Part 1: Peritoneal Cavity

Transect and reflect the muscles of the abdominal wall (on both left and right sides) to enter the abdominal cavity.

• On both sides, be sure the external abdominal oblique m. and the rectus abdominis m. have been transected and reflected.

• Transect both the internal abdominal oblique m. and transversus abdominis m. (near their lumbar origins) and reflect them ventrally to the level of the rectus abdominis m. (make a vertical cut just cranial to the hind limb as you reflect the muscle ventrally).

• On each side, make a cut that follows the costal arch to free the abdominal muscles from the caudal edge of the ribs and create a large ‘flap’ of muscle. (This cut should extend to the sternum.)

  • Connect these cuts (one from the left and one from the right) just caudal to the sternum to create one large flap of muscle that can be reflected caudally to enter the abdominal cavity.

The abdominal cavity is formed by the muscles of the abdominal wall, the ribs, and the diaphragm. It is lined by peritoneum, which encloses the peritoneal cavity.

• The peritoneal cavity is a closed space lined by a serous membrane called peritoneum; this will include parietal and visceral layers (as well as connecting peritoneum extending between the two layers and forming mesentery). (See DG Fig. 4-8)

  • Identify the parietal peritoneum, which is the layer lining the body wall (and was incised to open the peritoneal cavity).

  • Identify the visceral peritoneum, which is the layer surrounding/adhered to the surfaces of all of the organs within the abdominal cavity.

  • A third type of peritoneum is the connecting peritoneum that connects the parietal and visceral layers, or the visceral layers of adjacent organs. The connecting peritoneum forms peritoneal folds called ‘mesenteries’, ‘omenta’, or ‘ligaments’. (See DG Fig. 4-8) We will identify various parts of connecting peritoneum as we progress through this dissection.

Identify the falciform ligament which is a fold of connecting peritoneum passing from the umbilicus to the diaphragm and liver. It is usually fat-filled and easily identified on the deep face of the reflected ventral abdominal wall.

• Comment: In younger animals you may see the round ligament of the liver (aka umbilical v. remnant) extending from the free border of the falciform ligament to the liver (usually between the left medial and quadrate lobes); this structure is usually not seen in older animals.

• After identification of the falciform ligament, continue to follow the continuation of this fold of peritoneum caudally. Caudal to the umbilicus this fold is called the median ligament of the bladder. (See DG Fig. 4-11)

Identify the urinary bladder. If the urinary bladder is distended it will be found in the ventral abdomen extending cranially out of the pelvic inlet. If it is empty it will be contracted and look/feel like a small, firm ball and will be found in the pelvic inlet further caudally. (See DG Figs. 4-8, 4-9, 4-11, and 4-12)

• Identify the aforementioned median ligament of the bladder extending along the deep face of the ventral abdominal wall from the umbilicus to the ventral surface of the bladder.

  • Comment: In the fetus, the median ligament of the bladder contains the urachus and umbilical arteries, which degenerate in the adult.

• Identify the lateral ligaments of the bladder, which are usually filled with accumulated fat, and pass from the bladder to the pelvic wall. (The lateral ligaments of the bladder will contain the ureters and umbilical arteries as well.) (See DG Figs. 4-11 and 4-43)

After reflecting the ventral abdominal wall, the first thing you will likely see/notice in the abdominal cavity is the greater omentum (aka dorsal mesogastrium). The greater omentum is lacelike in appearance, with deposits of fat, and it represents the mesentery (connecting peritoneum) of the stomach. This greatly expanded structure folds on itself; the fold covers the ventral aspect of the abdominal viscera. The greater omentum passes from the dorsal body wall to the greater curvature of the stomach. (See DG Figs. 4-8, 4-9, A & B, and 4-11)

• Two areas of the fold of the greater omentum have regional names: the fold adjacent to the ventral body wall is the superficial leaf; the deeper part of the fold (adjacent to the abdominal organs) is referred to as the deep leaf. The space between these two leaves is called the omental bursa. (See DG Fig. 4-8)

  • Using your fingers, gently lift the greater omentum and peel apart the superficial and deep leaves to observe the omental bursa within.

  • Note that the omental bursa has a communication with the main peritoneal cavity via the epiploic foramen. This is an opening caudomedial to the caudate lobe of the liver, at the level of the cranial duodenal flexure which will be identified later in this lab after observing the lobes of the liver.

Using your fingers, gently peel the greater omentum away from the abdominal organs and reflect it cranially to continue identifying structures within the abdomen.