Dissection of the sheep heart is valuable because it is similar in size and structure to the human heart.Also, a dissection experience allows you to view structure in a way not possible with models and diagrams.Refer to textbooks, and other references as needed during this portion of lab.You will be working in groups of 3 or 4 for this dissection.
Obtain a preserved sheep heart, a dissection tray, dissecting instruments, and ALWAYS WEAR GLOVES while handling dissection materials.
Rinse the heart in cold water to remove as much of the preservative as possible, and to flush out any trapped blood clots.
Observe the texture of the pericardium.Although the parietal pericardium is missing, look for remnants of this membrane around the origins of the large blood vessels.
Locate the visceral pericardium that appears as a thin, transparent layer on the surface of the heart.Use a scalpel to remove a portion of this layer and expose the myocardium beneath.Also note the abundance of fat along the path of various blood vessels. This adipose tissue occurs in the loose connective tissue that underlies the visceral pericardium.You can carefully scrape some of this adipose tissue away to examine the coronary vessels.
Identify the base and apex of the heart.Visually locate the following: right atrium, auricle of right atrium, right ventricle, left atrium, left ventricle, and anterior interventricular sulcus.
Identify the pulmonary trunk and aorta extending from the superior aspect of the heart.Carefully clear away some of the fat between the pulmonary trunk and the aorta to expose the ligamentum arteriosum, a cordlike remnant of the ductus arteriosus [in the fetus, the ductus arteriosus allows blood to pass directly from the pulmonary trunk to the aorta, bypassing the nonfunctional fetal lungs].
Carefully remove some fat from the anterior interventricular sulcus and expose the blood vessels that pass along this groove. They include a branch of the left coronary artery and a cardiac vein.
Examine the base of the heart and locate the stumps of two relatively thin-walled veins that enter the right atrium. Demonstrate this connection by passing a slender probe or pipe cleaner through them.The upper vessel is the superior vena cava and the lower one is the inferior vena cava.Locate the aorta & compare the approximate diameter of the superior vena cava with the diameter of the aorta.Which is larger?Which has thicker walls?
Cut through the wall of the aorta until you see the aortic semilunar valve.Identify the two openings to the coronary arteries just above the valve.Insert a probe /pipe cleaner into one of these holes to see if you can follow the course of a coronary artery across the heart.
Try to identify the four thin-walled pulmonary veins entering the left atrium.It may or may not be possible to locate them, depending on how they were cut as the heart was removed.What do they convey to the left ventricle?
Insert a blade of the scissors into the superior vena cava and cut thru its wall so that you can view the interior of the right atrium.Do not extend your cut entirely through the right atrium or into the ventricle.Observe the right atrioventricular [AV] valve.How many flaps does it have?Pour some water into the right atrium and allow it to trickle into the right ventricle.Slowly and gently squeeze the right ventricle to watch the closing action of this valve. [If you squeeze too vigorously, you’ll get a face full of water!]Drain the water from the heart before continuing.
Find the pulmonary trunk and cut thru its anterior wall until you can see the pulmonary semilunar valve.Pour some water into the base of the pulmonary trunk to observe the closing action of this valve.How does its action differ from that of the atrioventricular valve?After observing semilunar valve action, drain the heart once again.Return to the superior vena cava, and continue the cut made into the wall through the right atrium and right AV valve into the right ventricle to the apex of the heart.Cut along the bottom of the ventricle, towards the lateral edge of the heart, making a similar cut at the base.Fold the wall back to observe the interior.
Observe the comb like ridges of muscle throughout most of the right atrium.This is called the pecitinate muscles [pectin = comb].Identify the papillary muscles in the right ventricle, and follow their attached chordae tendineae to the flaps of the AV valve.
Make a longitudinal cut throughout the aorta and continue it into the left ventricle.Notice how much thicker the myocardium of the left ventricle is than that of the right ventricle.Compare the shape of the left ventricular cavity to the shape of the right ventricular cavity.Are the papillary muscles and chordae tendineae observed in the right ventricle also present in the left ventricle?Count the number of cusps on the left AV valve.How does that compare with the number seen on the right AV valve?
In the left atrium, identify the 4 openings of the pulmonary veins. Pass a slender probe or pipe cleaner through each opening and locate the stump of its vessel.
As a review, locate and identify the stumps of each of the major blood vessels associated with the heart.
Discard of the specimen as instructed. Clean the dissecting tray and instruments before leaving the laboratory.