Section IV—Pleuroperitoneal Cavity, Viscera, and Urogenital System

Using a scalpel and just to one side of the midventral line, make a small, shallow incision—just large enough to insert a scissor blade—through the skin (and only through the skin), which is very thin.

FIGURE 6.10 Pleuroperitoneal cavity of a female frog in ventral view. The very large ovaries of this individual obscure many of the viscera.

on the other side of the body, finally producing two flaps that can be reflected laterally.

Begin reflecting the skin on one side. It tends to pull away easily but does adhere more strongly in several places. These represent the attachments of lymphatic sacs. Scrape away the connecting tissue, but stay close to the skin in doing so. You will reveal the pectoral and abdominal musculature. Proceed cautiously on the ven­trolateral surface, where the musculocutaneous vein extends anteroposteriorly. Leave the vein on the mus­culature; it lies just lateral to the lateral edge of the pec­toralis major (Figure 6.10). Anteriorly, it veers medially and passes deep to the pectoralis major. Follow the vein posteriorly as it reflects onto the deep surface of the skin, and note that it is formed by the coalescence of the numerous veins draining the skin.

Examine the abdominal musculature. On the midline, you will note the path of the ventral abdominal vein, which actually lies within the abdominal cavity and will be exposed presently.

FIGURE 6.11 Pleuroperitoneal cavity of a female frog in ventral view. The smaller ovaries expose many of the viscera.

anteroposterior incision, and reflect the resulting two flaps.

Follow the ventral abdominal vein as it passes into the cleft between the right and left lateral lobes of the liver, the large dark mass lying anteriorly in the pleuroperitoneal cavity. The liver is relatively wide and short, conforming to the shape of the body. Its lobes are usually subdivided to varying degrees. The left lobe usually extends further posteriorly due to the development of its posterior lobe, and the right lobe of the liver usually covers a smaller median lobe of the liver. Spread the lateral lobes of the liver to reveal the spherical, sac-like gall bladder, which lies just posterior to the passage of the ventral abdominal vein (Figure 6.11). Extending from this vein and the liver to the midvental body wall is the falciform ligament. Lift the body wall, and break through the falciform ligament to reveal the pericardium, a sac-like structure that contains the heart (see below) nestled between the anterior ends of the lateral lobes of the liver.

Examine the structures posterior to the liver.

FIGURE 6.12 Pleuroperitoneal cavity of a female frog in ventral view. Several structures have been removed from the left side to expose the urogenital system.

FIGURE 6.13 Pleuroperitoneal cavity of the male frog, with many of the viscera removed to expose urogenital system.

cycle. In females, the paired oviducts are large, highly convoluted tubes occupying much of the rest of the ventral part of the pleuroperitoneal cavity. Stretches of the digestive tract (see below) may be exposed among the coils of the oviducts, but these are generally nar­rower and slightly darker in color. In males large oviducts will not be present, and the coils belong to the digestive tract.

If your specimen is a female with very large or massive ovaries, remove them to provide a better view of the remaining structures, as shown in Figure 6.12. Grasp an ovary, reflect it laterally, and remove it by cutting though its mesentery, the mesovarium (Figure 6.12).

Examine the viscera. Find the stomach, tucked deep to the lateral side of the left lobe of the liver (Figure 6.11). It extends posteriorly on the left side of the pleuroperi­toneal cavity. Proximally it leads to the short, thick esophagus. Lift the proximal end of the stomach to reveal the left lung, far anterior in the pleuroperitoneal cavity (Figures 6.11 and 6.13). The right lung may be found dorsal to the right lobe of the liver. The lungs appear as small, contracted sacs in preserved specimens, but they are generally larger in live frogs. Return to the stomach. Distally it narrows and turns abruptly to the right and leads to the intestine. The intestine may be subdivided into the narrow, coiled small intestine fol­lowed by a short, wide large intestine that leads to the cloaca (Figure 6.12). The pancreas lies in the mesentery between the duodenum, the first part of the small intes­tine, and the stomach (Figure 6.11). The spleen is a dark, ovoid body lying in the mesentery farther distally and dorsal to the small intestine. Examine the posterior part of the pleuroperitoneal cavity to find the large, thin­walled urinary bladder. It empties into the ventral surface of the tube-like cloaca (Figures 6.11 and 6.13).

Examine an oviduct in a female specimen (Figure 6.12). Follow it anteriorly. As it passes anteriorly, it becomes narrower but remains highly coiled, and then, as it passes dorsal to the lung, straightens to reach its opening, the ostium, which lies just lateral to the peri­cardium and faces ventromedially. Ova enter the oviduct through the ostium and then pass posteriorly through the oviduct. At its other end, the oviduct widens and straightens to form the ovisac, which may contain masses of eggs. The ovisacs empty into the dorsal surface of the cloaca, just proximal to the level of the entrance of the urinary bladder.

The prominent kidneys lie on the dorsal wall of the pleu­roperitoneal cavity (Figures 6.12 and 6.13). They are dark, flattened, ovoid structures.

The ovaries in the female have already been identified. Identify the testes in a male. They lie on the ventral surface of the kidneys, and each is supported by its mesentery, the mesorchium (Figure 6.13). The testes are small, smooth, and ovoid structures, their light color in sharp contrast with that of the kidneys. Sperm pass from the testes through inconspicuous ductuli effentes in the mesorchium, and then enter the kidney to reach the archinephric duct.

Key Terms: Pleuroperitoneal Cavity, Viscera, and Urogenital System