Abdomen Lab 4b

Lab 4B Anterolateral Abdominal Wall

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.
Identify the superficial inguinal ring and the spermatic cord (male) and round ligament of the uterus (female).
Open the abdominal cavity.
Clean and separate the layers of the anterior abdominal wall.

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.

Figure 1.1 Tissue layers of the anterolateral abdominal wall

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.

Beginning the dissection

Using a scalpel, make cuts along the dotted 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.

FIGURE 1.2 Abdominal dissection lines

Anterior Abdominal Wall Musculature

Remove all the superficial fascia on the anterior abdominal wall (Figure 1.3, Gallery .) to identify the following:

External oblique m. - observe the direction of its striations, which is from superolateral to inferomedial (hands in pockets)
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.
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.

FIGURE 1.3 Anterior Abdominal Wall

The anterior part of the rectus sheath has been reflected on the left side to show the rectus abdominus muscle.

On the left side the deep fascia is intact, obscuring the view of the muscles of the body wall. On the right side the deep fascia has been removed to expose the external and internal oblique muscles, and the rectus abdominus.

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

GALLERY 1.2 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.

FIGURE 1.4 Detail of the inguinal ligament

Abdominal Wall Incisions

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.

GALLERY 1.3 Layers of the anterior abdominal 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.

Layers of the Anterior Abdominal Wall

Use a probe to separate the layers in the cut edges of the abdominal wall.

Identify the following:

External oblique m.- the most superficial of the three flat and anterolateral abdominal wall muscles; muscle fibers course in an inferomedial direction; its large aponeurosis covers the anterior part of the abdominal wall to the midline. Approaching the midline, the aponeuroses are intertwined, forming part of the linea alba, which extends from the xiphoid process to the pubic symphysis.
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.
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
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.

FIGURE 1.5 Muscles of the antero-lateral body wall.

FIGURE 1.6 Layers of the anterior abdominal wall

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