Abdomen Lab 4a

Lab 4A - Anterior Abdominal Wall and Overview of the Abdominal Viscera

Lab Overview

Dissect the rectus sheath, and understand the layers of the sheath superior vs. inferior to the arcuate line.
Relate the layers of the abdominal wall to the layers of the spermatic cord (male) and types of inguinal hernias (female and male)
Relate the contents of abdominal folds (ligaments) of the anterior abdominal wall to their adult or embryonic functions.
Explore the contents of the abdominal cavity by mobilizing and following the entirety of the digestive system.

The Anterior Abdominal Wall

Anterior Abdominal Wall- Rectus Sheath

Use blunt dissection to tease the layers apart, and identify the following: (Figure 2.1)

Rectus abdominis m. - long, flat muscle that extends the vertical length of the anterior abdominal wall; a paired muscle, separated in the midline by the linea alba; widens and thins as it ascends from the pubic symphysis to costal cartilages 5-7
Linea alba - a dense band of connective tissue formed from the fused aponeuroses of the external oblique, internal oblique, and transversus abdominus muscles. Runs from the xiphoid process to the pubic symphysis; separates the left and right rectus abdominis mm.
Rectus sheath - tendinous sheath surrounding the rectus abdominis m., formed by the aponeurosis of the external oblique, internal oblique, and transversus abdominis mm.; contains the superior and inferior epigastric vessels and anterior cutaneous neurovascular bundles

With scissors or a scalpel, make a vertical incision to open the anterior layer of the rectus sheath to display its contents; observe that, along its course, the rectus abdominis m. is intersected by three or four tendinous insertions.

In its lower quarter, the rectus sheath consists of an anterior layer only (formed by the aponeuroses of all three anterior abdominal wall muscles Figure 2.1.

At the arcuate line, the inferior edge of the posterior layer of the rectus sheath is distinct. Inferior to the arcuate line, the rectus abdominis m. is in direct contact with the transversalis fascia.

Note that the arcuate line is typically inferior to the umbilicus.

FIGURE 2.1 Cross-sections through the rectus sheath, superior (A) and inferior (B) to the umbilicus.

Clinical Correlation: Hernias of the abdominal wall (Figure 2.2)

Umbilical hernias are usually small protrusions of extraperitoneal fat and/or peritoneum and omentum and sometimes bowel. They result from increased intraabdominal pressure in the presence of weakness or incomplete closure of the anterior abdominal wall after ligation of the umbilical cord at birth, or may be acquired later, most commonly in women and obese people.

Diastasis recti is the condition in which left and right rectus abdominus muscles separate, and a gap may form along the linea alba. These gaps may be congenital, the result of the stresses of obesity and aging, or the consequence of surgical or traumatic wounds. In some cases they may require surgical repair to prevent a ventral midline hernia.

Hiatal hernias occur when abdominal contents (usually the stomach or part of the stomach) pass thorough the esophageal hiatus in the diaphragm and protrude into the thorax.

See the inguinal hernia clinical correlation in this Guide for details on inguinal hernias.

Epigastric Vessels Deep to the Rectus Abdominis Muscle

Bluntly dissect the rectus abdominis m. from the posterior layer of the rectus sheath on one side (Figure 2.4).

Identify the following:

Superior epigastric vessels - found on the deep surface of the rectus abdominis m.; terminal branches of the internal thoracic vessels; usually smaller than the inferior epigastric vessels
Inferior epigastric vessels - arise form the external iliac vessels, superior to the inguinal ligament; course superiorly in the transversalis fascia to reach the posterior surface of the rectus abdomninis m. in the vicinity of the arcuate line. The vessels run between the rectus abdominus and the posterior rectus sheath superior to the arcuate line

The superior and inferior epigastric vessels anastomose on the deep surface of the rectus abdominis m. Figure 2.3

FIGURE 2.3 Anastomosis of the superior and inferior epigastric arteries

FIGURE 2.4 Anastomosis of the superior and inferior epigastric arteries posterior to the rectus abdominis muscle.

Anterior Abdominal Wall- Internal Surface

Several structures run just superficial to the peritoneum, and their presence forms folds in the peritoneum. These folds are sometimes called folds, but may also be called ligaments. These “ligaments” have nothing to do with the ligaments that connect bones to other bones. They are simply folds in the peritoneum. Why are they sometimes called ligaments? I don’t know, I did not make that decision.

Identify the following structures on the internal surface of the abdominal wall (Figure 2.5):

Median umbilical fold (ligament) - unpaired fold in the midsagittal plane: formed by the urachus (obliterated allantois) and its peritoneal covering
Medial umbilical folds (ligaments) - paired folds lateral to the median umbilical fold; formed by the obliterated umbilical aa. and their peritoneal covering
Lateral umbilical folds (ligaments) - paired folds lateral to the medial umbilical folds; formed by the inferior epigastric vessels and their peritoneal covering
Falciform ligament - a peritoneal fold that runs between the umbilicus and the anterior body wall and the liver (studied further in another laboratory session). A remnant of the ventral mesentery.
Ligamentum teres hepatis - or round ligament of the liver- a thickened cord of tissue in the free inferior margin of the falciform ligament; formed by the obliterated umbilical v. and its peritoneal covering (Figure 2.6)
Deep inguinal ring - appears as an oval defect on the internal surface of the abdominal wall; located midway between the anterior superior iliac spine and the pubic symphysis, superior to the inguinal ligament and lateral to the inferior epigastric vessels (Figure 2.7)
- Ductus deferens (male)- exits the abdominal cavity via the deep inguinal ring
- Round ligament of the uterus (female)- exits the abdominal cavity via the deep inguinal ring

FIGURE 2.5 Internal view of the anterior body wall. Take some time tounderstand the perspective of this figure. It is the inside of the anteriorbody wall viewed from the inside. You will not see this view in yourcadaver because the body wall was cut open from the outside. Thisdrawing will help you put it all together.

FIGURE 2.6 Left lateral view of the falciform ligament and the round ligament of the liver (ligamentum teres).

FIGURE 2.7 Interior view of the anterior abdominal wall

FIGURE 2.8 Inguinal hernias (A,B)

FIGURE 2.9 Inguinal hernia (C)

Clinical Correlation: Inguinal hernias

An inguinal hernia is a protrusion of parietal peritoneum and viscera, such as the small intestine, through the abdominal wall in the inguinal region.

There are two major categories of inguinal hernia: indirect and direct. Indirect inguinal hernias follow the route of the spermatic cord, and pass through the length of the inguinal canal, so they may end up in the scrotum. Direct inguinal hernias push directly through the abdominal wall at the site of the superficial inguinal ring.

More than two-thirds of inguinal hernias are indirect (congenital) hernias, and most commonly occurring in males. Direct inguinal hernias are also referred to as acquired hernias, and occur in older individuals, often due to weakening of the abdominal wall and obesity (Figure 2.8, Figure 2.9).

Clinical Correlation: Blood supply and lympatic drainage of the testes

Because the testes descend from the posterior abdominal wall into the scrotum during fetal development, their lymphatic drainage differs from that of the scrotum, which is an out-pouching of the abdominal skin. Consequently, cancer of the testis metastasizes initially to the lumbar lymph nodes and cancer of the scrotum metastasizes initially to the superficial inguinal lymph nodes.

(Figure 2.10)

Overview of the Abdominal Viscera

Identify the following (Figure 2.11) within the abdominal cavity (do not cut):

Liver - positioned in the right upper quadrant
Gallbladder - attached to the inferior surface of the liver (may be absent due to surgical removal). The gallbladder may have stained some of the viscera green, brown, or black. This is normal.
Stomach - left upper quadrant, inferior to the left lobe of the liver
Spleen - left upper quadrant, posterior and lateral to the stomach; touches the diaphragm and the tail of the pancreas; slide your right hand into the peritoneal cavity, lateral to the stomach, to cup the spleen in your hand

Reflect the greater omentum (the fatty apron; Figure 2.13) superiorly to expose the intestines and identify the structures in the following list. Note that it may be necessary to bluntly dissect or cut omental adhesions during reflection. Appreciate that most of the duodenum (the first segment of the small intestine) and the head of the pancreas are retroperitoneal and cannot be seen at this time. Now, identify:

Jejunum - second segment of the small intestine
Ileum - third segment of the small intestine
Ileocecal junction - right lower quadrant; union of the ileum to the cecum
Cecum and the vermiform appendix - right lower quadrant; the appendix is located on the inferior border of the cecum (appendix may be absent due to surgical removal)
Ascending, transverse, descending and sigmoid colon - extend from the right lower quadrant to the left lower quadrant.

FIGURE 2.11 Gross structures of the abdominal cavity.

FIGURE 2.13 Pull the greater omentum superiorly for a better view of the contents of the abdominal cavity.

Foregut- Stomach

To better view the structures of the foregut you may want to resect part of liver and cut the diaphragm (as shown in Figure 2.15) Ask a member of the teaching staff to assist you.

Identify the following (Figure 2.14):

Esophagus - the terminal and empties into the cardia of the stomach
Cardia - territory of the cardiac sphincter, formed by the right crus of the diaphragm; region where the esophagus enters the stomach
Greater curvature - inferior border of the stomach
Lesser curvature - superior border of the stomach
Fundus - dilated. superior part of the stomach related to the left dome of the diaphragm
Body- region between the fundus and the pylorus
Pyloric sphincter- make an incision along the long axis of the pyloric region to observe the pyloric sphincter (thick band of smooth muscle)

FIGURE 2.14 Regions of the stomach, and internal view

FIGURE 2.15 Anterior view of abdomen (liver removed)

Intestines

Distal to the pyloric sphincter the gut tube becomes the first part of the small intestine, the duodenum. The duodenum is the shortest region of the small intestine, but it receives bile from the liver and digestive enzymes and bicarbonate ions from the pancreas. Both the liver and the pancreas are derived from the foregut (endoderm and splanchnic lateral plate mesoderm), and are still connected to it by their respective ducts, which empty into the duodenum.

The duodenum becomes secondarily retroperitoneal in its middle part. Note where it disappears into the retroperitoneum, and then move the transverse colon and greater omentum out of the way to find the spot where the duodenum emerges from the retroperitoneum and transitions to the jejunum.

Mobilize the jejunum and the ileum to appreciate the anatomy of the mesentery, and follow the small intestine to its termination at the ileocecal junction. There is no grossly visible transition between the jejunum and the ileum

The ileum attaches to the cecum at the iliocecal junction. The cecum is a pouch that is suspended from the ascending colon. Mobilize the cecum and search for the appendix on its postero-medial surface.

The ascending colon is secondarily retroperitoneal, and therefore it is directly attached to the posterior body wall (it has no mesentery). Follow the ascending colon superiorly to the right (hepatic) flexure, where it transitions to the transverse colon.

The transverse colon is suspended by the greater omentum and the transverse mesocolon, so the greater omentum must be reflected superiorly to see it.

The transverse colon typically ascends as it moves toward the left side of the abdominal cavity. It terminates at the splenic (left colic) flexure, where it transitions to the descending colon.

The descending colon is secondarily retroperitoneal, so it will be applied to the posterior body wall. It has no mesentery, same as the ascending colon. Just superior to the pelvis the descending colon transitions to the sigmoid colon.

The sigmoid colon does have a mesentery. It travels toward the midline and transitions to the rectum, which is the terminal part of the large intestine.

The first part of the rectum is intraperitoneal but as it passes into the pelvis it become retroperitoneal.

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