Lab 6 - Anterior Body Wall & Abdominal Viscera
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 intercostal muscles. Note the relationship of the abdominal muscles to the rectus abdominis and consider the clinical impacts of the arrangement.
Expose and examine the visceral surface of the abdominal body wall and examine the organs that are relatively more ventral (closest to the body wall).
- Examine the liver.
- Examine the stomach.
- Open the lesser omentum and identify the three major structures lying within it: the hepatic artery, portal vein, and bile duct. Examine the gall bladder.
- Examine the duodenum, pancrease, and spleen.
- Examine the colon and mobilize the digestive tract. You will remove the gut in the next lab.
1. SKIN AND SUPERFICIAL FASCIA
- Make a midline incision through the skin from the sternum to the pubis.
- 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 leaving the umbilicus in place.
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 just below the plane of the navel.
- Make a transverse incision towards the anterior superior iliac spine. Palpate this structure if you are unable to visually identify it.
- 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 mediall border of the rectus abdominis 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 abdominis.
- Cut the rectus abdominis muscle on either side just beneath the umbilicus.
- Using a blunt probe, separate the muscle away from the posterior wall of the rectus 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).
- Identify the greater omentum, the apron that covers much of the abdominal viscera (MOVIE -courtesy of UMich).
- Not the liver's position and size and its 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.
- Using a scalpel with a new blade, carefully cut through the liver medial to the falciform ligament to remove the left lobe of the liver. Cut all the way through, careful not to cut any surrounding structures. This will expose the lesser omentum and the structures contained within.
- On the left lobe that you have removed, 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 from the esophagus to the duodenum, outlining the greater and lesser curvatures.
- Note 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).
6. LESSER OMENTUM AND GALL BLADDER
- Insert a finger into the omental foramen (aka epiploic foramen or foramen of Winslow), just lateral to the lesser omentum. This is the entrance to the omental bursa (lesser peritoneal sac) (MOVIE -courtesy of UMich).
- Dissect the free edge of lesser omentum and identify the three major structures lying within it: the hepatic artery proper, 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.
- You may notice the efferent nerves from the celiac plexus (formed from the greater splanchnic nerve) running to the liver with the hepatic arteries.
- Examine the hepatic artery proper, which is a branch of the common hepatic artery. The hepatic artery proper divides into the right and left hepatic arteries in the lesser omentum to supply the liver.
- The portal vein lies deep to the hepatic artery proper and the bile duct. Identify this structure of the lesser omentum.
- 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, from the gall bladder, hepatic ducts, from the liver. Both of these structures empty into the common bile duct, the third main structure in the lesser omentum.
- Trace the common bile duct to the duodenum. Variations in the duct are common.
7. DUODENUM, PANCREAS, AND SPLEEN
- Raise the left side of the stomach, reach your hand posteriorly, and bring the spleen forward. At the hilum of the spleen you will find the the large splenic artery.
- 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 and is embedded in this structure.
- Mobilize the second part of the duodenum
- Pull this part of the duodenum 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 common hepatic artery and left gastric artery.
- With the celiac trunk exposed, you may be able to find one or both of the important celiac ganglia, which form a 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 contributions 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.
- Lift up (superiorly) the greater omentum to see the small intestines and colon.
- Trace the mobile part of the small intestine from the duodenojejeunal junction to the ileocecal junction.
- Identify the cecum, appendix, ascending colon, transverse colon, left colic flexure, and descending colon.
- Beginning with the cecum and ascending colon, carefully pull the fixed parts of the colon away from the posterior abdominal wall with your fingers.
- With the gut fully mobilized, review the courses of the superior mesenteric artery and inferior messentaric artery. Trace the course of the inferior mesenteric artery to the parts of the colon past the left colic flexure (MOVIE -courtesy of UMich).
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 outpocketing 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?
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