Dissection of the Sheep Heart EFE Animal Science EFE Veterinary Science

I <3 Heart Dissection! Before beginning, assemble a dissection tray, scissors, blunt probe, gloves for each person, and a couple of paper towels.

You will be given a preserved sheep’s heart It will likely be a bit deformed from vacuum packing. Gently “smoosh” it into shape and squeeze out any extra preservative fluid.

Next, remove excess fat from the outside surface The fat is whitish, firm and is most easily removed by picking at it with your fingers.

It is not necessary to remove all of the fat So don’t get too obsessed over it! You may wish to remove it by blunt dissection: slip the closed scissors tip under the fat, then spread the blades.

Next, orient the heart. The base is the broad part (top), and the apex is the narrow (bottom) part. This is the ventral side (note that you cannot see the auricles!)

Locate the auricles (earlike projections) Orient the heart with the auricles downward on the tray (ventral side up toward you).

You will begin your first cut in the vena cava This is the large vessel that empties into the right atrium and auricle. Probe with a finger to find it.

Cut from the vena cava toward the apex Learn good habits: Surgical scissors are always held by the thumb and 4th (ring) finger.

Spread the heart open and locate the following structures (label with dissection pins): Right auricle, right atrium, fossa ovalis (remnant of the foramen ovale), interatrial septum,

Locate and label, continued: Right ventricle, moderator bands, trabeculae carneae, pulmonary artery, right atrioventricuar (AV) valve

Locate and label, continued Right AV valve = tricuspid (in humans), chordae tendonae, papillary muscles

Locate and Label right auricle right atrium fossa ovalis (remnant of the foramen ovale) interatrial septum, right ventricle moderator bands trabeculae carneae pulmonary artery right atrioventricuar (AV) valve = tricuspid (in humans) chordae tendonae papillary muscles

Now continue your cut around in a “U” shape Picking up where you left off at the apex, and continue up the base and up the coronary artery

Remove the flap; the right ventricle is now fully open and structures are visible. The probe is indicating the interventricular septum

Chordae tendinae of the right AV valve

Papillary muscle in the right ventricle

Fossa ovalis (remnant of the foramen ovale) This is an oval depression on the interatrial septum. Before birth, the foramen ovale allows blood to bypass the lungs.

The fossa ovalis

Now locate the pulmonic (semilunar) valve This prevents backflow of blood from the pulmonary arteries. Note the lack of chordae tendinae.

Next, turn your heart over. The right side (where you have already cut) faces downward toward your dissection tray.

Begin your cut at the left atrium And continue as far as possible toward the apex of the heart. What do you notice about the thickness of the ventricular wall?

At the apex, turn and cut upward toward the aorta at the heart base. The aorta is the large, thick-walled vessel. (Are you holding your scissors correctly?!)

Now, one last cut. Start where you began the last cut, in the left atrium. Continue around and through the pulmonary artery and up the aorta. Lift the flap.

You have exposed the aortic semilunar valve. This prevents backflow from the aorta and ensures that blood moves forward throughout the body.

Moderator band in the left ventricle This prevents the ventricle from over-distention. These are more numerous and thicker than in the right ventricle. Why?

Locate and Label Pulmonary veins, left atrium, left auricle, interatrial septum, aorta, aortic (semilunar) valve

Locate and label Left atrioventricular (AV) valve = bicuspid valve in humans

Locate and Label pulmonary veins left atrium left auricle interatrial septum aorta aortic (semilunar) valve left atrioventricular (AV) valve = bicuspid valve in humans

Compare the size and wall thicknesses of the two ventricles Compare the size and wall thicknesses of the two ventricles. Why is there a difference? Be certain that, in addition to naming the structures, you are able to trace the path of blood flow through the heart.

Now, don’t you <3 the heart, too?!