Section IV—Body Cavity, Viscera, and Vessels

The sternum and pectoral musculature must be removed to examine the structures within the body cavity, includ­ing viscera and blood vessels. Unfortunately, removal of the sternum and muscles as described below often oblit­erates much of the extensive respiratory system, the air sacs that form auxiliary air pathways in the avian res­piratory system.

Return to the task of exposing the body cavity. Clear the connective tissue from the lateral part of the pec­toral muscles, just posterior to the axillary regions, but do not damage any vessels. Make a longitudinal inci­sion through the abdominal muscles, just to one side of the midventral line, and, using stout scissors, follow anteriorly along the lateral margin of one of the pec­toral muscles, cutting through the ribs as you do so. Lift the sternum as much as possible and clear the connect­ing tissue until you see the vessels that pass to the sternum, so that you will avoid damaging them. Using a sharp scalpel, cut through the anterior part of the pec­torals, as shown in Figure 8.14.

Note the heart (Figure 8.14) lying anteriorly on the midline. Its four main chambers include the left and right atria (sing., atrium) and left and right ventricles (a very small sinus venosus is also present but will not be identified). The lungs, right and left, are tucked laterally to the heart. Probe to find them, and partially remove the serosa covering them to see their spongy texture. Posterior to the heart are the lobes of the liver; note that

FIGURE 8.13 Photographs and interpretive illustrations of trunk of the pigeon to show the arrangement of major airs sacs of the respiratory system. (a) Ventral view. (b) Left lateral view (see page 243).

the right lobe is considerably larger than the left. Tucked deep to these lobes, on the left side, is the gizzard, the very muscular, distal portion of the complex, two-part stomach of birds. The glandular proventriculus, the anterior part of the stomach (though it is actually a modification of the distal part of the esophagus), con­tinues posteriorly from the esophagus and lies deep to the heart, as does the posterior part of the trachea. These structures will be seen shortly (Figure 8.15). The highly coiled small intestine is relatively long, in con­trast to the short and straight large intestine (Figures 8.16 and 8.17). The division between these structures is marked by the presence of a pair of small diverticula, the colic ceca.

Return to the heart (Figure 8.14). The ascending aorta arches anteriorly from the left ventricle and gives off the paired brachiocephalic arteries before curving to the right and posteriorly as the aortic arch.

FIGURE 8.13 Continued

Most of the regions supplied by these arteries are drained by veins that ultimately enter the right or left anterior vena cava, each of which is formed by the confluence of three large vessels, the jugular, subclavian, and pectoral veins. The jugular lies along the lateral surface of the neck, draining the head and neck. The right jugular is usually larger than the left. The subcla­vian vein is a short segment that mainly receives the axillary vein, which in turn receives the basilic vein from the arm. It is the basilic and not the brachial vein that follows the brachial artery. The brachial vein is rep­resented by an anterior branch of the axillary that divides into two narrow vessels along the anterior margin of the brachial muscle.

The pulmonary trunk leaves the right ventricle and almost immediately splits into left and right pulmonary arteries (Figures 8.14 and 8.15) to the lungs. The left

FIGURE 8.14 Trunk of the pigeon in ventral view. Pectoral musculature and sternum removed to reveal heart, viscera, and vessels.

pulmonary artery is easily apparent and can be found passing dorsal to the left brachiocephalic artery. There are two right and two left pulmonary veins that return blood from the lungs. These vessels enter the left atrium separately. They are not easy to find, but will be seen when the heart is removed (Figure 8.15).

Lift the heart and note the great vessels, just described, associated with it. Cut each vessel, and remove the heart. This will expose the lungs and allow you to follow the trachea and esophagus posteriorly (Figure 8.15). The trachea is held open by cartilaginous rings. More posteriorly it bifurcates into left and right bronchi, which extend into the lungs. The syrinx is the sound-producing organ located at the base of the trachea. Examine the cut vessels, and identify the pul­monary arteries and pulmonary veins. The latter exit the lungs just posterior to the entrance of the bronchi. There

FIGURE 8.15 Trunk of the pigeon in ventral view.

are two main veins on each side but it may be difficult to discern them. The pulmonary arteries enter the lungs somewhat anterior to the bronchi.

Remove the left lobe of the liver, being careful not to injure vessels external to it. Once it has been removed, you will be able to follow the esophagus to the stomach. In birds, the stomach is a complex, two-part organ. The more proximal proventriculus secretes strong hydrochloric acid and digestive enzymes that begin the chemical digestion of food. The thicker, muscular gizzard performs most of the mechanical breakdown of food.

Next, examine the intestines. Note that they are bound by mesentery. Various vessels will be seen either sup­plying or draining the viscera. For the time being, iden­tify the large hepatic portal vein (Figure 8.15) that extends between the viscera and the right lobe of the liver. Lift the lobe to note the entrance of the vein into the liver. Cut the portion (and only that portion) of the right lobe that is posterior to the entrance of the hepatic portal vein. You will now be able to follow the intesti­nal tract more easily. The duodenum, the first part of the small intestine, arises from the junction of the proventriculus and gizzard. It makes a long loop, con­sisting of ascending and descending portions that lie close together. Between them is the narrow, elongated pancreas. Follow the remaining part of the small intes­tine. It passes into the short and straight large intestine, which continues posteriorly into the cloaca. A pair of small colic ceca (sing., cecum) mark the division between the small and large intestines.

Return to the duodenum. Carefully dissect the connec­tive tissue between its ascending and descending portions and the pancreas. You will reveal several struc­tures, including the bile ducts and pancreatic ducts (Figure 8.15). There are three of the latter, but only two are readily apparent. Also, you will reveal a large vein, the pancreaticoduodenal vein, accompanied by the smaller pancreaticoduodenal artery, associated with the pancreas.

Carefully uncoil the intestines to observe that the posterior mesenteric vein, a branch of the hepatic portal system, extends posteriorly and eventually joins the venous system of the posterior abdominal cavity (see Figures 8.16-8.18). Once the hepatic portal system has been studied, find the point where the posterior vena cava, which returns blood from most of the body pos­terior to the heart, passes through the left lobe of the liver. This occurs dorsal to the entrance of the hepatic portal vein. Cut through the vena cava and remove the remainder of the liver. Cut through the distal end of the proventriculus, just dorsal to its union with the gizzard. Find the celiac and anterior mesenteric arteries (Figure 8.16) as they emerge from the aorta, just to the right of the distal end of the proventriculus. The ovoid, dark-colored spleen lies in this region as well and can vary in size. Follow the celiac artery, noting its main branches. Cut through them and the anterior mesenteric artery. Also cut through the posterior end of the small intestine, just anterior to the colic ceca, and the posterior mesenteric vein. Then remove the digestive tract.

Clear away connective tissue to expose the urogenital structures and the vessels lying on the dorsal wall of the abdominal cavity. Each kidney is superficially subdi­vided into three lobes by the vessels that pass through it. Males (Figure 8.16) possess a pair of approximately bean-shaped testes. Each lies on the anterior lobe of a kidney. In nonbreeding males, the testes are much smaller than in breeding males and may be difficult to identify. The vas deferens carries semen from the testis to the cloaca. It leaves, slightly expanded, the dorsomedial side of the testis and extends posteriorly to the cloaca as a relatively straight, narrow tube. It is slightly wider and may be convoluted in breeding males. It passes for most of its length along the medial side of the ureter, the wider tube that carries urine from the kidneys to the cloaca. The ureter emerges from between the anterior and middle lobes of the kidneys. Near the cloaca, the vas deferens crosses the surface of the ureter and extends to the cloaca, lateral to the ureter.

In females (Figure 8.17) only the left-side reproductive organs are present, those of the right side having degen­erated soon after their initial formation. The ovary lies on the anterior end of the left kidney. Its morphology and size varies in accordance with the breeding season. The ovary contains numerous spherical follicles in various stages of development. Mature ova pass from the ovary and enter the oviduct through its anterior opening, the ostium. The oviduct is a relatively straight tubular structure (Figure 8.17) that becomes large and convoluted. It conducts the ovum to the cloaca and is subdivided into glandular portions that perform specific roles (such as secreting the shell membrane or the shell itself), but these portions are not identifiable grossly. The left and right ureters of the female are in the same position as in the male. A vestigial posterior portion of the right oviduct may be present along the posterome­dial end of the right ureter.

Return to the posterior vena cava and follow it posteri­orly (Figures 8.16-8.18). It is formed at the level of the anterior part of the kidneys by the confluence of the right and left common iliac veins. An anterior renal vein, draining the anterior lobe of the kidney, enters each common iliac. The common iliac turns laterally between the anterior and medial lobes of the kidney. As it does

FIGURE 8.16 Posterior end of the pleuroperitoneal cavity of the male pigeon in ventral view. Gizzard and small intestine removed to show urogenital structures, viscera, and vessels.

so, it receives the posterior renal vein, a large vessel that extends posteriorly on the kidney with the ureter and vas deferens in the male and the oviduct (left side only) in the female. The posterior renal vein arises from several branches in the posterior lobe of the kidney and receives a branch from the middle lobe of the kidney.

The posterior renal portal vein enters the posterior lobe of the kidney. To follow its path, as well as that of many other veins in this region, kidney tissue must be removed (Figure 8.18). Begin by exposing the posterior portions of the posterior renal vein. Then follow the posterior renal portal vein as it passes anteriorly through the kidney, deep to the posterior renal vein. Between the posterior and middle lobes of the kidney, the renal portal receives the ischiadic vein, which is the main vein of the hind limb. The paired ischiadic arteries lie ventral to the ischiadic veins and are easily seen. Trace the

FIGURE 8.17 Posterior end of the pleuroperitoneal cavity of the female pigeon in ventral view. Gizzard and small intestine removed to show urogenital structures, viscera, and vessels.

FIGURE 8.18 Posterior end of the pleuroperitoneal cavity of the (male) pigeon in ventral view. The right-side kidney has been dissected to reveal the pattern of renal and renal portal veins.

origin of one from the descending aorta. The artery sup­plies the middle and posterior lobes of the kidney. Follow the ischiadic vein laterally, but do not damage the superficial and narrow pubic vein and artery lying on the musculature along the pubic bone. The ischiadic vein, accompanied by the ischiadic artery, passes through the ilioischiadic foramen (Figure 8.6) as it enters the abdominal cavity from the hind limb.

Continue to follow the posterior renal portal vein. Between the anterior and middle lobes of the kidney, the renal portal vein unites with the external iliac vein to form the common iliac vein. The anterior renal portal vein enters almost directly opposite that of the posterior renal portal vein. Follow the external iliac vein laterally to its tributary, the femoral vein, which drains most of the anterior part of the thigh. The main vein of the hind limb is the ischiadic vein, but most of the blood is diverted to the femoral vein by the ischiofemoral anas­tomosis, a large vessel between the ischiadic and femoral veins. It appears as a large branch, larger indeed than the femoral, passing posteriorly deep to musculature. You may follow it by cutting through the musculature. Lastly, note the anterior of the pubic vein, draining the lateral abdominal wall, near the origin of the external iliac. The external iliac artery lies deep to the external iliac vein. Carefully probe to find it. It gives rise to the femoral artery, supplying the hind limb, and the pubic artery, supplying the lateral abdominal wall. Trace the external iliac artery to its origin from the descending aorta. Further anteriorly, note the anterior renal artery, which supplies the anterior and middle lobes of the kidney.

Examine the posterior end of the abdominal cavity. The posterior renal portal vein is formed by the confluence of the narrower interior iliac vein and the wider, transversely oriented interiliac anastomosis. The pos­terior mesenteric vein, which was cut during removal of the intestines, arises from the middle of the interiliac anastomosis and passes anteriorly as part of the hepatic portal system. A small branch accompanies the poste­rior mesenteric artery to the posterior end of the large intestine. The narrow median caudal vein enters the interiliac anastomosis opposite the origin of the posterior mesenteric vein. The internal iliac vein drains the posterolateral region of the abdominal cavity. The internal iliac artery accompanies the internal iliac vein. Beyond the origin of the internal iliac arteries, the descending aorta continues posteriorly as the median caudal artery.

Key Terms: Body Cavity, Viscera, and Vessels