Thorax Lab 1a

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.

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.

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.

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

Reflection of the Pectoralis Major Muscle

Free the inferior border of the pectoralis major muscle from the underlying tissue by blunt dissection and/or scissor spreading.

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:

1. Thinner muscles can be quite fragile and you don’t want to tear the muscle to pieces.

2. 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.

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.

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.