Human Dissection Techniques – Advanced Anatomy Lab #2

THORAX DISSECTION

  1. Lungs and Pleural Sacs
  2. Heart and Pericardial Sacs

Remove lateral and anterior portions of the ribcage, to expose heart and lungs within the thoracic cavity.  Cut through the ribs and the intercostal muscles around the perimeter of the rib cage on each side.  Make your cuts or hole as large as possible by following the margin of the rib cage, and then going down as far laterally as you can along the side.  Essentially, this is along the mid-axillary line; You will have nice exposure for your thoracic organs.  Get your fingers under the rib cage and peel the layer of tissue, the parietal pleura away from its attachment to the under side of the rib cage.  Leave as much of that intact as possible as you remove the bone.  Remove the ribs and the intercostals muscles on each side.  The parietal pleura should be clear.  Cloudiness, opaque, or thick regions could indicate previous infection within the sac.  Visceral pleura are delicate; They are immediately on the surface of the lung to provide a friction-free movement as lungs fill with air and release during exhalation.  Remove sternum from midline.  This may require a saw to cut the superior and inferior ends of the sternum.  A chisel and hammer may work as well.  Be sure to bluntly separate the soft tissue from the deep surface of the sternum.  Keep the vessels intact (internal thoracic or internal mammary arteries), which supply the chest wall and continue to the abdominal wall to provide collateral circulation.

Now we can see the lungs and the heart.  We did preserve the internal thoracic artery and vein on either side.  Remove the lung.  Be careful that you do not cut the phrenic nerve which innervates the diaphragm; It travels between the lung and the heart.  To remove the lung, push it laterally and the heart in medially, so you can see the region of the lung called the hilus.  This is where a variety of structures enter or exit the lung, such as the airway, blood vessels, nerves, and lymphatics.)  Make a clean cut at the hilus of the lung.  Use a knife or a scalpel.

Once the lungs are removed, the right and left lungs can be easily distinguished, because they have different numbers of lobes.  On the right lung the oblique fissure separates the inferior lobe from the superior and middle lobes.  The horizontal fissure separates the superior lobe from the middle lobe.

Lungs tend to be darker on the posterior side of the lung, due to blood collection; the cadaver has been laying on its back and blood has collected in the posterior area due to gravity.  Turn the lung over to look at the hilus and to study some of the structures there.

Pulmonary arteries – of intermediate thickness – provide deoxygenated blood to the lung

Pulmonary veins – thinnest wall

Bronchus – cartilaginous – thickest wall – must remain patent at all times

Hilar lymph nodes –

Bronchial circulation – which supplies a secondary blood supply to the lungs

 

Remember R   A    L    S

On the right lung, the pulmonary artery is anterior to the bronchus.  Pulmonary veins are more inferior structures at the hilus.

Left lung – Separated by an oblique fissure.  Superior lobe has an appendage called the lingula.

On the left lung, the pulmonary artery is superior to the bronchus.  Two pulmonary veins are more inferior.  Hilar lymph nodes are very black.  They contain some of the pollutants that have passed through the lungs.  Tiny vessels around the bronchus are the bronchial vessels.  (A couple of mm in diameter)  There is a dual blood supply to the lung.

Place the left lung back into the chest so that we can discuss the pleural sac that surrounds the lung.  The visceral layer of the pleural sac is immediately on the surface of the organ.  The outer parietal layer has different names depending on which surface it is coating.  The portion of the parietal pleura that is adherent to the inside of the rib cage is called the costal pleura; the diaphragmatic pleura coats the superior surface of the diaphragm.  On the midline, between the lung and the heart, is the mediastinal pleura, which is also a portion of the parietal pleura.

Look at how much of the thoracic cavity the lung occupies.  It doesn’t occupy the entire height of the cavity; It falls a little short.  There are some places where parietal pleura rub against another layer of parietal pleura.  A good example is where the diaphragmatic pleura and the costal pleura rub against each other, in an area called the costodiaphragmatic recess.  This region can be important clinically, as a physician can insert a needle to withdraw fluid that has accumulated in the pleural cavity.  Using this area reduces the risk of puncturing the lung itself.  Now that both lings are removed, we can explore the mediastinum, which includes the heart.  Like the lungs, the heart is surrounded by a sac called the pericardial sac.  This sac reduces the friction as the heart enlarges and contracts again.  We need to cut through the parietal pericardium to expose the heart.  We will see the visceral pericardium right on the heart.  Be careful not to cut the phrenic nerve, which innervates the diaphragm.  Keep the internal thoracic vessels intact as well.

Make and “X” shaped cut within the pericardial sac and flip back the flaps to expose the heart.  You are now looking at the visceral pericardium.  There is a moderate amount of adipose tissue on this layer.  We will have to dissect through that to expose the coronary vasculature of the heart.

Identify two sinuses within the pericardial cavity –

  • These are the structures that form due to the way the pericardium attaches to and reflects off of the surface of the heart.

1)        Oblique sinus – Put your hand behind the heart within the pericardial cavity; You can slide it only a certain distance posterior to the heart and then you reach a dead end.

This is where the visceral and parietal pericardium become continuous with each other at the surface of the heart.  We cannot go any further because of the oblique sinus.

Similarly, at the superior end of the heart and just to the left of it, we can slide fingers through and get through and get to the other side–  to the  — 2)  Tranverse Sinus.

Transverse sinus  – more clinically significant than the oblique sinus.  Surgeons use this transverse sinus to clamp off the outflow vessels surgically; which are the aorta and the pulmonary trunk.

Slide 2 fingers behind the “two vessels” and come out on the opposite side.  (This region is called the transverse sinus.)  Remove heart from the mediastinum by cutting through the great vessels, or major inflow and outflow vessels of the heart.

Start by pulling the heart superiorly and cutting through the IVC, which is bringing the blood back to the heart from the regions inferior to the diaphragm.  There is no real length to the IVC.  It comes through the diaphragm and immediately enters the RA of the heart.

Cut through aorta and pulmonary trunk at the transverse sinus region.  Make all these incisions within the pericardial cavity to ensure that no internal structures are damaged.  We will need to investigate the internal structures in later dissections.

Last large vessel needing to be cut is the superior vena cava, which returns blood from upper limb and head back to the RA.  We have already cut through the PV when we removed the lungs, so we need to bluntly dissect those from their penetration to the pericardial cavity so we can pull the heart out.

Pectinate Muscles –

Left Atrium is on the posterior side of the heart.

Cutting the heart sagitally will expose both ventricles

R —thinner—only pumps next door to the lungs

L—thicker—-pumps all over body

Inside Ventricles –

Structures prevent backflow of blood

Tricuspid valve

Chordi tendonae – prevents valves from flipping inside out

Tribeculae carnae

Bicuspid (aka mitral valve)

Septomarginal Band – aids in proper contraction of right ventricle.

Let’s understand how well the heart is protected by the sternum and the thoracic cage.  The heart is positioned asymmetrically within the chest, so that on the right side it projects appr. 1 inch toward the right from the edge of the sternum.  It is important to be able to predict injury caused by a puncture wound.  In this case the right atrium would be affected by a potential puncture wound, to the anterior chest wall.  On the left side the heart projects appr. 3 inches from the left side of the sternum.  The R and L ventricles could be damaged from a puncture wound.

Landmark used to show boundary between right and left ventricle – anterior interventricular artery

Aka L A D – Left Anterior Descending Artery

In order to see the origin of the CA from the aorta, we need to fold down the pulmonary trunk and pin it out of the way.  We want to see the origin of the coronary arteries from the aorta.  Now, as soon as the aorta exits the left ventricle, we can see that it has 2 major branches heading in either direction —R and L coronary artery.  Dissect these by removing the superficial tissue off the surface of the heart by bluntly dissecting it with forceps or a probe by scraping off the fat.  LCA – very short, bifurcating almost immediately into the AIA (anterior interventricular artery) and the circumflex artery, which will wrap around to reach the posterior side of the heart.  Again, the 2 major branches of the left CA are the anterior interventricular artery, aka LAD – Left Anterior Descending, and the circumflex artery.

The LAD is one of the most commonly obstructed vessels in the coronary circulation, and likely the most commonly by-passed.  Looking at the right coronary artery, multiple branches come off as it wraps around the heart.

For now focus on major branches –

Right Marginal Artery

RCA continues to the posterior surface of the heart, and it sends a branch down called the posterior interventricular artery.  There will be some collateral circulation and anastomoses between the posterior interventricular artery and the anterior interventricular artery branch of the left coronary.  After these CA’s supply the myocardium with its O2 and nutrients, that deoxygenated, nutrient poor blood needs to be returned to the RA.  A set of cardiac veins will be responsible for that.  The vessel called the coronary sinus is going to receive all of the de-oxygenated blood from the cardiac veins.  You can match cardiac veins with the major branches of coronary arteries.

SO, traveling with the posterior interventricular artery, we have the middle cardiac vein, which is passing superiorly, joining the coronary sinus, and then together, empty into the RA.  Returning to the anterior side of the heart, we can see the longest cardiac vein, which travels with the branches of the left coronary artery and carries the deoxygenated blood back around to the posterior side of the heart, into the coronary sinus and then into the right atrium.

Internal anatomy of the heart –

Start with the atria

Right atria with the SVC and the IVC delivering the deoxygenated blood back to the heart.  Using a pair of scissors to cut between the vena cava, you can open the right atria and look at some internal structures.  The inside is smooth.  The fossa ovale is a depression located within the septum, separating the right and left atria from each other.  The fossa ovale represents an opening that was present between the two atria during embryonic development.  Usually, this closes off after birth, so that we establish the normal adult circulation.  Inside the RA are rough areas called pectinate muscles.  (Ridges of muscles).

Flip the heart over to look at the left atria, which is on the posterior side of the heart.  4 pulmonary veins are returning oxygenated blood from the lung back to the heart.  Cutting through the left atrium, there is nothing remarkable to see.  Cut the heart sagitally to investigate the ventricles.  Interventricular artieries will be used as a guide to cut from the apex of the heart up to the base.  (Cut perpendicularly to the interventricular arteries.)  Do not cut into two pieces.  Use forceps to remove blood clots and rinse.

Right ventricle – much thinner because it only has to pump blood next door to the lungs.

Left ventricle – much thicker because it has to pump blood to the entire body.

The interventricular septum, which separates the right and left ventricle has an inferior muscular portion and at the superior edge there is a much thinner membranous portion.

Inside ventricles – the main function of these structures is to prevent backflow while the heart contracts.

Right side – tri-cuspid valve and leaflet—The leaflet has strands attached called chordi tendonae.  (Strands of connective tissue)

These tendons are used by papillary muscles to prevent valves from everting or flipping inside out as the ventricle contracts.  The papillary muscles use the chordi tendonae to prevent eversion of the tri-cuspid valve.  There are additional ridges of muscle in the wall of the ventricle called tribeculae carnae; Similar structures exist in the left ventricle.  Papillary muscles attach to chordi tendonae and are attached to the bicuspid or mitral valve, AV valve that will prevent blood from flowing backwards, from LV into LA during ventricular contraction.

Myocardium is brown.  (No MI)  Problems could affect the conduction system, which cannot be seen grossly or histologically.  The conduction system controls the contraction.

RV only – The septomarginal band extends from the interventricular septum to the papillary muscle.  It does contain a portion of the conduction system.

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