Dissecting a sheep heart is a valuable exercise for understanding the anatomy of the mammalian circulatory system. The sheep heart shares a four-chambered structure with the human heart, making it an excellent comparative model for study. This similarity allows for a hands-on exploration of the chambers, valves, and major vessels that govern blood flow. Following the dissection procedure reveals the intricate, three-dimensional organization of cardiac tissue and how the heart functions as a powerful, muscular pump.
Essential Preparation and Safety Protocols
Before beginning the dissection, establish a safe and prepared workspace. Gather all necessary implements, including a dissecting tray, a scalpel, dissecting scissors, and a metal probe. The sheep heart specimen is typically preserved, often with a glycol solution, and should be handled with care.
Personal protective equipment is mandatory to prevent exposure to the preservative and guard against accidental injury from sharp instruments. Safety gear includes disposable gloves and protective eyewear, such as safety goggles. Ensure the heart is thawed if necessary and placed securely in the dissection tray for examination.
External Examination and Orientation
Properly orienting the heart is the first step before making any incisions. Identify the apex, the pointed, inferior end of the heart, and the base, the superior end where the great vessels attach. The coronary groove, or interventricular sulcus, is a diagonal line of fatty tissue and blood vessels that runs across the surface, marking the division between the left and right ventricles.
To distinguish the anterior from the posterior surface, look for this sulcus, as it typically runs diagonally across the anterior side. Confirm the left and right sides by gently squeezing the lower chambers; the left ventricle will feel significantly firmer and more muscular than the right. This thickness difference is due to the left side’s function of pumping blood to the entire body.
Locate the auricles, the dark, wrinkled flaps of tissue covering the atria at the top of the heart. Identify the major vessels near the base: the pulmonary artery is usually the most anterior vessel, and the aorta is generally found just behind it. The superior and inferior vena cava openings, which lead into the right atrium, can often be probed on the posterior side.
Making the Incisions and Exploring Internal Structures
Right Side Incision and Examination
The first incision should focus on opening the right side of the heart to examine the pulmonary circuit structures. Carefully insert the probe into the pulmonary artery and guide it down into the right ventricle. Use the dissecting scissors to cut along the path of the probe, extending the incision down the anterior wall of the right ventricle toward the apex.
After opening the right ventricle, pull the tissue apart to expose the interior chamber. Look for the tricuspid valve, which regulates blood flow between the right atrium and the right ventricle. Notice the delicate, white, string-like structures called the chordae tendineae, or “heartstrings,” which anchor the valve flaps to muscular projections on the ventricular wall. These muscular projections are the papillary muscles, which prevent the valve from inverting when the ventricle contracts.
Left Side Incision and Examination
Next, turn your attention to the left side of the heart to explore the systemic circuit components. Insert the probe into the aorta, the largest vessel, and guide it into the left ventricle. Make a deep incision down the wall of the left ventricle, parallel to the interventricular septum, extending from the base towards the apex.
This cut will immediately highlight the difference in wall thickness between the two ventricles. Once the left ventricle is opened, locate the bicuspid, or mitral, valve, which separates the left atrium from the ventricle. Like the tricuspid valve, the mitral valve is tethered by chordae tendineae to papillary muscles within the chamber.
Locating the Semilunar Valves
To find the semilunar valves, insert the probe into the pulmonary artery again. Look for three small, membranous pockets just inside the vessel opening; these form the pulmonary semilunar valve. Repeat this process by probing the aorta to find the aortic semilunar valve, which prevents blood from flowing back into the left ventricle. Finally, observe the interventricular septum, the muscular wall dividing the two ventricular chambers.