Reading: Fetal Pig Dissection

The fetal pig that you will dissect has been injected with a colored latex (rubber) compound. The arteries have been filled with red latex and the veins with blue. An incision was made on the side of the neck to enable the injections. The incision can be seen in the first photograph below.

Several different pig dissections were used to obtain the photographs below. As a result, a structure shown in one photograph may look different than the same structure shown in another photograph.

Click on any of the photographs to view enlargements. Links to high-resolution, unlabeled photographs are also provided for many of the photographs.

Orientation

The following words will be used to help identify the location of structures.

Anterior refers to the head end. If a structure is anterior to another, then it is closer to the head.
Posterior refers to the tail end.
Dorsal refers to the back side. The pig in figure 1 is lying on its dorsal side.
Ventral is the belly side. It is opposite the dorsal side. The pig in figure 1 below has its ventral side up.

External Structures

Obtain a fetal pig and identify the structures listed in figure 1. Use figures 1–4 below to identify its sex.

Use your pig and also a pig of the opposite sex to identify the structures in the photographs below. The word “urogenital” refers to an opening that serves both the urinary (excretory) and the reproductive systems.

Figure 1. Female: injection site, nipples, umbilical cord

Figure 2. Female: genital papilla, urogenital opening, anus

Figure 3. Male: scrotum

Figure 4. Male: urogenital opening, penis, anus

Preparation and Initial Cuts

Tie one front leg of the animal with a string that passes underneath the dissecting pan to the other leg. Repeat this with the back leg.

The first step is to tie the pig to the dissecting pan so that it remains in place for easy viewing. A string tied to one front leg of the animal passes underneath the dissecting pan to the other leg. A string passing under the pan also holds the two back legs in place.

Figure 5.

Insert one blade of scissors through the body wall on one side of the umbilical cord and cut posteriorly to the base of the leg as shown in figure 6. Continue cutting from the anterior end of this cut so that it resembles an upside-down U. Your finished cut will be anterior to the navel and along each side of the navel. The flap of body wall that contains the navel can be folded posteriorly to reveal the internal organs of the abdomen.

One blade of the scissors is inserted through the body wall on one side of the umbilical cord and a cut is made posteriorly to the base of the leg.

Figure 6.

The cut is extended from the anterior end around the navel and then posterior again so that it resembles an upside-down U. the finished cut is anterior to the navel and extends in a posterior direction on each side of the navel.

Figure 7.

The flap of body wall that contains the navel can be folded posteriorly to reveal the internal organs of the abdomen

Figure 8.

Extend a single cut along the midline of the ventral surface of the animal to about 2 cm. from the chin. Cut completely through the body wall in the abdominal area but keep the cut shallow in the neck region.

A single cut is extended along the midline of the ventral surface of the animal to about 2 centimeters from the chin. In the abdominal area, this cut is completely through the body wall but in the neck area, care must be taken to keep it shallow so that the underlying glands are not destroyed.

Figure 9.

A cut is made on the side of the animal from the point just posterior to the diaphragm dorsally. A similar cut is made on the other side. These two cuts will enable you to spread open the abdominal cavity.

Figure 10.

Mouth and Neck Region

Use a scalpel to cut the sides of the mouth so that the bottom jaw can be opened for easier viewing (see figure 11). You will need to cut through the musculature and the joint that holds the lower jaw to the skull.

A scalpel is used to cut the sides of the mouth so that the bottom jaw can be opened for easier viewing. It is necessary to cut through the musculature and the joint that holds the lower jaw to the skull.

Figure 11.

Open the jaw wide enough so that the glottis and epiglottis are exposed. The epiglottis projects up through the soft palate into a region called the nasopharynx. The hard palate and soft palate separate the nasal and oral cavities. When breathing, air passes through the nasal passages to the pharynx. The pharynx is the space in the posterior portion of the mouth that both food and air pass through. From the pharynx, it passes through the glottis to the trachea.

The jaw is opened wide enough so that the glottis and epiglottis are exposed. The epiglottis projects up through the soft palate into a region called nasopharynx. The hard palate and soft palate separate the nasal and oral cavities. When breathing, air passes through the nasal passages to the pharynx. The pharynx is the space in the posterior portion of the mouth that both food and air pass through. From the pharynx, it passes through the glottis to the trachea.

Figure 12. Hard palate, soft palate, glottis, epiglottis, and tongue.

Carefully, peel the skin away from the incision in the neck region using a blunt probe (a needle or the point of scissors will do if a blunt probe is not available). Use the probe to peel away muscle tissue until the thymus gland on each side of the trachea is exposed.

Use a probe to separate the two lobes of the thymus gland and to further separate the musculature over the trachea. The thyroid gland is darker and lies between the posterior ends of the two lobes of the thymus gland.

The skin is carefully peeled away from the incision in the neck region using either a blunt probe, a needle, or the point of scissors. The muscle tissue around the thymus gland is also peeled away until the thymus gland on each side of the trachea is exposed.

Figure 13. Thymus

A blunt probe is used to separate the two lobes of the thymus gland and to further separate the musculature over the trachea. The thyroid gland is darker and lies between the posterior ends of the two lobes of the thymus gland.

Figure 14. The surrounding tissues have been separated to reveal the thyroid gland.

Continue separating the tissue with a probe until the trachea and esophagus are exposed. The esophagus is dorsal to the trachea. The large hard structure attached to the trachea is the larynx. It contains the vocal chords.

In the photograph below, the heart and blood vessels of the neck region have been removed so that the trachea can be seen more clearly. You should not remove these structures yet because you will need to identify the blood vessels later in the dissection.

A probe is used to continue separating the tissue in the neck region until the trachea and esophagus are exposed. The esophagus is dorsal to the trachea. the large hard structure attached to the trachea is the larynx. It contains the vocal chords. In this photograph, the heart and blood vessels of the neck region have been removed so that the trachea can be seen more clearly.

Figure 15. Esophagus, larynx, trachea, bronchus, and lungs.

Respiratory System

Observe how the diaphragm attaches to the body wall and separates the abdominal cavity from the lung (pleural) and heart (pericardial) cavities (figure 16 and 18 below). Contraction of the diaphragm forces air into the lungs.

You have already seen the nasopharynx, hard palate, soft palate, epiglottis, glottis, trachea, and larynx. Follow the trachea to where it branches into two bronchi and observe that each bronchus leads to a lung. The left lung contains three lobes and the right lung contains four. Each lung is located in a body cavity called a pleural cavity.

The diaphragm attaches to the body wall and separates the abdominal cavity from the lung (pleural) and heart (pericardial) cavities. Contraction of the diaphragm forces air into the lungs.

Figure 16. Diaphragm.

In this photograph, the diaphragm has been cut so that the body wall can be spread open to reveal the lungs.

Figure 17. Lungs

18. Lungs, diaphragm.

Figure 19. Lungs, diaphragm (cut)

The heart and major blood vessels have been removed in this photograph to reveal details of the respiratory system. You have already seen the nares, hard palate, soft palate, epiglottis, glottis, trachea, and larynx when you viewed the mouth and neck region. The trachea branches into two bronchi and each bronchus leads to a lung. The left lung contains three lobes and the right lung contains four.

Figure 20. Esophagus, larynx, trachea, bronchus, and lung.

Digestive System

You have already seen how the esophagus leads from the pharynx through the neck region. Using a probe, trace follow the esophagus to the stomach. Identify the small intestine and large intestine. Find the posterior part of the large intestine called the rectum and observe that it leads to the anus. Locate the cecum, a blind pouch where the small intestine joins the large intestine.

Identify the liver. Lift the right lobe and find the gallbladder. This structure stores bile produced by the liver. Find the bile duct that leads to the small intestine. The pancreas is located dorsal and posterior to the stomach. It extends along the length of the stomach from the left side of the body (your right) to the point where the stomach joins the small intestine. Lift the stomach and identify this light-colored organ.

The spleen is an elongate, flattened, brownish organ that extends along the posterior part of the stomach ventral to (above) the pancreas.

The cecum is a blind pouch where the small intestine joins the large intestine. It houses bacteria used to digest plant materials such as cellulose. The cecum is large in herbivores but much of it has been lost during evolution in humans. The appendix in humans is the evolutionary remains of a larger cecum in human ancestors.

Food passes through the esophagus to the stomach, small intestine, and large intestine. The first part of the small intestine is the duodenum. Secretions released from the pancreas and gall bladder empty into the duodenum.

Figure 21. Duodenum, gallbladder, liver, lungs, large intestine, pancreas, small intestine, stomach. The liver has been lifted to reveal the gallbladder.

Figure 22. bile duct, gallbladder, large intestine, liver, and small intestine. The liver has been lifted to reveal the gallbladder.

Figure 23. Large intestine, liver, small intestine, spleen, and stomach.

The stomach and spleen have been moved to the right to show the pancreas underneath.

Figure 24. small intestine, large intestine, pancreas, spleen, stomach – The spleen has been moved aside to reveal the pancreas.

Figure 25. The stomach and liver are lifted to show the pancreas.

Figure 26. Large intestine, pancreas, small intestine, spleen, and stomach.

The cecum is a blind pouch where the small intestine joins the large intestine. It houses bacteria used to digest the plant materials such as cellulose. The cecum is large in herbivores, but much of it has been lost during evolution in humans. The appendix in humans is the evolutionary remains of a larger cecum in human ancestors.

Figure 27. cecum, large intestine, liver, small intestine, spleen. The cecum is found at the point where the small intestine joins the large intestine.

Figure 28. Large intestine, liver, small intestine, spleen, and stomach.

Circulatory System

Figures 29 and 30 summarize the circulatory system of a mammal.

Blood flow between the lungs and body as its oxygenation levels change. When oxygenated blood becomes unoxygenated, it flows from the rest of the body to the lungs. Blood flows at a low pressure from other areas of the body through the vena cava, to the right atrium, and into the right ventricle, entering through the right atrioventricular valve (also known as the tricuspid valve). The blood leaves the right ventricle through the right semilunar valve at a high pressure, and travels through the pulmonary artery into the lungs. It then becomes oxygenated. The oxygenated blood flows at a low pressure through the pulmonary vein to the left atrium, to the left ventricle through the left atrioventricular valve (also known as the mitral or bicuspid valve). The blood then leaves the left ventricle at a high pressure through the left semilunar valve and travels through the aorta to the rest of the body.

Figure 29. Circulatory system

Another diagram, very similar to the one just before. However, the two ventricles are next to each other, which more accurately reflects the structure of the heart.

Figure 30. Circulatory system

The drawing below shows some of the major arteries that carry blood to the body. Blood vessels that branch from the aorta carry blood to most of the body.

The pulmonary artery is capable of delivering a large amount of blood to the lungs but the lungs are not needed to oxygenate the blood of a fetus, so most of the blood is diverted to the aorta. This diagram shows that the ductus arteriosus connects the pulmonary artery to the aorta and diverts blood that would otherwise go to the lungs.

Shortly after birth, the ductus arteriosus closes and blood in the pulmonary artery goes to the lungs instead of the body.

Blood passes from the left ventricle through the aortic arch and aorta to the body. The first branch of the aorta is the brachiocephalic artery. The second branch is the left subclavian artery which goes to the left front leg. The right subvclavian carries blood to the right front leg and the carotids carry blood to the head.

The pulmonary trunk is attached to the heart. It splits off into the pulmonary artery and the ductus arteriosus. The ductus attaches to the aorta, just below the aortic arch. The aorta is also attached to the heart. The aorta arches (the aortic arch) and runs down the body. There are branch points on the aorta for the right and left renal (to kidney) and for right and left external iliac (to legs). The right subclavian, brachiocephalic, and left subclavian (to the left forelimb) arteries all branch from the aorta before the aortic arch. The Brachiocephalic diverges into two arteries: the right and left common carotids (to the head).

Figure 31. Major arteries

The pericardium is a membrane that surrounds the heart and lines the pericardial cavity. It contains a lubricating fluid and isolates the heart from body movements such as the expansion and contraction of the nearby pleural (lung) cavity.

To view details of the aortic arch, ductus arteriosus, and pulmonary artery, it will be helpful to remove the left lung. With the left lung removed, the heart can be pushed to the right side to reveal the aorta and other blood vessels shown in figures 33–42.

The diaphragm has been cut from the body walls. The pericardium is a membrane that surrounds the heart and lines the pericardial cavity. It contains a lubricating fluid and isolates the heart from body movements, such as the expansion and contraction of the nearby pleural (lung) cavity. The pericardium in the photograph has been cut, revealing the heart within.

Figure 32. Diaphragm, heart, lungs, and pericardium

Figure 33. Aortic arch, coronary artery, left atrium, left ventricle, pulmonary artery, right atrium, right ventricle.

Blood passes from the left ventricle through the aortic arch and aorta to the body. The first branch of the aorta is the brachiocephalic artery. The second branch is the left subclavian artery, which goes to the left front leg.

Figure 34. Aorta, aortic arch, left atrium, brachiocephalic artery, ductus arteriosus, lung, pulmonary artery, pulmonary trunk, left subclavian artery,

The first branch of the aorta is the brachiocephalic artery. The second branch is the left subclavian artery, which goes to the left front leg. The right subclavian carries blood to the right front leg and the carotids carry blood to the head.

Figure 35. Aorta, aortic arch, left atrium, brachiocephalic artery, left common carotid artery, right common carotid artery, ductus arterious, pulmonary artery, pulmonary trunk, left subclavian artery, right subclavian artery, trachea, left ventricle

Figure 36. Aortic arch, left atrium, brachiocephalic artery, left common carotid artery, right common carotid artery, larynx, pulmonary trunk, left subclavian artery, right subclavian artery, left ventricle.

The anterior vena cava receives blood from the anterior part of the body and carries it to the right atrium.

Figure 37. Anterior vena cava, coronary artery, larynx, posterior vena cava, right subclavian vein, trachea.

Figure 38. Anterior vena cava, coronary artery, right external jugular vein, right internal jugular vein, larynx, lungs, right subclavian vein, trachea

Figure 39. Anterior vena cava, posterior vena cava.

The posterior vena cava receives blood from the posterior portion of the body and carries it to the right atrium.

Figure 40. Heart, liver, lung, posterior vena cava, thymus, thyroid

Figure 41. Aorta, colon, kidney, posterior vena cava, renal artery, renal vein, testicular artery, testis, umbilical artery, ureter, urinary bladder, vas deferens. The renal artery passes blood from the aorta to the kidney. The renal vein returns blood from the kidney to the posterior vena cava.

42. aorta, colon (large intestine), diaphragm, heart, kidney, lung, renal artery, posterior vena cava, renal vein, small intestine, spleen, stomach, ureter

Figure 42. Aorta, colon (large intestine), diaphragm, heart, kidney, lung, renal artery, posterior vena cava, renal vein, small intestine, spleen, stomach, ureter.

Figure 43. External iliac artery, kidney, large intestine, posterior vena cava, renal vein, small intestine, testis, umbilical artery, ureter, urinary bladder.

Figure 44. Left atrium, brachiocephalic artery, left common carotid artery, right common carotid artery, coronary artery, external iliac artery, external iliac vein, posterior vena cava, pulmonary trunk, renal artery, renal vein, left subclavian artery, right subclavian artery, umbilical artery, left ventricle.

Excretory System

Figure 45. Aorta, colon (large intestine), diaphragm, heart, kidney, lung, renal artery, posterior vena cava, renal vein, small intestine, spleen, stomach, ureter.