Thorax Lab 3A

Lab 3A: Heart and Back (left side)

Before you begin:

Today’s lab 3A includes:

bisecting the heart and studying the internal structures of the heart.
dissection and study of the left side of the back (leaving the right side for Group B).

You may find it useful for one part of your group to work on bisecting the heart while the other starts on the back dissection.

Dissecting the back has two steps: removing the skin and then reflecting the superficial back muscles to reveal the deep (intrinsic, epaxial) muscles of the back.

The key learning goal for the back is to be able to distinguish between the deep (intrinsic, epaxial) back muscles that are innervated by the dorsal=posterior rami of the spinal nerves and the superficial back muscles that are innervated by the ventral=anterior rami of the spinal nerves.

Short List of Structures - 3A

Heart Bisection and Internal Anatomy

Orientation

The heart and lungs have now been removed from the body and separated. Maintaining your understanding of the orientation of the heart as you spin it around is difficult without its relationship to the thorax for reference. The aorta has been cut close to the heart so you won’t see the whole aortic arch. Identify the aorta (thick walled) and the pulmonary trunk (=pulmonary artery, slightly thinner walled). Hold the heart in your hand with the anterior aspect of the heart facing you and your hand is cradling the posterior aspect. Your right is the patient’s (heart’s) left. Then try holding the heart in anatomical position up in front of your own thorax, so your right is the patient’s (heart’s) right. Note that the figure to the right shows the posterior aspect of the heart.

Isolated heart (posterior view)

Inspection of the removed heart

Identify the following features on the removed heart:

Superior and inferior vena cava- aligned along the right border of the heart.
Pulmonary trunk- bifurcates inferior to the aortic arch.
Ascending aorta- the right atrium may overlap the ascending aorta.
Pulmonary veins- four located posterior and inferior to the pulmonary trunk. The two right pulmonary vv. are positioned between the superior and inferior venae cavae.
Coronary sulcus- oblique groove that separates the atria from the ventricles, filled by coronary vessels and adipose tissue.
Anterior interventricular groove- filled by the left anterior descending (anterior interventricular) coronary a., great cardiac v., and adipose tissue.
Posterior interventricular groove- filled by the right posterior interventricular coronary a., middle cardiac v., and adipose tissue.
Apex of the heart- the inferolateral portion of the left ventricle. It lies posterior to the fourth or the fifth intercostal space about one hand width from the midsagittal line.
Base of the heart- opposite the apex, it includes the atria and the great vessels.

NOTE: The aorta was cut close to the heart, so you won’t see the aortic arch as it is shown in some figures..

Isolated heart (anterior view)

Structure and function of the heart

Most of the substance of the heart is ventricle. The ventricles do the pumping work of the heart, so they are large and thick-walled. By comparison the atria are small and thin-walled. The apex of the heart is the muscular, rounded tip of the heart, and is part of the left ventricle. In anatomical position the apex points to the left and infero-laterally. The base of the heart is opposite the apex.

The most anterior of the great arteries of the heart is the pulmonary trunk. Just inferior to the pulmonary trunk is the wall of the right ventricle. This means that the anterior surface of the heart is mostly right ventricle. Also on the anterior surface of the heart to the left, you’ll find the great cardiac vein mostly embedded in fat. Deeper in the fat is the accompanying left anterior descending (LAD, or anterior interventricular) coronary artery. These vessels run superficial to the interventricular septum, which separates the left and right ventricles.

The ascending aorta is slightly posterior and to the right of the pulmonary trunk. The superior vena cava is to the right of the aorta. The position of the left ventricle can be verified by placing a finger into the aorta and following it down posterior to the pulmonary trunk and through the aortic valve. The position of the right atrium can be verified by placing a finger into the superior or inferior vena cava.

To find the left atrium, turn the heart so that you’re viewing its posterior surface and look for the pulmonary veins (typically 4, 2 upper and 2 lower). The auricles (“ears”) are the small, wrinkled flaps of tissue that project from both atria.

Isolated heart (anterior view)

Sectioning the Heart

Now that we’ve explored the external anatomy of the heart, let’s take a look inside.

You will bring your heart to a tray located at the back of the lab and use one of the large, sharp knives provided to section the heart. Your cut should lie perpendicular to the plane of the interventricular septum (this is an internal structure, to visualize where this is externally, locate the anterior and posterior interventricular vessels) so that you cut through all four chambers and will see them in cross section (see image to the right).

Here is a trick that we like to use to orient to the plane of the interventricular septum:

Locate the anterior interventricular artery (LAD) and place your thumb over it
Locate the posterior interventricular artery and place your index finger over it
Now look between your thumb and index finger, this is where you will cut the heart = perpendicular to the plane made by your thumb and index finger running from the apex of the heart through the middle of the atria

It’s much easier to show you how to make this section than to describe it, so talk to one of the teaching staff when you’re ready to make the cut.

The heart will be filled with coagulated blood. Clean out all that you can by hand, and then rinse the two halves of the heart in one of the buckets of water provided. Do not rinse your heart at the sink.

The Sectioned Heart

After your heart is rinsed, make sure you’re able to orient yourself to both halves and are able to identify the chambers, valves, and the roots of the great vessels.

Sectioned heart - anterior diagrammatic view

Sectioned heart - blood flow

PUTTING IT ALL TOGETHER

You should be able to trace the path of a blood cell from any point in the circulation. For example, you should be able to describe the blood flow from the right atrium through the tricuspid valve, right ventricle, etc. all the way through the circulation to its return to the right atrium via the vena cava.

Internal structures of the right atrium

The atria are thin walled and not as distinctive as the ventricles. The right atrium receives venous blood from the superior vena cava (SVC), inferior venae cava (IVC) and the coronary sinus. Examine the two halves of the right atrium in your sectioned heart and identify the following structures:

Pectinate muscles- rough, muscular ridges in the anterior wall.
SA node- not grossly visible, but located in the anterior superior atrial wall where the SVC enters the right atrium.
Coronary sinus- opens into the right atrium between the apertures of the tricuspid valve and the IVC.
Interatrial septum- wall separating the right atrium from the left atrium.
Fossa ovalis- a depression in the interatrial septum superior to the IVC (marking the position of the foramen ovale in the fetus).
Tricuspid valve- observe the superior borders of the three cusps.
Atrioventricular (AV) node- not grossly visible, but located in the atrioventricular septum above the opening of the coronary sinus.

Internal structures of the right atrium

HELPFUL HINTS

The superior and inferior venae cavae are not shown in the dissection image to the left, but you should be able to find these structures. Put a probe or finger through their trunks and see where they enter the right atrium. You will not be able to see the SA or the AV nodes.

Internal structures of the right ventricle

The ventricles are thick walled, powerful muscular chambers. The right ventricle receives deoxygenated blood from the right atrium and pumps it to the lungs. Examine the two halves of the right ventricle in your sectioned heart and identify the following structures:

Interventricular septum- has muscular (inferior) and membranous (superior) regions.
Trabeculae carneae- irregular muscular ridges on the inner surfaces of the walls of the ventricle.
Papillary muscles- muscular pillars attached to the tricuspid valve leaflets via chordae tendineae.
Conus arteriosus- smooth surface of the right ventricle immediately inferior to the pulmonary trunk.
Pulmonary semilunar valve- observe the anterior, left, and right cusps (named for their anatomic position). Cant find this? See the instructions below:

If you followed the directions when sectioning the heart, you may notice the aorta is visible along with the aortic semilunar valve, but the pulmonary semilunar valve is hidden. This is normal. If you would like to find this valve, simply pass a finger or probe through the pulmonary trunk until it emerges into the right ventricle. If you would like to see this valve, make a vertical cut with a scalpel to transect the pulmonary trunk until the valve is visible. Ask a member of the teaching staff if you have trouble with this step.

The left heart

The left atrium is much simpler than the right atrium. Identify the following features in the left atrium:

Pulmonary veins- 4 veins bringing oxygenated blood to the left atrium from the lungs.
Interatrial septum- wall separating the right atrium from the left atrium.
Fossa ovalis- a depression in the interatrial septum superior to the IVC (marking the position of the foramen ovale in the fetus).
Bicuspid (mitral, left AV) valve- observe the superior borders of the two cusps.

The left ventricle has much thicker walls than the right ventricle. Identify the following features in the left ventricle:

Bicuspid (mitral, left AV) valve- two papillary mm. and chordae tendineae are attached to the two cusps of the bicuspid valve
Trabeculae carneae- irregular muscular ridges on the inner surfaces of the walls of the ventricle.
Papillary muscles- muscular pillars attached to the bicuspid valve leaflets via chordae tendineae.
Interventricular septum- has muscular (inferior) and membranous (superior) regions.
Aortic valve- has right, left, and posterior cusps. The fibrous thickenings at the midpoint of the edge of each cusp are called nodules.

Internal structures of the left ventricle

Back: Surface Anatomy and Skin Removal

Dissect the LEFT SIDE of the back only for Lab 3A

Place the cadaver prone (face down). On thin bodies, the following structures will appear as projections. On heavier bodies they will be more difficult to locate. Identify the following:

Acromion of the scapula
Spine of the scapula
Medial margin of the scapula
Superior angle of the scapula
Inferior angle of the scapula
Midaxillary line: this line is used as a reference points for needle insertion during a thoracentesis.
Iliac crest
Sacrum

Midaxillary line

Posterior surface landmarks

Skin - Incisions

Cut along the dotted lines (left side of the back for Lab 3A, right side for Lab 3B). You should basically be removing the rest of the skin from the thorax and shoulder.
The incisions should pass through the skin and superficial fascia, leaving the deep fascia and underlying muscle intact.
The depth of your incisions will vary depending on the amount of fat in the superficial fascia.
Cut the string holding the bag over the head to get as close to point (A) as possible.
Place the removed skin flaps in the bucket.

HELPFUL HINTS

Removing skin from the back of the thorax is usually more difficult than removing it from the front. The superficial fascia is often discolored (dark), so it is harder to distinguish from the deeper structures. Some potentially helpful dissection techniques are offered in the following section.

Posterior skin incisions

Skin - Reflection

To reflect the skin of the back:

Grasp a corner of the skin, using forceps ("I" in the figure to the right).
Keep pulling up while using a scalpel to cut (reflect) the skin and superficial fascia from the underlying deep fascia and muscles.
Once enough skin is reflected you may use a scalpel to cut a “button-hole” through the skin. Place a finger inside the “button-hole” and pull up ("II" in the figure to the right).
Look at the neurovascular bundles that course between the deep facia of the musculature and superficial facia being reflected.
As you reflect the superior flap, look for muscle fibers of the trapezius mm. STOP here, do not cut any deeper than this layer.
As you reflect the inferior flap, look for the thoracodorsal = thoracolumbar fascia, a broad, flat, shiny tendon of the latissimus dorsi mm. STOP here, do not cut any deeper than this layer!

HELPFUL HINTS

One trick is to cut the skin into narrower strips with additional mediolateral cuts. This makes it easier to pull the superficial fascia into tension. Be careful and patient, this portion of the lab will take awhile.

Dissection technique for reflecting skin. Note that the thoracolumbar fascia = thoracodorsal fascia.

Muscles of the Back

Superficial (extrinsic) Back muscles

Remove any remaining superficial facia by grasping the fascia with hemostats and using a scalpel to cut the fascia away from the deeper, underlying muscle. Identify the following:

Trapezius m. - identify its attachments to the base of the skull, nuchal ligament, spines of the C7 - T12 vertebrae, spine of the scapula, acromial process of the scapula, and the lateral part of the clavicle.
- Observe the superior, medial, and inferior divisions (their muscle fibers course in different directions).
Latissimus dorsi m. - identify its attachments to the vertebrae from T7 to the sacrum via the thoracolumbar fascia (the muscle inserts laterally into the humerus, which will be seen later).
Thoracolumbar fascia - a thick layer of deep fascia that is the fused aponeuroses of several mm. It attaches to the spinous processes from the T7 vertebra to the sacrum.
Triangle of auscultation - bound by the medial margin of the scapula and by the trapezius and latissimus dorsi mm.
Lumbar triangle - bounded by the external oblique m., latissimus dorsi m., and the iliac crest.

Note: The superficial back mm. are located on the back but act on the upper limbs and will therefore be studied further in the upper limb lab sessions.

HELPFUL HINTS

The triangle of auscultation is a point on the back where there is little muscle between the skin and the wall of the thorax. It is an especially good point to listen to lung sounds.

Superficial muscles of the back

Trapezius muscle

Reflect the trapezius m. on both sides of the back in the following manner to allow access to deeper structures and to preserve its nerve and blood supply:

Grasp the most inferior attachment of the trapezius mm. (spinous process of T12 vertebra) with a hemostat or forceps.
Free the deep surface of the trapezius mm. from the underlying muscles, using a scalpel and your fingers. Be careful, the trapezius mm. can be extremely thin.
Insert your fingers deep to the free lateral and inferior border of the trapezius mm. You may feel the fat and loose connective tissue separating the trapezius mm. from other, deeper musculature.
Using a scalpel, cut the medial attachment of the trapezius mm. from the vertebral spines beginning at T12 and moving superiorly. Keep your cuts as close to the vertebral spines as possible). You may need to loosen the muscle with your fingers as you proceed.
- Do not cut the rhomboid mm., which are immediately deep to the trapezius mm. and attached to the medial margin of the scapula.
Superior to the T1 vertebra, cut the trapezius mm. from the nuchal ligament and base of the skull.
With the muscles attached only to the spine of the scapulae and the clavicles, reflect the trapezius mm. laterally toward the shoulder.

Study the deep surface of the reflected trapezius mm. to identify the following nerve and vessels travelling together within the connective tissue:

Spinal accessory nerve (cranial nerve XI, CN XI) - clean and follow the nerve to observe the numerous branches into the musculature of the trapezius mm. The nerve exits the skull via the jugular foramen, but do not attempt to follow the nerve back to the skull. This area will be dissected in a further laboratory session.
Superficial branch of the transverse cervical a. - enters the deep superior surfaces of the trapezius mm.

Reflection of the trapezius m.

Deep surface of the trapezius m.

Rhomboid and Levator Scapulae Muscles

Rhomboid major and minor mm. - observe muscle attachments along the spines of C7-T5 vertebrae and the medial margin of the scapula; the two muscles may be fused and appear as one

Place your fingers deep to the rhomboid major and minor mm. Dissect, using your scalpel, the muscles' medial attachments from the spinous processes of C7 T5 and reflect the rhomboid mm. laterally (leave them attached to the scapula)

- Levator scapulae m. - do not dissect the levator scapulae m. at this time; however, observe its attachments on the superior angle of the scapula and transverse processes of C1 -C4 vertebrae

Muscles of the back

Figure 9.20 Rhomboid and levator scapulae mm.

Latissimus Dorsi Muscle

To dissect the latissimus dorsi m., follow these procedures for both sides of the body (Figure 9.21):

Insert your fingers deep to the superior border of the latissimus dorsi m. and gently lift away from the deeper musculature
With your scalpel, cut the latissimus dorsi m. along the spinous processes from T7 vertebra to the sacrum (top of the iliac crest).
Make horizontal incisions from the sacrum along the iliac crest
Grasp the medial, inferior edge of the latissimus dorsi m. and begin to lift the muscle; note how the lateral fibers of the latissimus dorsi m. interdigitate with the external oblique m. inferior to the serratus anterior m. Blunt dissection (using your hand or a blunt instrument) will allow lateral reflection of the latissimus dorsi m.

The thoracodorsal n., a., and v., supplying the latissimus dorsi m. are located on the deep superior surface of the latissimus dorsi m. and will be studied in a further laboratory session.

HELPFUL HINTS

The latissimus dorsi m. arises from the thoracolumbar fascia that covers the deep vertebral musculature (erector spinae mm.) of the back, so try to avoid cutting the deeper musculature.

9.12 Reflecting the latissimus dorsi m.

Serratus anterior muscle

The serratus anterior muscle overlies the lateral part of the thorax and forms the medial wall of the axilla. This broad sheet of thick muscle was given its name because of the sawtooth (serrated) appearance of its fleshy slips or digitations.

The serratus anterior attaches to the medial border of the scapula and anchors the scapula, keeping in closely approximated to the chest wall and enabling other muscles to use it as a fixed bone for movements of the humerus.

After reflecting the lattissimus dorsi muscle, you should be able to visualize the serratus anterior running anteriorly along the lateral body wall. You can also visualize this muscle by lifting up on the scapula, assuming the trapezius and rhomboid muscles have been reflected.

Intermediate Back Muscles

After the superficial back muscles are dissected and reflected the intermediate back muscles are readily accessible. Identify the following thin, accessory respiratory muscles (Figure 9.22):

Serratus posterior superior m. - located deep to the levator scapulae and rhomboid mm. and superficial to the splenius capitis m,; attaches along the spines of C7 - T3 vertebrae and the ribs
Serratus posterior inferior m. - located deep to the latissimus dorsi m. and attached along the spines of T11 - L3 vertebrae and the ribs; may be attached to the deep surface of the latissimus dorsi m.

HELPFUL HINTS

The serratus posterior muscles are very thin. Often the serratus posterior inferior is found on the deep surface of the reflected latissimus dorsi.

9.22 Intermediate back mm.

Intrinsic (Deep) Back Muscles

Reflect the serratus posterior superior and inferior mm. along their vertebral attachments to reveal the deep muscles. Additionally you will need to reflect a heavy layer of deep fascia.

Identify the erector spinae musculature consisting of the following muscles:

Iliocostalis m.: most lateral of the erector spinae group.
Longissimus m.: middle muscle group of the erector spinae musculature
Spinalis m.: Most medial of the erector spinae group.

All of these deep back muscles are innervated by the dorsal (posterior) rami of thoracic spinal nerves (Figure 9.23). You may be able to see the ends of these posterior rami poking up through the muscles. The posterior rami run all the way up into the removed skin, where they provide sensory input from the skin in the dorsal midline.

Group B will have to remove the erector spinae entirely (not just reflect them) to do a laminectomy and expose the spinal cord. So get a good look at the erector spinae now!

Superior view of spinal column

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