Pre-lab Exercise: Surface Anatomy of the Anterior Abdominal Wall
Primary Lab Objectives and Goals:
Remove the skin and superficial fascia of the abdomen.
Examine the muscles, aponeuroses, nerves and vessels of the anterior abdominal body wall. Understand the layers of abdominal muscles that are homologous to the intercostals. Note their relationship to the rectus abdominis and consider the clinical impacts of the arrangement.
Expose and examine the visceral surface of the abdominal body wall and consider the organs that are relatively more ventral (closest to the body wall).
Identify divisions of the digestive tract. Carefully trace your way from the stomach through the duodenum, ileum, jejunum, cecum, and then the ascending, transverse and descending colon (the sigmoid colon may be hard to find). Understand the flow of digestive contents.
- Locate the superior mesenteric artery. Understand that this artery defines midgut structures.
- Examine the liver and stomach and their mesenteries. It is important here to understand blood flow to foregut structures from the celiac artery.
- Examine the omental bursa and foramen. Understand the relation of these structures to the stomach and to the vessels going to the liver.
- Open the lesser omentum and identify the three major structures lying within it: the hepatic artery, portal vein, and bile duct.
- Examine the spleen (a foregut structure) and small intestine. Try to see and understand the anatomical differences in each section of small intestine.
- Mobilize the digestive tract and note the locations of the three main arteries that supply the gut: the celiac, superior mesenteric and inferior mesenteric. Understand that the celiac artery supplies and defines foregut structures, whereas the superior mesenteric artery defines midgut structures and the inferior mesenteric defines hindgut. You will remove the gut in the next lab.
- Examine the pancreas and duodenum and understand the contributions that each receive from the celiac and superior mesenteric arteries.
1. SKIN AND SUPERFICIAL FASCIA
- Make a midline incision through the skin from the sternum to the pubis; use scissors when possible.
- Encircle the umbilicus and leave it attached to the body wall.
- Reflect the skin and superficial fascia laterally to both sides. Remove all the skin and superficial fascia over the abdomen.
2. ABDOMINAL MUSCLES
Note: The three layers of abdominal muscles: external oblique, internal oblique, and transversus abdominis, are homologous with the three layers of the intercostal muscles.
- Identify the External Oblique muscle.
- Use your scissors to transect the fibers of the external oblique muscle approximately 2 inches lateral to where it joins the rectus sheath (the blue dotted line on the figure below).
HINT: "Transect" means to cut across the structure generally, unless otherwise noted, perpendicular to the long axis of the structure.
Make sure that you make your cut through the external oblique muscle and NOT through the rectus sheath. It will be much easier to find the different layers of the oblique muscles this way.
- Continue this cut downward to a point about four inches below the navel.
- Make a transverse incision over to the anterior superior iliac spine.
- Reflect the External Oblique to expose the anterior ends of the lower intercostal spaces. This will also expose the Internal Oblique.
- Reflect the Internal Oblique to expose the third and deepest layer of abdominal wall musculature, the Transversus Abdominis.
Examine one of the hanging skeletons. Notice how close the iliac crest comes to the bottom of the rib cage. The approximation of the pelvis to the rib cage limits the potential flexibility of the lumbar vertebral column. This makes it easier for the lateral abdominal-wall muscles to control the sway of the trunk.
- Note that the aponeurosis of the Internal Oblique splits to form a sheath around the Rectus Abdominis called the rectus sheath.
- Find the lateral border of the Rectus muscle and open the sheath with a longitudinal incision just lateral to the linea alba (the midline union of the aponeurosis forming the rectus sheath).
- Reflect the anterior wall of the sheath to reveal the Rectus Abdominis muscle; note the tendinous intersections of the Rectus.
- Transect the Rectus Abdominis.
HINT: When you transect a muscle you need to decide where it is best to do so. Mostly you can cut somewhere in the middle of the muscle. If there are several muscles, you may want to cut at different levels so you can “reconstruct” each muscle with less confusion.
- Dissect the superior and inferior portions of the muscle away from the posterior wall of the sheath. Try not to pierce the transversalis fascia, the deepest layer of the abdominal body wall, inferior to the arcuate line, which defines the end of the posterior reinforcement of the sheath by fascia from abdominal muscles.
Within the rectus sheath, you will find the superior and inferior epigastric arteries running longitudinally deep to the rectus abdominis and supplying this muscle. The superior epigastric artery is a continuation of the internal thoracic artery in the thorax. The inferior epigastric is a branch of the external iliac artery. Near the lower end of the Rectus abdominis, the inferior epigastric artery enters the rectus sheath by passing through the transversalis fascia. The superior and inferior epigastric arteries form an anastomosis within the rectus sheath, providing a potential alternative pathway for blood to flow from the thoracic region to the lower parts of the body.
3. ANTERIOR ABDOMINAL WALL
- Transect abdominal muscles along the rib cage inferolaterally. At the mid axial line (below armpit) continue your cut inferiorly to the iliac crest (as shown by the blue dotted line). Reflect the abdominal wall tissue inferiorly.
HINT: As you reflect the anterior abdominal wall, you will find a ventral mesentery, the falciform ligament, running from the liver to the body wall. Starting superiorly, carefully cut the falciform ligament close to the surface of the liver (so that a small flap remains attached to the abdominal wall) and work your way inferiorly to mobilize the body wall.
- Once the body wall is fully reflected, examine the inner surface and locate the four embryonic structures (mere vestiges in the adult) that are attached to the navel from behind: the ligamentum teres of the liver (in the free edge of the falciform ligament), the two medial umbilical folds corresponding to the obliterated umbilical arteries, and the single midline median umbilical fold corresponding to the obliterated allantois (urachus).
4. ABDOMINAL CAVITY
- Briefly examine the liver (we will return to it later), its position and size and the relationship with the inferior surface of the diaphragm.
- Explore the reflections of the peritoneum from the diaphragm to the liver with your hands (MOVIE -courtesy of UMich).
- Note how hard it is to slip your hands behind the liver. The visceral peritoneum covering the liver reflects onto the lower surface of the diaphragm. The portion of the liver in direct contact with the diaphragm is known as the bare area. This peritoneal reflection holds the liver to the diaphragm, keeping the liver in place.
- Note the boundaries of the coronary ligaments, reflections of peritoneum attaching liver to the inferior surface of the diaphragm.
- Find and palpate the bare area of the liver where the coronary ligaments do not cover the liver.
- Briefly identify the main divisions of the digestive tract (liver, stomach, small and large intestines)
- Identify the greater omentum, the apron that covers much of the abdominal viscera (MOVIE -courtesy of UMich). Carefully raise the greater omentum and reflect it back over the lower thorax.
- Using you hands to mobilize structures and follow them, establish the continuity of the various parts of the colon from the cecum to the sigmoid colon.
- Compare the pattern of the arteries within mesentery of the jejunum and ileum. The jejunum has few arterial arcades and longer vasa recta compared to the ileum.
- Trace the mesenteries back to their attachments to the posterior abdominal wall (MOVIE -courtesy of UMich).
- Find the superior mesenteric artery running in the root of the mesentery by tracing the course of the duodenum. This artery is the axis of rotation in the primary intestinal loop (MOVIE -courtesy of UMich).
- Carefully cut and remove the left lobe of the liver, exposing the lesser omentum.
- Examine the internal structure of the liver and its segmental vessels.
- Trace the ligamentum teres of the liver (the obliterated umbilical vein found earlier) to the portal vein, running in the caudal edge of the lesser omentum (MOVIE -courtesy of UMich).
- Trace the outline of the stomach
- Note the peripheral arterial anastomoses and the gastric regions (fundus, body, and pylorus) (MOVIE -courtesy of UMich).
- Follow the lesser omentum from the lesser curvature of the stomach to the porta hepatis.
During development and rotation of the gut, the stomach and spleen move from a midline position (shown in the embryo on the left below) to lie on the left side of the body, whereas the liver ends up on the right side (shown in the adult on the right below). The connected mesenteries between these organs are also rotated to form the greater and lesser omenta (and the greater and lesser omental bursae, or sacs), which you will explore as you continue through this dissection (MOVIE -courtesy of UMich).
7. OMENTAL FORAMEN AND BURSA
- Replace the structures that you have disturbed and fold the greater omentum back down over the abdominal viscera.
- Insert a finger into the omental foramen (aka epiploic foramen or foramen of Winslow), the entrance to the omental bursa (lesser peritoneal sac) (MOVIE -courtesy of UMich).
- Explore the recesses of the omental bursa. The omental bursa can also be explored by lifting up the transverse colon and cutting a small window in the underside of the transverse mesocolon (MOVIE -courtesy of UMich).
- Separate peritoneal adhesions with care, and try to avoid tearing the lesser omentum when your hand enters the omental bursa.
The liver is connected to the gut by the lesser omentum. The lesser omentum encloses two vessels carrying blood to the liver (hepatic artery, portal vein) and a duct which drains the exocrine secretions of the liver (bile duct). These structures are accompanied by minor vessels supplying and draining the extra-hepatic parts of the system of biliary ducts, including the gall bladder. It is very important for surgeons to be aware of these structures when resolving problems associated with the stomach or posterior abdominal wall via the lesser sac.
8. LESSER OMENTUM
- Dissect the free edge of lesser omentum and identify the three major structures lying within it: the hepatic artery, portal vein, and bile duct that enter the liver via the porta hepatis (MOVIE -courtesy of UMich).
NOTE: to dissect the lesser omentum you will want to first make sure you have identified the omental foramen. Near the foramen you can palpate the vascular and nervous structures within the omentum. Once you know where all those are, you can dissect the layers of the omentum. See the figure below in which a paper is placed in the omental foramen.
- Note the efferent nerves running to the liver with the hepatic arteries.
- Study the common variations in the pattern of the arteries traversing the lesser omentum, and determine whether any of these variants are present in your specimen.
- Free the gall bladder from the visceral surface of the liver (MOVIE -couresty of UMich).
- Open the gall bladder which may contain gallstones.
- Probe the connections of the cystic duct, hepatic ducts, and bile duct.
- Trace the bile duct to the duodenum. Variations in the duct are common.
9. SPLEEN AND SMALL INTESTINE
- Lift the left side of the stomach and examine the spleen (MOVIE -courtesy of UMich).
- Note where the spleen lies in relation to the ribs.
- Trace the mobile part of the small intestine from the duodenojejeunal junction to the ileocecal junction.
- Locate the appendix (if present) in your specimen (MOVIE -courtesy of UMich).
- Replace folds of skin for a moment and note the relationship of that to McBurney’s point 2/3 of the way down a line from umbilicus to iliac crest.
The mobile portions of the gastrointestinal tract (e.g. stomach, most of the small intestine, and some of the large intestine) have a dorsal mesentery, which connects them to the posterior abdominal wall. The fixed portions of the tract (parts of the duodenum and colon) have secondarily fused with the posterior wall and lost their dorsal mesentery. During this lab, you will remobilize those fixed portions
- Beginning with the cecum and ascending colon, carefully pull the fixed parts of the colon away from the posterior abdominal wall with your fingers.
- Trace the course of the inferior mesenteric artery to the parts of the colon past the left colic flexure (MOVIE -courtesy of UMich).
With the gut fully mobilized, you should review the courses of the superior and inferior mesenteric arteries.
- Note the different lengths and densities of the vasa recta in various regions of the small and large intestines.
11. PANCREAS AND SPLENIC ARTERY
- Earlier you dissected the lesser omentum. Now open the omental bursa and raise the stomach to expose the duodenum, and the body and tail of the pancreas (MOVIE -courtesy of UMich).
- Note that the splenic artery runs dorsal to the pancreas.
- Mobilize the second part of the duodenum
- Pull it forward and free it from the head of the pancreas.
- Clean off the peritoneum overlying the pancreas.
- Examine the pancreas
- Pull the pancreas forward
- Trace the splenic artery from the celiac trunk to the stomach and spleen (MOVIE -courtesy of UMich).
- Identify the other two major branches of the celiac trunk, the hepatic and left gastric arteries.
Note that portions of the stomach, duodenum and pancreas are supplied by branches from the celiac trunk AND superior mesenteric artery. In contrast, the liver and spleen are supplied by branches of the celiac trunk only.
- With the celiac trunk exposed, you should be able to find one or both of the important celiac ganglia, which form a ganglionated plexus around the celiac trunk.
- Look for the vagal trunks (you saw them in the thorax earlier) bearing visceral parasympathetics through the esophageal hiatus in the diaphragm. Trace conrtibutions from the vagal trunks to the celiac plexus.
Note that similar autonomic plexuses and ganglia may be found at the root of the superior and inferior mesenteric arteries for distribution to corresponding regions of the small and large intestines.
However, the celiac, superior, and inferior plexi and ganglia will probably be best observed after the abdominal viscera have been removed, which will be done during the next lab session.
1. Meckel's Diverticulum
In the embryo, the midgut is continuous through the navel with the rudimentary yolk sac. The vitelline duct connecting gut and yolk sac ordinarily degenerates in later fetal life, but a vestige of it sometimes persists in the form of a blind out pocketing or appendix of the ileum, called Meckel's diverticulum. Occasionally, a Meckel's diverticulum can become strangulated or may begin hemorrhaging.
2. Vitelline Fistula
A less common and more serious malformation is persistence of a patent vitelline duct; the resulting vitelline fistula permits fecal discharge through the navel, and the ileum may prolapse through the fistula.
Note the following potential weak points in the walls enclosing the abdominal viscera:
- The vertebrocostal triangle in the diaphragm, where the embryonic pleuroperitoneal membrane often is not reinforced by hypaxial musculature
- the umbilicus, through which two diverticula of the gut and three major blood vessels pass in the fetus;
- the inguinal canal, through which the male gonads descend into the scrotum;
- the femoral canal, through which the superficial lymphatics of the hindlimb pass back into the abdomen on their way toward the lumbar lymph trunks.
Increased intra-abdominal pressure may force a loop of the abdominal gut through any of these weak points, producing a hernia. A congenital hernia may be found at birth at any of the first three points listed above. Excluding a congenital defect in the abdominal wall or diaphragm, what causes of increased intra-abdominal pressure might result in a diaphragmatic, umbilical, inguinal or femoral hernia? Why are hernias more common among men than among women?
A vagotomy (surgical division of the vagus nerve) is sometimes performed to treat ulcers. Why would this be effective?