Thorax Lab 1A

Lab 1A: Superficial Thorax

Learning objectives

Practice dissection techniques for removing skin and superficial fascia (subcutaneous fat), properly revealing and reflecting muscles, and using the bone saw to transect bone.
Understand the importance of lymphatic drainage of the breast as a route for cancer metastasis. [Note: be sure to visit a table with a female cadaver if you have a male!]
Be able to distinguish between different types of (string-like) structures: nerves, arteries and veins.
Learn how to clean and trace arteries and nerves back to where they branch from and out to targets, in order to make positive identifications.

Short List of Structures - 1A

Female Cadaver: Orientation and Incisions

Before You Begin...

If you have a female cadaver, begin with this section. It will guide you through the removal of the skin and superficial structures that form the female breast. If you have a male cadaver, be sure to observe the dissected female breast prior to the end of lab.

During all of your dissections, you should take care to cover those parts of the body you are not working on. Get in that habit now.

This section begins with a review of the osteology to orient you. You will be asked to find the same bony structures that you found on the articulated skeleton on your cadaver before making any incisions.

Although the figures in this guide show dotted lines for the incisions, you do not need to make these marks on your cadaver before cutting.

The cadaver should be supine for this dissection.

To orient yourselves, locate the following structures. Keep in mind that on thin bodies, these structures will appear as projections; on heavier bodies, you will need to palpate to locate these structures. Remember that bony structures can also be found on an articulated skeleton. Use the labeled figure to help you.

Surface anatomy of the anterior chest (female)

Skin incisions

Make cuts with the scalpel along the dotted lines as shown in the figure. A to B cuts along the midline over the sternum. A to C cuts along the clavicles. B to D cuts along the inferior border of the rib cage. Abduct the upper limbs and reflect the skin as far posterior as possible.

Start at point A or B, where bone underlies the skin. As you work laterally, be sure you don’t cut so deep that you damage the underlying muscle. How will you know how deep is too deep? That’s one of the things you’re learning today: how to recognize muscle!

Skin Incisions (female)

After making the incisions, use your toothed forceps to pull up a corner of the skin, and then use your scalpel to separate the skin and the superficial fascia from the underlying muscles. Note that as you do this, you will also be removing the breast. Remove the skin and breasts from both sides.

As you remove the skin, notice any cutaneous neurovascular bundles that course through the superficial fascia toward the skin.

The breast may be dissected now, or you may choose to do this dissection later in the lab. Store the breasts in a separate bag on the shelf beneath your table. If any of the groups near you are working on a male cadaver, please share your findings with them.

After making your incisions, begin removing the skin and fascia. Start at one of the corners. Note the muscle fibers visible in the corner.
Once you see the red fibers of the pectoralis major m., you will know you have gone deep enough. Leave the pectoralis major m. intact.
Continue removing the skin and fascia by putting tension on the skin and using a scalpel to cut along the natural plane of separation.
The thickness of the skin and fascia will vary from body to body. Do not cut through the underlying muscle.
After the skin and breast have been removed (you should remove both), identify the pectoralis major m., serratus anterior m. and anterior portion of the deltoid m.

Breast Dissection

The breast has no capsule and is located in the superficial fascia, superficial to the pectoralis major m.

Use the scalpel to make a sagittal incision through the nipple, and then continue to cut through the entire breast. Identify the following structures:

Probe the fatty tissue deep to the nipple and areola to locate any glandular tissue of the mammary glands, and any of the fifteen to twenty lactiferous ducts that drain milk from the glandular tissue to the nipple. Most of the cadavers are significantly post-menopausal and the glandular tissue and ducts will be difficult to see.

Next, look for suspensory ligaments of the breast. To better observe the suspensory ligaments, you may find it helpful to pull the nipple to put tension on (straighten) the ligaments. You may also see lymph nodes in the breast tissue. Understanding the lymphatic drainage of the breast is important in understanding the metastasis of breast cancer.

Cut deeply along the dotted line to make a sagittal section through the breast.

Once the sagittal section is made, look for suspensory ligaments, glandular tissue, and lactiferous ducts. Glandular tissue and lactiferous ducts will be difficult to see.

Clinical Correlation: Lymphatic Drainage of Breast

Breast cancer typically spreads by means of lymphatic vessels (lymphogenic metastasis), which can carry cancer cells from the breast to the lymph nodes, mainly those in the axilla. The cancer cells lodge in the nodes, producing nests of tumor masses (metastases). Abundant communication among lymphatic pathways and axillary, cervical, and parasternal lymph nodes may also cause metastases from the breast to develop in the supraclavicular lymph nodes, the opposite breast, or the abdomen.

Male Cadaver: Orientation and Incisions

Before You Begin...

During all of your dissections, you should take care to cover those parts of the body you are not working on. Get in that habit now.

This section begins with a review of the osteology to orient you. You will be asked to find the same bony structures that you found on the articulated skeleton on your cadaver before making any incisions.

Although the figures in this guide show dotted lines for the incisions, you do not need to make these marks on your cadaver before cutting.

The cadaver should be supine for this dissection.

To orient yourselves, locate the following structures. Keep in mind that on thin bodies, these structures will appear as projections; on heavier bodies, you will need to palpate to identify these structures.

Surface anatomy of male pectoral region

Make cuts with the scalpel along the dotted lines shown in figures 5.1 and 5.2 in the gallery. A to B cuts along the midline over the sternum. A to C cuts along the clavicles. B to D cuts along the inferior border of the rib cage. Abduct the upper limbs and reflect the skin as far posterior as possible.

Start at point A or B, where bone underlies the skin. As you work laterally, be sure you don’t cut so deep that you damage the underlying muscle. How will you know how deep is too deep? That’s one of the things you’re learning today: how to recognize muscle!

After making the incisions, use your toothed forceps to pull up a corner of the skin, and then use your scalpel to separate the skin and the superficial fascia from the underlying muscles. Remove the skin from both sides of the chest. As you remove the skin, notice any cutaneous neurovascular bundles that course through the superficial fascia toward the skin. Put the skin flaps into the tissue bucket under your table.

After making your incisions, begin removing the skin and fascia. Start at one of the corners. Note the muscle fibers visible deep to the skin and fascia
Once you see the red fibers of the pectoralis major m., you will know you have gone deep enough. Leave the pectoralis major m. intact.
Continue removing the skin and fascia by putting tension on the skin and using a scalpel to cut along the natural plane of separation. A cutaneous neurovascular bundle is visible in this photo, you’ll see many of them throughout your dissections.
Note the visible pectoralis major m., serratus anterior m. and anterior portion of the deltoid m. (this is a female).
Make initial skin incisions along the dotted lines.

Anterior Wall of the Thorax

In both female and male cadavers, we will now proceed to dissect the muscles, arteries, veins and nerves of the anterior thoracic wall. With a scalpel or sharp scissors, remove any remaining superficial fascia to clarify the borders of the pectoralis major muscle. Continue to reflect the skin off the shoulder and the proximal anterior aspect of the arm to see the anterior part of the deltoid muscle. This will also allow you to see the full extent of the pectoralis major m.

The cephalic v. runs through the deltopectoral triangle, which is bordered by the pectorals major m., the deltoid m., and the clavicle.

At this point, identify the pectoralis major muscle and appreciate both the clavicular head and the sternal head (named for their bony attachments). You should also be able to identify the deltoid muscle, although you will only be able to see the anterior half (attached to the clavicle).

Along the line where the pectoralis major and and the deltoid meet, identify the deltopectoral triangle, a triangular space bordered by the clavicle medially, the clavicular head of the pectoralis major muscle inferiorly, and the anterior border of the deltoid muscle superiorly.

Look for the cephalic vein in the deltopectoral triangle. Using blunt dissection or scissors spreading, explore the deltopectoral triangle to find this vessel. Note that the cephalic v. may be covered by muscle fibers from the deltoid or pectoralis major m. The size of the cephalic v. varies between individuals and you may not find it in your cadaver.

Continue to clean away connective tissue on the lateral aspect of the thorax to expose slips of the serratus anterior muscle. The serratus anterior arises from individual muscle bellies from the upper eight ribs, runs posteriorly between the ribs and the scapula (deep to the latissimus dorsi muscle) and inserts on the medial border of the scapula. At this point, you will only be able to see its attachments to the ribs.

Also, you can look for the long thoracic nerve, which runs along the serratus anterior m. and provides its motor innervation.

Cephalic vein

Cutaneous nerves of the thoracic region

Reflection of the Pectoralis Major Muscle

Free the pectoralis major muscle from the underlying tissue using blunt dissection first, followed by a cut starting along the inferior medial edge where it attaches to the sternum (A; righthand photo) moving superiorly (B; righthand photo).

Take care to avoid tearing the medial pectoral nerve with blunt dissection; the nerve is located just deep to the center of the pectoralis major muscle belly (the hand in the image to the right is quite close to this nerve) so stay lateral.

Insert your fingers deep to the inferior border of the pectoralis major muscle and work your fingers back and forth to break fascial connections and separate the pectoralis major muscle from the underlying tissue. This requires a combination of force and finesse for two reasons:

Thinner muscles can be quite fragile and you don’t want to tear the muscle to pieces.
You do want to try to preserve the nerves and vessels that enter the deep surface of the muscle.

When you are ready to reflect the muscle, use a scalpel or scissors to cut along the sternal and clavicular attachments. In the region of the clavicle, be careful to avoid damaging the underlying nerves and blood vessels. Branches of these nerves and blood vessels enter the deep surface of the pectoralis major; try to keep them intact until you identify them.

Free the pectoralis major m. from its deep attachments to the rib cage before cutting its medial and superior connections away from the bone to reflect it.

As you are reflecting the pectoralis major muscle, peek underneath and identify the pectoralis minor muscle.

Remove the adipose between the clavicle and the pectoralis minor muscle. As you do, look for the medial pectoral nerve, which pierces the pectoralis minor (providing motor innervation as it passes through) and then enters the pectoralis major m. (and provides motor innervation to part of the pectoralis major). Often, the part of the medial pectoral n. that runs between the pectoralis major and minor is torn when the pectoralis major is reflected. In that case, you’ll see a wispy stub of the medial pectoral n. emerging from the pectoralis minor.

At the cranio-medial border of the pectoralis minor, look for the lateral pectoral nerve, which enters the deep surface of the pectoralis major to provide motor innervation to that part of the muscle not innervated by the medial pectoral n.

Be sure to reflect the pectoralis major muscle on both sides.

Medial and lateral pectoral nn., and pectoral branches of the thoracoacromial trunk.

Medial and lateral pectoral nn. Note that this is a dissection on the left side with the cranial direction to the left of the image.

HELPFUL HINTS

You’ll need to spend some time cleaning the nerves and arteries in this region to actually see them. A glimpse of an artery that is otherwise buried in adipose is not the same as exposing the length of that structure and identifying where it is coming from, and where it is going to.

Do not use your scalpel to poke around in this region! Scissor-spread to loosen up fat and connective tissue, and then pull that tissue out with your forceps.

Reflection of the Pectoralis Minor Muscle

At this point, detach the right and left pectoralis minor muscles from their medial attachment along ribs 3 through 5 and reflect them laterally. Use fingers, forceps, and scissor spreading to expose the axillary vein on the left and right sides, and deep to it, the axillary artery. In order to visualize the axillary artery you will need to cut and reflect the axillary vein, but first observe its path and connection to the cephalic vein.

The axilla is the anatomical term for the region of the body we typically call the armpit. The axillary a. is the continuation of the subclavian a. in the axillary region (armpit). The axillary a. becomes the brachial a. when it enters the arm (see figure).

The thoracoacromial trunk (=thoracoacromial artery) originates from the axillary artery. The thoracoacromial trunk is typically short (2-4 mm) and divides into four branches according to their respective fields of distributions. The four branches of the thoracoacromial artery are Pectoral, Deltoid, Acromial and Clavicular branch.

It is unlikely that you will find all four branches of the thoracoacromial trunk, but do try to identify one or two of them. Find one structure that you think may be a branch and follow it back to the axillary artery and out to its target muscle for positive identification of the branch. Repeat for a second candidate artery. Does your cadaver have a definitive thoracoacromial trunk, or do these branches come directly off the axillary artery? For example, the dissection image to the right shows a cadaver with no distinct thoracoacromial trunk. Instead those terminal branches arise individually directly from the axillary artery. Compare this with the diagram above and to the right, which shows the classic configuration of the thoracoacromial trunk with four terminal branches,

The thoracoacromial trunk, a branch of the axillary artery. Note the 4 terminal branches of the thoracoacromial trunk.

An actual view of the axillary artery and several of its branches. Identification of the branches of the thoracoacromial trunk can only be confirmed by following small arteries to their targets.

HELPFUL HINTS

There is a great deal of variation in the vasculature of the human body. You may find that your cadaver has a single thoracoacromial trunk and four distinct branches, or only three branches, or perhaps five! You may find that one or more of the four named branches comes directly off of the axillary artery, instead of the thoracoacromial trunk. You will see this kind of variation throughout your dissections. Be sure to appreciate, and celebrate, this great diversity.

This is a good time to start to learn how to distinguish between nerves, arteries and veins

Veins are very thin-walled and often still contain blood that may be liquid or coagulated. Thus, they usually look purplish or dark blue.
Arteries are thicker walled and feel a bit stretchy (elastic) when you try to stretch them. They usually appear white and will collapse when you squeeze them between your fingers because they are hollow.
Nerves are also whitish. Larger nerves can feel stiffer than arteries and do not compress when you squeeze them. Smaller nerves can be quite delicate.
Arteries and nerves can sometimes be difficult to tell apart. Cleaning and following them back to their origins can often provide a definite identification (e.g., an artery branching from a larger artery).
Using a sharp scalpel to make a longitudinal incision can also help distinguish artery from nerve—if it is hollow (has a "lumen”), then it is an artery. Making the cut longitudinally will preserve the connections of the artery or nerve while still making it possible to check for a lumen.