Abdomen Lab 4B

Lab 4B - Anterolateral Body Wall and its Layers

Lab Overview

Identify surface features of the abdominal region.
Remove the skin from the anterior abdominal wall to expose the external oblique muscle, rectus sheath and inguinal ligament.
Dissect the rectus sheath, and understand the layers of the sheath superior vs. inferior to the arcuate line.
Identify the superficial inguinal ring and the spermatic cord (male) and round ligament of the uterus (female).
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.

Short List of Structures - 4B

Muscles of the abdominal wall
Superficial and Deep Inguinal Rings
Inferior epigastric a. and v. (Lateral umbilical fold)
Falciform Ligament

Anterior Abdomen: Surface Orientation

Before you start today’s dissection, take a moment to identify some surface structures of the abdomen. Use one of the mounted skeletons in the lab to locate the bony landmarks before trying to find them on the cadaver

Identify the following surface features:

HELPFUL HINTS

On thin bodies, these structures will appear as projections and will be easily palpated. On heavier bodies, the structures will be less obvious and may require more forceful palpation to feel them.

The Anterior Abdominal Wall

Your goal with this part of the dissection is to understand the layout of the nine layers of tissue that make up the anterolateral abdominal wall. You’ll be able to see most of the layers, but some are very thin and may not be separable from other layers. Understanding these layers is necessary for understanding the pathology of abdominal hernias. From superficial to deep the layers are:

Skin
Camper’s Fascia- fatty layer of the superficial fascia, variable in its thickness, depending on whether the cadaver is thin or fat
Scarpa’s Fascia- membranous layer of the superficial fascia
External oblique m. - enveloped by deep fascia
Internal oblique m. - enveloped by deep fascia
Transversus abdominis m. - enveloped by deep fascia
Transversalis fascia
Extraperitoneal fat
Parietal peritoneum

HELPFUL HINTS

You should know these layers (Figure 1.1), though you may not be able to definitively identify all of them in your dissection.

Camper’s and Scarpa’s fascia are simply named parts of the same fascial layers you’ve been referring to as the superficial fascia; the fat that’s under the skin. Campers is the loose fatty tissue, Scarpas is the layer of dense connective tissue just deep to the fat that lies superficial to the muscles of the body wall.

The transversalis fascia is very difficult to differentiate from the adjacent fascia of the transversus abdominis muscle.

Tissue layers of the anterolateral abdominal wall

Beginning the dissection

Using a scalpel, make cuts along the dashed lines (not the red lines!). If your cadaver is thin, take care to avoid damaging structures deep to the skin by pulling up on the skin with forceps.

For the vertical cut, make your incision along the midline as much as possible. For the horizontal cuts, cut from the pubic symphysis across the top of the thighs.

Reflect the skin and superficial fascia (fat) flaps laterally to expose the underlying musculature.

Be especially careful in the region of the inguinal ligament, indicated by the red lines. The inguinal ligament is the inferolateral margin of the external oblique aponeurosis, and is the site of inguinal hernias, which are very common in men.

Abdominal dissection lines

Abdominal Wall Musculature

Remove all the superficial fascia on the anterior abdominal wall and use a probe to separate the layers to identify the following:

External oblique m. - observe the direction of its striations, which is from superolateral to inferomedial (hands in pockets)
Internal oblique m.- intermediate muscle layer deep to the external oblique m.; the striations course at right angles to the external oblique m. in a superomedial direction. Anteriorly, the internal oblique aponeuroses contribute to the linea alba at the midline.
External oblique aponeurosis- the tendon of the external oblique m. An aponeurosis is a broad, sheet-like tendon. The external oblique aponeurosis forms part of the anterior lamina (layer) of the rectus sheath. The inferior border of the external oblique aponeurosis forms the inguinal ligament, take care not to destroy the inguinal ligament during dissection.
Rectus sheath- a tendinous sheath formed by the aponeuroses of 3 muscles; the external oblique, the internal oblique, and the transversus abdominis. The rectus sheath surrounds the rectus abdominis m. You'll be able to see the rectus sheath more fully when the abdomen is opened.
Transversus abdominis m. - deep muscle layer (deep to the internal oblique); the striations course, for the most part, in the transverse plane. Anteriorly, the aponeuroses contribute to the linea alba at the midline.
Rectus abdominis m.- courses longitudinally from the sternum and costal margin to the pubis. These muscles are within the rectus sheath, and the paired left/right muscles are separated by the linea alba
Tendinous intersections- superior, middle, and inferior tendinous slips that anchor the anterior layer of the rectus sheath to the rectus abdominis m. Form the indentions responsible for 6-pack abs.
Linea alba- strong tendinous seam between the two rectus sheaths. This is where the aponeuroses of the oblique and transversus muscles meet in the midline.
Transversalis fascia- a thin continuous layer of fascia between the transversus abdominus m. and rectus abdominus m. (superficially) and the extraperitoneal fat (deep). Difficult to isolate.
Extraperitoneal fat- located between the transversalis fascia and parietal peritoneum
Parietal peritoneum- serous lining on the internal surface of the abdominal wall, the deepest layer of the abdominal wall.

Anterior Abdominal Wall

Muscles of the antero-lateral body wall.

The external oblique muscle has been reflected to view the deeper internal oblique muscle.

Both external and internal oblique muscles have been reflected to reveal the transversus abdominis msucle.

Superficial Inguinal Ring

Find the thickened inferior region of the external oblique aponeurosis that is the inguinal ligament. The ligament runs between the ASIS and the pubic tubercle, so those are useful landmarks to guide you. As the ligament approaches the pubic tubercle it divides into an elongate Y-shape. such that the region between the arms of the Y is thinner than the rest of the ligament. That thin region is the superficial inguinal ring, the medial end of the inguinal canal (Gallery 1.2)

Dissect, using forceps, the following:

Superficial inguinal ring- located at the medial (superficial) end of the inguinal canal approximately 2.5 cm superolateral to the pubic tubercle; a triangular passage through the external oblique aponeurosis that is traversed by the spermatic cord (male) or round ligament of the uterus (female); the borders of the superficial inguinal ring are the following:
- Medial crus - fibers of the external oblique aponeurosis attached to the pubic symphysis
- Intercrural fibers - crossing (intercrural) fibers in the external oblique aponeurosis that hold the medial and the lateral crura together; prevent further widening of the superficial inguinal ring
- Lateral crus - fibers of the external oblique aponeurosis attached to the pubic tubercle
Spermatic cord / round ligament of the uterus - passes through the superficial inguinal ring to reach the perineum (scrotum in males, labia majora in females)
Inguinal ligament - the inferior border of the external oblique aponeurosis between the anterior superior iliac spine laterally and the pubic tubercle medially

FIGURE 1.4 Detail of the inguinal ligament

Inguinal ligament and the superficial inguinal ring

Figure of the inguinal ligament, superficial inguinal ring, and the spermatic cord (the thinner uterine ligament exits the superficial ring in females).

The round ligament of the uterus exits from the superficial inguinal ring and inserts out on the labia majora. Round ligament pain is common in pregnancy.

Opening the Abdominal Cavity

With a scalpel, carefully make incisions along the dashed lines seen in the figure below. The midline cut should go from xiphoid process to pubic symphysis. The lateral cuts should be just above the level of the umbilicus.

Be careful that you do not cut into the intestines. Lift up the body wall with forceps while you cut, to avoid cutting into any of the abdominal viscera. Start along the midline (linea alba) about 2 cm superior to the umbilicus. Cut a 2 cm long incision that is deep enough that you are certain you are in the abdominal cavity. Then use forceps to lift the body wall as you cut the rest of the incisions.

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.

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

Anastomosis of the superior and inferior epigastric arteries

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)
- To visualize the deep inguinal ring you will need to reflect the thin layer of parietal peritoneum that covers it
- 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

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.

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

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.

Another view of an Indirect Inguinal Hernia (through inguinal canal)

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)

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