Dissection 5: Pectoral Region and Axilla
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Introduction: For today’s introduction I will copy for you a definition from The
Origin of Medical Terms by Henry Allen Skinner…
axilla Latin – armpit. The derivation is uncertain. It has
been suggested that it is a compound word formed from
axis-alae, meaning the axis of the wing, because the arm or
wing revolves from this point.
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Pectoralis Major: Superficial muscle of the thorax, innervated by medial and
lateral pectoral nerves. Pectoralis Major has two distinct heads and both are
capable of internal rotation and adduction of the humerus. You may have also
noticed when a person who has exerted themselves is trying to catch their breath
they instinctively place their arms on a stable surface. What they are doing is
stabilizing their shoulders so that the pectoralis major can act as an accessory
muscle of respiration by reversing the parts of the muscle that act as origin and
insertion. (Contractions of the muscle will pull the thoracic cage out as opposed to
pulling the humerus in toward the thorax.)
o Clavicular Head: A flexor of the humerus when the shoulder is in neutral
(arm at rest by your side)
o Sternocostal Head: An extensor of the humerus when the shoulder is
flexed.
Deltoid & Deltopectoral Triangle: The cephalic vein lies in the deltopectoral
triangle on its way to empty into the axillary vein. (Axillary vein comes from the
union of the basilic vein and the companion veins of the brachial artery) Once the
fat is cleared out of this triangle you may be able to see some of the branches of
the thoracoacromial trunk. You can find the deltopectoral triangle on yourself by
standing in front of a mirror, taking off your shirt and flexing you shoulder to 90o.
It is that small depression between the pectoralis major and the deltoid located at
about the middle of the clavicle. In some anatomy texts you may see this same
region described as the infraclavicular fossa.
Lateral Pectoral Nerve: (C5, C6, C7) Look for this entering the deep surface of
the clavicular head of pectoralis major. Also see if you can find its contribution to
the sternocostal head as well as a communicating branch to the medial cord of the
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brachial plexus (BP). This communicating branch will cross over the anterior
surface of the axillary artery. The lateral pectoral nerve is named as it is because it
comes from the lateral cord of the brachial plexus, not because it is more lateral in
comparison to the other pectoral nerve.
Thoracoacromial trunk: (a branch of axillary artery): Found deep to pectoralis
minor it gives off four immediate branches (1. acromial 2. pectoral 3. deltoid 4.
clavicular) No, you do not have to identify each of the branches, I listed them for
the overachievers in the class, you know who you are.
Medial Pectoral Nerve (C8,T1) Its name comes from its origin at the medial cord
of the brachial plexus. Look for this nerve entering the deep surface of pectoralis
minor, piercing it, and then entering the deep surface of pectoralis major. The
branches that pass between the pectoral muscles will be thin. Thin enough that
they look like fascia, so don’t separate pectoralis major from pectoralis minor
with a scalpel.
Pectoralis minor: Attaches superiorly to the coracoid process of the scapula and
inferiorly to ribs 3-5. Found deep to the pectoralis major it makes a “bridge”
under which travels the axillary artery and the cords of the brachial plexus. (Did I
just write under? I meant deep to) The pectoralis minor also serves as the
landmark whose medial and lateral borders divide the three parts of the axillary
artery.
Cords of the brachial plexus (BP): There is a lateral, medial, and posterior cord
of the brachial plexus. They are named for their orientation around the axillary
artery (in anatomical position of course) Knowing this fact really helps when you
are trying to orient yourself to the different parts of the BP. Seriously dude, you
should know this, it will make identifying the terminal branches so much easier.
Lateral cord of the BP: This cord gives off one branch that we already saw, the
lateral pectoral nerve. The lateral cord terminates as the musculocutaneous nerve,
and gives off a contribution to the median nerve, sometimes called the lateral root
of the median nerve. (Don’t confuse this with the roots of the BP)
Medial cord of the BP: This cord gives off three branches in this order from
medial to lateral. 1. medial pectoral nerve 2. medial brachial cutaneous 3. medial
antebrachial cutaneous. The medial cords terminal branch is the ulnar nerve, and
the medial cord also gives off a contribution to the median nerve, sometimes
called the medial root of the median nerve. (again, not to be confused with the
roots of the BP)
o Axillary Artery: Continuation of subclavian artery that changed its name
as it crossed rib 1. Described in 3 parts…
o Part 1: Lateral border of rib one to the medial border of pectoralis minor.
Only branch - Superior Thoracic Artery: Not much to see here, it supplies
the first two intercostals & maybe some of serratus anterior. Generally a
tiny vessel.
o Part 2: Medial border of pectoralis minor to lateral border of pectoralis
minor. Two branches - Thoracoacromial trunk & Lateral Thoracic Artery:
Thoracoacromial trunk was addressed previously. Lateral thoracic artery
travels inferiorly to enter superficial surface of serratus anterior. In some
cadavers, lateral thoracic artery may be a branch of subscapular artery or
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the thoracodorsal artery. So be aware of these two common variations. In
females this artery may be larger as it is a major source of blood to the
breast.
o Part 3: Lateral border of pectoralis minor to inferior border of teres major.
Three branches - Subscapular artery (which will bifurcate into the
thoracodorsal artery and circumflex scapular aa.), and the Anterior and
Posterior Circumflex Humeral Arteries. These last two form an
anastomoses around the surgical neck of the humerus. The posterior is
often larger than the anterior. Confirm which is which by determining
which travels with the axillary nerve. (Think back to quadrangular space)
Subclavius: Small muscle inferior to the clavicle, try not to reflect it back with
clavicular head of pectoralis major. Innervation: Nerve to Subclavius; if only all
nerves were named so simply.
Axillary nerve: (C5,C6) As mentioned earlier it travels with an artery in the
quadrangular space. Ahh yes, that was the posterior circumflex humeral artery. If
you follow the nerve medially and you will see it coming off of the posterior cord
as one of the posterior cords two terminal branches.
“M” = musculocutaneous nerve, lateral root of median nerve, medial root of
median nerve, ulnar nerve. This sentence makes considerably more sense after
you see the completed dissection.
Serratus Anterior: Attaches to the vertebral border of the anterior surface of the
scapula, then lies on the thorax until it attaches to the ribs anteriorly. Along with
the antagonistic activity of the axioscapular musculature it secures the scapula
onto the thorax. Damage to its nerve supply, the long thoracic nerve, causes
scapular winging.
Long Thoracic Nerve: This nerve emerges from the roots of the BP (C5, C6, C7)
One of the only nerves to lie on the superficial surface of the muscle it innervates,
it is exposed to various mechanical trauma such as sustaining an unfortunate knife
wound in response to your beer fueled observation about that bikers lady friend.
Lesson: If you are not going to flee from a person brandishing a knife, then use
your arms to protect your sides and your hands to protect our face. When the
nerve suffers a lesion, the clinical result is scapular winging that was first
described in the literature by French surgeon Dr. Alfred Velpeau in 1837. He
named the condition scapula alata.
Intercostobrachial nerve: (T2) This is a branch of the 2nd intercostal nerve, it
emerges from the 2nd intercostal space (between ribs 2 &3) at the mid axillary
line, penetrates the serratus anterior and enters the axilla and the arm. It may
communicate with the medial brachial cutaneous nerve. It is not part of the
brachial plexus. Think of this nerve next time you are tickling someone in the
armpit as that is where its sensory distribution of the intercostobrachial nerve is
found. The axilla is also where you find T2 dermatome.
Subscapularis: You get a better look at this rotator cuff muscle and its nerve
supply (Upper & Lower Subscapular nerves) in this dissection. Remember it is an
internal rotator of the humerus. You will be able to differentiate the upper from
the lower subscapular nerve by following the lower subscapular nerve to teres
major as well as subscapularis.
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Tendons of latissimus Dorsi and Teres Major: These two almost blend together
as they approach their attachments to the humerus. Both are strong internal
rotators and teres major is a landmark for a name change in the main artery of the
arm (Axillary a.  Brachial a.) Teres major is actually fairly easy to see in a
shirtless person. Ask one of the TAs to demonstrate. It has a fusifirm appearance
traveling between the inferior angle of the scapula and the proximal humerus.
Fusiform? You may need to look that up.
Thoracodorsal artery and nerve: (C6, C7, C8) We see these two again but now
you can follow them from either the subscapular artery or the posterior cord of the
BP to latissimus dorsi.
Upper and Lower Subscapular Nerve :(posterior cord to subscapularis) You
can determine which is which by looking to see which of the two pierces teres
major. (C5, C6) Upper Subscapular (C5, C6) Lower Subscapular (spinal levels
according to Gray’s Anatomy 39th edition)
Radial Nerve: (C5, C6, C7, C8, T1) What vessel are you going to use to confirm
the identity of this nerve? I hope you said profunda brachii. They will be diving in
between the humerus & long head of the triceps to travel in the radial / spiral
groove of the humerus together. (Remember triangular interval?)
Corcobrachialis: This muscle is actually part of the next dissection, but it serves
as a good landmark for identifying the musculocutaneous nerve (C5, C6, C7)
because the musculocutaneous nerve pierces the belly of this muscle.
Short head of biceps brachii: Like coracobrachialis and pectoralis minor, it has
an attachment to the coracoid process of the scapula by way of what is described
as a conjoint tendon. We will talk more about the biceps in our next dissection.
Anatomical Note: While we have now seen some examples of anatomical variation, it is,
in my opinion, during this dissection that we will see the most variations. What is
important here is that you have a good understanding of what the normal anatomy should
be and from that point, reason out what it is you are looking at when you observe a
variation. The most common variations are typically described in atlases like Netters or
Grants but if you see something really unusual, then you may want to see what you can
find on the internet. The odds are most variations have been observed before and
documented by prudent anatomists. Try to avoid looking up things like this on Wikipedia
since you never know what you are getting on that website. Instead use the DYC online
journal database and find a peer reviewed source to help explain any variations you may
find in your dissection.