line.

Note: The transversalis fascia is deep fascia that covers the inner surface of the transversus abdominis m., and extends medially to the anterior abdominal wall.

Photo 13. Transversalis fascia & arcuate line

Examine the deep surface of the anterior abdominal wall

Find these structures:

• • Median umbilical fold

  • Medial umbilical folds

  • Lateral umbilical (epigastric) folds

  • Deep inguinal ring

  • Inguinal canal

  • Inguinal triangle

12.) Cut through the five layers of the anterior abdominal wall. Be mindful to not damage the structures deep to the wall (visceral peritoneum, GI tract, liver, etc.). Separate all layers from the costal margin, and reflect the entire anterior abdominal wall inferiorly.

Photo 14. Procedural: inferiorly reflect the abdominal wall

Note: The lateral abdominal wall is composed of seven layers. From superficial to deep, these layers are: skin, subcutaneous tissue, external oblique m., internal oblique m., transversus abdominis m., transversalis fascia, and parietal peritoneum.

Photo 15. Layers of the abdominal wall

13.) On the deep surface of the abdominal wall, identify the three types of peritoneal folds: median umbilical fold, medial umbilical folds, and lateral umbilical (epigastric) folds.

Note: The median umbilical fold is located in the midline (extending from umbilicus to the apex of the urinary bladder) and is created by the median umbilical ligament (a remnant of the urachus).

Note: The medial umbilical folds are located lateral to the median umbilical fold. The fossae in between are the supravesicular fossae.The medial umbilical folds are created by the medial umbilical ligaments (obliterated umbilical aa.).

Note: The lateral umbilical folds are located lateral to the medial umbilical folds. The fossae in between are the medial inguinal fossae. Direct inguinal hernias are associated with the medial inguinal fossae. The fossae lateral to the lateral umbilical folds are the lateral inguinal fossae, and these are associated with indirect inguinal hernias. The lateral umbilical folds are created by the inferior epigastric vessels.

Photo 16. Umbilical folds

14.) Lateral to the lateral umbilical folds (inferior epigastric vasculature), locate the deep inguinal rings.

Note: The deep inguinal ring is the internal aperture of the inguinal canal through which the round ligament of the uterus or spermatic cord enters the inguinal canal. The ring is an invagination of the transversalis fascia. Indirect inguinal hernias enter the inguinal canal through the deep inguinal ring.

Photo 17. Deep inguinal ring

15.) Identify the inguinal triangle. This triangle is visible from both a superficial and deep view of the abdomen.

Note: The boundaries for the inguinal triangle are inferior epigastric vessels (superolaterally), lateral border of rectus abdominis (medial), and inguinal ligament (inferiorly).

Note: Direct inguinal hernias enter the inguinal canal through the ‘weak fascia’ (e.g. transversalis fascia) in the inferomedial portion of the inguinal triangle.

Photo 18. Inguinal triangle

Locate the diaphragm

Find this structure:

• • Diaphragm

16.) View the diaphragm superiorly. Also take note of the diaphragm from an inferior approach, but realize that further dissect will necessitate destruction of the diaphragm.

Photo 19. Diaphragm

Note: The diaphragm is the muscular division between the thoracic and abdominopelvic cavities. The muscular fibers of the diaphragm originate circumferentially from the thoracic outlet/inferior thoracic aperture (an uneven ring following the inferior margins of the 12th and 11th ribs, the costal cartilages, and the xiphisternal junction), and insert on the central tendon.

Note: There are three major apertures of diaphragm:

• caval opening, at T8 (contents: inferior vena cava, right phrenic n., & lymphatics)

• esophageal hiatus, at T10 (contents: esophagus, vagal trunks, esophageal br. of left gastric a., & lymphatics)

• aortic hiatus, at T12 (contents: descending aorta, thoracic duct, & br. of the azygos v.).

Observe peritoneum & abdominal viscera

Find these structures:

• • Falciform ligament

  • Greater omentum

    • Omental apron

  • Small intestine mesentery (“The” mesentery)

  • Transverse mesocolon

  • Peritoneal cavity

  • Large intestine

    • Cecum

      • Ileocecal valve

      • Appendix

    • Ascending colon

    • Right colic (hepatic) flexure

    • Transverse colon

    • Left colic (splenic) flexure

    • Descending colon

    • Sigmoid colon

    • Semilunar folds of colon

    • Haustra of colon

    • Taeniae coli

    • Omental appendices

  • Small intestine

    • Duodenojujeunal flexure

    • Jejunum

    • Ileum

    • Ileocecal valve

    • Circular folds

17.) Examine the anterior aspects of the abdominal cavity. Identify the liver and stomach, which are covered with visceral peritoneum and considered intraperitoneal. Conceptualize the peritoneal cavity.

Note: The peritoneum, a serous membrane lining the abdominopelvic cavity, consists of two layers: visceral and parietal peritoneum. Parietal peritoneum lines the walls of the cavity, whereas visceral peritoneum covers viscera suspended within the cavity. Between the parietal and visceral peritoneum is a dynamic potential space, the peritoneal cavity.

Note: Intraperitoneal refers to viscera covered by visceral peritoneum and suspended within, but not inside the peritoneal cavity. Retroperitoneal refers to organs located posterior to the peritoneum. Structures may either be primary or secondary retroperitoneal, depending on their developmental history. A primary retroperitoneal structure (i.e. kidneys, inferior vena cava, aorta, proximal rectum, ureters, and suprarenal glands) develops and remains retroperitoneal, whereas secondary retroperitoneal structures (i.e. most of the duodenum, the ascending and descending colon, and most of the pancreas) begin development intraperitoneal, but eventually are pulled retroperitoneal.  

18.) Locate and identify the falciform ligament between the right and left lobes of the liver.

Note: The falciform ligament is a peritoneal ligament (reflection) between the anterior abdominal wall and the liver. This ligament surrounds the round ligament of the liver (the remnant of the umbilical vein).

Photo 20. Reflected anterior abdominal wall

19.) Identify the greater omentum, and differentiate a specific portion of the greater omentum: omental apron of the gastrocolic ligament.

Note: Omenta are folds of peritoneum which bind viscera to other viscera, or to the abdominal wall. The omenta are derived from the dorsal and ventral mesogastria (mesenteries), which anchored the primitive gut tube to the abdominal wall.

Note: Derived from the dorsal mesogastrium, the greater omentum is largely rooted on the stomach. It consists of several specialized peritoneal connections to viscera, such as the omental apron, a ‘free-hanging’ double layer of the gastrocolic ligament, which hangs inferiorly from the greater curvature of the stomach anterior to the jejunum and ileum, before looping back to the transverse colon.

Photo 21. Abdominal cavity

20.) Reflect the omental apron superiorly to locate, and identify the intestines.

Photo 22. Small intestine

21.) Examine the jejunum and ileum.

Note: The small intestine consists (proximally to distally) of the duodenum, jejunum, and ileum. The duodenum connects the stomach to the jejunum, and will be studied with the pancreas in a later lab. The midportion of the duodenum is retroperitoneal, thus the duodenum is anchored to the posterior abdominal wall. The small intestine mesentery (aka ‘THE’ mesentery) anchors the jejunum and ileum to the posterior abdominal wall.

Photo 23.  Jejunum, ileum, & mesentery

22.) Examine the mesentery (of jejunum and ileum) and its attachment to the posterior abdominal wall.

Note: The mesentery (of the jejunum and ileum) is a double layer of peritoneum encasing fatty connective tissues and neurovasculature serving the alimentary canal. The root of the mesentery anchors in an oblique line on the posterior abdominal wall, starting adjacent to the duodenojejunal flexure to the right sacro-iliac joint.

23.) On plastinated specimens, investigate the internal components of the small intestine: circular folds and ileocecal valve.

Note: The mucosa of the small intestine are characterized by circular folds, except in the first part of the duodenum and terminal portions of the ileum. Functionally, these folds serve to increase surface area for absorption and segmentalize the intestine.

Photo 24. Circular folds

Note: Ileocecal valve is located at the junction of the ileum of the small intestine and cecum of the large intestine. The valve is composed of a localized thickening of muscle, and serves to impede reflux and regulate transit of the small intestine.

Photo 25. Ileocecal junction

Photo 26. Ileocecal valve

24.) Run the length of the ileum to its terminus and junction with the large intestine. Examine the length of the large intestine, and identify its features.

Photo 27. Large intestine

Photo 28. Large intestine

Note: The large intestine is comprised of several regions, proximal-to-distal: the cecum & appendix, ascending colon, R. colic (hepatic) flexure, transverse colon, L. colic (splenic) flexure, descending colon, sigmoid colon, and rectum.  

Note: The ascending and descending colons are secondarily retroperitoneal. The proximal two-thirds of the rectum is also retroperitoneal.

Note: Fusion fascia are double layers of connective tissue formed by secondarily retroperitoneal structures colliding with the posterior body wall. Fusion fascia consist of what was previously parietal peritoneum and the visceral peritoneum on the posterior aspect of a structure fusing. The visceral peritoneum of the anterior structure then becomes the parietal peritoneum. Fusion fascia are clinically important because they allow for greater facility in either mobilizing or accessing a structure and are devoid of neurovasculature.

Photo 29. Cecum & appendix

25.) Carefully free the ascending and descending colons from the posterior body wall. Closely examine a length of one of these structures (or the transverse colon).

Note: The outer longitudinal layer of the muscularis of the colon condenses into three bands of muscle: the taeniae coli. Deviations of fibers from the free taenia coli form circumferential bands around the large intestine at regular intervals. These bands give the large intestine a sacculated appearance, and these sacs are known as haustra. Appendages of the large intestine are omental appendices, fat-filled outpouchings from the peritoneum.

Note: In your donor, you may notice diverticula, small outpouchings of the intestinal wall. Diverticulosis is common in older adults.

Photo 30. Features of the large intestine

Photo 31. Diverticula

26.) On plastinated specimens, locate semilunar folds of the colon.

Note: Semilunar folds of colon are caused by the haustra of the colon. Unlike the circular folds of the small intestine, the semilunar folds do not form complete rings.

Photo 32. Semilunar folds of colon

27.) Examine the transverse mesocolon.

Note: The transverse mesocolon is a double layer of peritoneum connecting the transverse colon to the posterior abdominal wall.

Photo 33. Transverse mesocolon

Locate superior mesenteric artery, vein, and branches

Find these structures:

• • Superior mesenteric a.

    • Jejunal & ileal aa.

    • Ileocolic a.

      • Appendicular a.

    • Right colic a.

    • Middle colic a.

  • Marginal a.

  • Superior mesenteric v.

28.) Starting at the root of the mesentery, and using blunt dissection, strip the visceral peritoneum from the intestines to expose the superior mesenteric a. (SMA) & intestinal aa. beneath the right side of the mesentery.

Note: The SMA supplies the derivatives of the midgut (distal duodenum through the proximal two-thirds of the transverse colon).

Note: The SMA has several major branches:

• inferior pancreaticoduodenal aa. (distal duodenum & pancreas)

• jejunal and ileal (intestinal) aa. 

• ileocolic a. (distal ileum, cecum, & appendix)

• right colic a. (ascending colon)

• middle colic a. (transverse colon).

N.B. The branches are listed by the order of the structures of the midgut they supply, not by their branching order off the SMA. Keep in mind that the SMA branches from the abdominal aorta inferior to the celiac trunk. It becomes intraperitoneal as it emerges between the pancreas and the duodenum (posterior to the stomach).

Note: The marginal a. is considered a branch of the superior mesenteric a., but it is an anastomotic arcade of the terminal branches of the SMA and IMA serving the large intestine. Relatively short straight aa. (arteriae rectae) bridge the distance from the marginal a. to the large intestine.

Note: The distal small intestine is supplied by a series of SMA branches called the jejunal and ileal (intestinal) aa. Intestinal branches exit the SMA linearly, form elaborate anastomotic arcades, and relatively long straight arteries (arteriae rectae) deliver blood to the jejunum and ileum. These intestinal arteries (and their venous counterparts) are sandwiched in the mesentery of the small intestine.

Photo 34. Procedural: removing the mesentery

Photo 35. Intestinal aa.

29.) Continue to remove the peritoneum to uncover the colic branches of the SMA.

Note: The ‘colic’ aa. typically deliver blood from the SMA to the marginal a.(arcade) in the vicinity of their target regions. Straight aa. then deliver the blood a short distance to the large intestine. The ileocolic a. is considered the terminal branch of the SMA, but often it appears as a branch of the SMA leading to the vicinity of the ileocecal junction, and the SMA will continue to descend inferiorly, giving off intestinal aa. Carefully follow all branches in this area, and often you may find anastomoses among these branches with the marginal a.  

Photo 36. Colic arteries of SMA

Note: The hepatic portal system is a series of veins collecting blood from capillary beds of the alimentary canal and delivering the blood to capillary beds of the liver. Nearly all veins of the GI drain into the hepatic portal system. The major architecture of the hepatic portal system is the union of the superior mesenteric v. with the splenic v. to form the hepatic portal v.

Photo 37. Superior mesenteric v.

Locate inferior mesenteric vasculature

Find these structures:

• • Inferior mesenteric a.

    • Left colic a.

    • Sigmoid aa.

    • Superior rectal a.

  • Inferior mesenteric v.

30.) Reflect the small intestine laterally to the right side of the peritoneal cavity, exposing the descending colon, sigmoid colon, and rectum.

Photo 38. Distal large intestine

31.) Bluntly remove the peritoneum covering the abdominal aorta and inferior mesenteric a.

Note: The IMA supplies blood to the derivatives of the hindgut (the distal third of the transverse colon through to the rectum).

Note: The IMA has several major branches:

• left colic a. (descending colon)

• sigmoid aa. (sigmoid colon)

• superior rectal a. (rectum)

Note: Do not trace the superior rectal a. distally. We will revisit blood supply of the rectum when dissecting the pelvis.

Photo 39. Inferior mesenteric a.

Photo 40. Inferior mesenteric v.