Diagrams of the digestive, respiratory and circulatory systems usually show each one alone, neatly coloured, as if it floated by itself. Inside a real body, the systems are packed together, wrapped in membranes, and connected by blood vessels and muscles. A dissection shows how they actually fit. The fetal pig is the classic specimen because pigs are mammals with organ systems arranged almost exactly like ours, and the specimens are a by-product of the pork industry: they come from pregnant sows already slaughtered for food, not from animals raised for dissection. In this lesson you will work through a virtual dissection step by step, as if the specimen were on the tray in front of you. A frog can be used the same way, and the differences are noted where they matter.
Orientation and technique
| Term | Meaning in a four-legged animal | Example |
|---|---|---|
| Anterior (cranial) | Toward the head | The heart is anterior to the stomach. |
| Posterior (caudal) | Toward the tail | The bladder is posterior to the liver. |
| Dorsal | Toward the back | The kidneys lie dorsal to the intestines. |
| Ventral | Toward the belly | The umbilical cord is on the ventral surface. |
| Medial / lateral | Toward / away from the midline | The heart is medial to the lungs. |
| Proximal / distal | Closer to / farther from where a limb attaches | The hoof is distal to the knee. |
| Left / right | The specimen's own left and right, not yours | The stomach lies on the pig's left side. |
- Prepare: wear gloves and goggles; rinse off excess preservative; place the pig ventral side up in the tray and tie the legs apart.
- Cut the skin, not the organs: lift the skin with forceps and cut with scissors, tips pointed up, from the throat to just anterior to the umbilical cord, then around the cord on both sides and toward the hind legs.
- Make side cuts: add lateral cuts just posterior to the front legs and anterior to the hind legs, then fold back the flaps of body wall to open the cavity.
- Separate tissues gently: use a blunt probe to tease membranes apart instead of cutting; once a structure is cut it cannot be put back.
- Work from the surface inward: identify and examine each organ in place before moving it, and remove organs only when instructed.
The abdominal cavity: digestion
The umbilical cord connected the fetus to the placenta. In cross-section it shows two umbilical arteries and one umbilical vein; the vein carried oxygen-rich, nutrient-rich blood from the mother's side of the placenta, because the fetal lungs and gut were not yet working. Opening the abdomen, the large, dark brown, lobed organ just posterior to the diaphragm is the liver, the largest internal organ. Lift its lobes to find the small greenish gallbladder, which stores bile. The stomach is a J-shaped sac on the pig's left side; the long, thin, dark spleen curves along its outer edge (it belongs to the immune and circulatory systems, not digestion). Follow the stomach to the small intestine, a long coiled tube held in place by a thin, blood-vessel-rich membrane, the mesentery. Nestled between the stomach and the first loop of the small intestine is the pale, granular pancreas. The small intestine joins the large intestine, which in pigs is wound into a distinctive spiral, then continues as the rectum to the anus.
The thoracic cavity: breathing and circulation
A dome-shaped sheet of muscle, the diaphragm, separates the abdominal cavity from the thoracic cavity (chest). Cut carefully through the ribs on one side of the sternum to open the chest. In the middle sits the heart, wrapped in a thin sac, the pericardium. On either side are the spongy, lobed lungs. In the neck, the trachea (windpipe) is easy to recognize by its stiff C-shaped rings of cartilage, which hold it open; follow it down to where it branches into the two bronchi. The soft, collapsible esophagus lies dorsal to the trachea, so you must lift the trachea to see it. The thymus, a large gland that helps develop the immune system, is spread over the upper heart and neck of a fetus and shrinks after birth. Remove the heart and cut it open: the left ventricle wall is visibly thicker than the right, because it pumps blood around the whole body.
How the systems connect
| Connection | Path | Why it matters |
|---|---|---|
| Hepatic portal system | Capillaries of the stomach and intestines → hepatic portal vein → capillaries of the liver → hepatic vein → inferior vena cava | Absorbed nutrients pass through the liver first, which stores glucose as glycogen and removes toxins before blood reaches the rest of the body. |
| Diaphragm | Contracts and flattens → chest volume rises → air flows into the lungs | Links the skeletal muscle system to breathing; the esophagus and major vessels pass through it. |
| Pulmonary circuit | Right ventricle → pulmonary arteries → lung capillaries → pulmonary veins → left atrium | Delivers oxygen-poor blood to the alveoli and returns oxygen-rich blood to the heart. |
| Systemic circuit | Left ventricle → aorta → body capillaries (including the gut and liver) → venae cavae → right atrium | Carries oxygen and the nutrients absorbed by the digestive system to every cell. |
Ethics and alternatives
- Shows real textures, sizes, colours and variation that models cannot.
- Builds hands-on skill with instruments, useful for careers in health and veterinary science.
- Fetal pigs come from animals already slaughtered for food, so none are killed for the lab.
- Respects objections to using animals, and avoids exposure to preservatives.
- Virtual dissections and 3-D models can be repeated, reversed and zoomed, and are cheaper.
- Studies comparing students who used good alternatives with those who dissected often find similar test results.
Scientists who use animals follow the Three Rs: Replace animals with alternatives where possible, Reduce the number used, and Refine methods to minimize harm. In Canada, the Canadian Council on Animal Care sets standards for animals used in research and teaching. Whichever method you use, treat the specimen with respect, follow safety rules, and dispose of materials properly.