Anatomy of the pectoral and scapular regions

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Introduction0:00–0:29

The pectoral and scapular regions consist of various structures that include muscles, blood vessels, and nerves, which all act together to make our upper limbs functional.
The muscles of the pectoral region are divided into groups based on their locations. There are axial appendicular muscles, which extend between the axial and appendicular skeletons, and the scapulohumeral muscles, which specifically connect the scapula to the humerus.
First, let's take a look at the axial appendicular muscles, which are divided into two large groups, the anterior and posterior groups of muscles.

Anterior axio-appendicular muscles0:29–0:47

The anterior axial appendicular group is composed of four muscles the pectoralis major, pectoralis minor, the subclavius, and the serratus anterior.
The pectoralis major is a relatively flat fan-shaped muscle which covers the upper half of the thorax. This muscle has two proximal attachments, also called heads.

Pectoralis major0:47–3:23

The first one is the clavicular head, which attaches proximally to the medial half of the clavicle. Below it, there's another much larger part called the sternocostal head, which attaches proximally to the anterior surface of the sternum and the superior six costal cartilages.
Both the clavicular and the sternocostal heads converge distally and then attach to the lateral lip of the intertubercular groove of the humerus.
Suprailaterally, the clavicular head of the pectoralis major lies adjacent to the deltoid muscle. These muscles don't come completely together, thus creating a narrow gap between them that's called the deltopectoral groove, which is where the cephalic vein passes.
Additionally, inferolaterally, the sternocosal head of the pectoralis major twists and forms the anterior axillary fold.
The pectoralis major muscle is innervated by two nerves, the lateral pectoral nerve and the medial pectoral nerve. The lateral pectoral nerve receives fibers from the anterior rami of C5, C6, and C7, and the medial pectoral nerve receives fibers from the anterior rami of C8 and T1, and they're also connected by a communicating branch.
Both of these nerves arise from the brachial plexus, and their names tell which cords they're from. The lateral pectoral nerve takes a journey to reach its destination.
First, it arises from the lateral cord and then passes across the axillary artery and vein. Before piercing the clavic pectoral fascia to finally gain access to the deep surface of the clavicular head.
On the other side, the medial pectoral nerve arises from the medial cord and curves anteriorly, passing between the axillary artery and vein.
It then pierces pectoralis minor and then the sternocostal head of pectoralis major, innervating both of these muscles. All right, so when these two heads of the pectoralis major muscle contract simultaneously, they produce abduction and medial rotation of the arm.
Additionally, each muscle head can also contract independently. When the clavicular head contracts alone, it causes flexion of the arm, while the contraction of the sternocoal head extends it back from a flexed position.
Alright, now let's talk about the pectoralis minor muscle, which is a triangular muscle situated deep to the pectoralis major in the anterior axillary wall.

Pectoralis minor3:23–4:28

You can think of these two muscles as cousins, where the major covers and gives comfort to the minor one. The base of the pectoralis minor muscle attaches to the anterior surface of the 3rd, 4th, and 5th ribs at the point where they meet their costal cartilages.
The fibers of pectoralis minor converge towards its apex slaterally into a tendon that attaches to the coracoid process of the scapula.
The pectoralis minor is innervated by the medial pectoral nerve. And when the pectoralis minor contracts, it pulls on the coracoid process inferiorly and anteriorly against the thoracic wall, providing stability to the scapula by holding it tight to the thorax.
It also helps during deep inhalation because this muscle can elevate the ribs as it contracts, thus increasing the volume of the thoracic cavity so more air can get into the lungs.

Subclavius4:28–5:20

Next, in the anterior axial appendicular group is the subclavius, which is this small round muscle lying horizontally inferior to the clavicle and superior to the first rib.
It's best seen when the arm is in the anatomical position. Proximally, the subclavius attaches to the first rib and its costal cartilage, while distally it attaches to the inferior surface of the middle third of the clavicle.
This is where this muscle gets its name. The subclavius is innervated by the nerve to the subclavius, which receives fibers from the anterior rami of C5 and C6 via the superior trunk of the brachial plexus.
The subclavius helps to stabilize the shoulder by anchoring and depressing the clavicle and also adding some strength to stabilize the acromioclavicular joint when you hit those heavy weights at the gym.
Last but not least, there's the serratus anterior muscle, which forms the medial wall of the axilla and lies over the lateral part of the thorax.

Serratus anterior5:20–6:43

This is a fitting name for the muscle because serratus means saw, and the way this muscle's fibers are arranged on its anterior border gives it a saw-like appearance.
The muscle originates on the external surfaces of the lateral parts of the upper 8 ribs and then prolong laterally and posteriorly to insert onto the whole length of the anterior aspect of the medial border of the scapula along with the inferior angle of the scapula.
The serratus anterior is innervated by the long thoracic nerve, which originates from the anterior rami of C5, C6, and C7.
When the serratus anterior contracts, it protracts the scapula, meaning it pulls it anteriorly and laterally along the back.
This happens when you're trying to reach for something anteriorly or when making a punching motion, which is why this muscle is also referred to as the boxer's muscle.
The serratus anterior also helps to anchor the scapula by holding it close to the thoracic wall, like when doing push-ups or pushing a car.
Last but not least, this muscle also causes the scapula to rotate upwards, thus tilting its glenoid cavity like when abducting your arm above the head.

Quiz6:43–7:09

OK, so now let's pause and reflect a bit on what we've seen so far. In the meantime, can you recall the muscles of the anterior axial appendicular group and their innervations?
The blood supply of both the pectoralis major and minor comes from the pectoral artery, which is one of the four branches of the thoracoacromial artery.

Vascular supply7:09–8:34

The thoracoacromial artery originates from the second part of the axillary artery, which is the part of the axillary artery that lies posterior to the pectoralis minor.
After branching off from the axillary artery, the thoracoacromial artery curls around the supromedial border of the pectoralis minor and then pierces the costo coracoid membrane of the clavi pectoral fascia.
After it emerges on the other side of the fascia, that's when it divides into four branches in which the first is the pectoral artery.
The pectoral branch passes between the pectoralis major and minor, where it provides branches that supply these two muscles.
Next, the thoracoacromial artery gives the clavicular branch, which courses superiorly and medially to supply the subclavius muscle and the sternoclavicular joint.
The thoracoacromial artery also gives the acromial branch, which runs superior to the coracoid process, towards the acromion.
Finally, towards the lateral side, the thoracoacromial artery gives the deltoid artery, which then crosses over the pectoralis minor muscle and then passes alongside the cephalic vein in the deltopectoral groove between the pectoralis major and deltoid muscle.
At this level, the deltoid artery provides branches that supply both of these muscles. Now let's move to the muscles of the posterior axio appendicular group.

Posterior axio-appendicular muscles8:34–8:49

They're divided into two subgroups. The first subgroup consists of two superficial muscles, the trapezius and latissimus dorsi.

Trapezius8:49–10:07

The trapezius is this large triangular muscle extending from the posterior aspect of the neck to the superior half of the back.
There's two trapezius muscles in the back, which when seen together look like a trapezium. Proximally, the trapezius attaches to the medial third of the superior nuchal line, the external occipital protuberance, the nuchal ligament, and the spinous processes of the C7 to T12 vertebrae.
Distally, the trapezius attaches to the lateral third of the clavicle, the acromion, and the spine of the scapula. Based on the orientation of its fibers, the trapezius is divided into the ascending part.
The middle part And the descending part This muscle is innervated by the spinal accessory nerve, which is cranial nerve 11, as well as by branches from the anterior rami of C3 and C4.
Contraction of the descending part results in the elevation of the scapula, like when shrugging the shoulders. The contraction of the middle part retracts the scapula, while the contraction of the ascending part pulls the scapula downward, lowering the shoulder.
And finally, when the ascending and descending fibers work together, it results in superior rotation of the scapula on the thoracic wall.
The second superficial muscle is latissimus dorsi, which extends from the trunk to the humerus, acting directly on the glenohumeral joint and indirectly on the pectoral girdle.

Latissimus dorsi10:07–11:01

The latissimus dorsi is proximally attached to the iliac crest, thoraccolumbar fascia, the inferior 6 thoracic vertebrae, and the inferior 3 or 4 ribs.
Distally, the latissimus dorsi attaches to the floor of the intertubercular groove of the humerus. The latissimus dorsi is innervated by the thoracodorsal nerve, which receives fibers from the anterior rami of C6, C7, and C8.
The contraction of latissimus dorsi extends and medially rotates the humerus, like when trying to scratch at that annoying itch in the middle of your back.
It's also a powerful abductor and is the muscle used when pulling yourself up during rock climbing, chin-ups, or movements such as swimming or chopping wood.

Levator scapulae11:01–12:06

The second subgroup of the posterior axial appendicular muscle group consists of two deep muscles, which are the levator scapulae and the rhohomboids.
The levator scapulae lies in the neck deep to the sternocleidomastoid muscle and the trapezius. Proximally, it attaches to the posterior tubercles of the transverse processes of the C1 to C4 vertebrae and then descends to distally attach to the medial border of the scapula superior to the root of the scapular spine.
The levator scapulae is innervated by the dorsal scapular nerve, which arises from the anterior rami of C4 and C5, as well as by direct branches from the anterior rami of the C3 and C4 cervical nerves.
Levator roughly translates as lifter, so when this muscle contracts, it elevates the scapula. It also fixes the scapula and holds it in place to resist downward forces like when carrying a heavy piece of luggage up the stairs.
It also rotates the glenoid cavity inferiorly by rotating the scapula downwards. Now let's have a look at the rhomboids, the muscles that are immediately located deep to the trapezius.

Rhomboids12:06–14:03

There are two of them, the rhomboid minor and the rhomboid major, which is below the minor. The proximal attachment of the rhomboid minor is to the nuchal ligament and the spinous processes of the C7 and T1 vertebrae.
Distally rhomboid minor attaches to the medial border of the scapula at the level of the spine of the scapula. The much larger rhorhomboid major proximally attaches to the spinous processes of the T2 to T5 vertebrae and distally to the medial border of the scapula inferior to the spine of the scapula.
Both rhomboids are innervated by the dorsal scapular nerve, which receives fibers from the anterior rami of C4 and C5. Now to help remember which vertebrae the levator scapulae and the rhomboid muscles originate from, you can recall the 4224 rule.
The levator scapulae originates from the first four vertebrae, C1 to C4. Then none of these muscles originate from the next two vertebrae, which is C5 and C6.
Then rhomboid minor originates from the next two vertebrae, which is C7 and T1. And finally, rhomboid major originates from the next four vertebrae, which is T2 to T5.
So, starting at the top of the vertebral column, 4 vertebrae for the origin of levator scapulae, 2 with no attachment, 2 for rhomboid minor, and 4 for rhohomboid major, 4224.
OK, now back to the rhomboids. Rrhomboid minor and major always contract together, producing scapular retraction and downward rotation of the scapula.
They also assist the serratus anterior in holding the scapula against the thoracic wall. These movements enable us to forcibly lower the arm like when driving a stake with a sledgehammer.
All right, now let's shift gears and then have a look at the scapulohumeral muscles which pass from the scapula to the humerus.

Scapulohumeral muscles14:03–14:36

These six muscles, also referred to as the intrinsic shoulder muscles, are the deltoid, the teres major, and the 4 rotator cuff muscles.
The rotator cuff muscles are the supraspinatus, infraspinatus, teres minor muscle, and subscapularis muscles. They're widely known as the sits muscles, and they're powerful rotators of the humerus.
Take note that the small t represents the teres minor. Let's have a closer look at the deltoid first, which forms the rounded contour of the shoulder.

Deltoid14:36–15:53

Proximally, the deltoid attaches to the spine of the scapula, the acromion process, and the lateral third of the clavicle.
Distally, it attaches to the deltoid tuberosity of the humerus. The deltoid is divided into three parts the clavicular.
A chromial And spinal parts. It's innervated by the axillary nerve, which receives fibers from the C5 and C6 spinal segments.
When the three parts contract simultaneously, it results in arm abduction when the arm is already held at 15 degrees of abduction.
The initial 15 degrees of abduction are done by the supraspinatus muscle. The parts of the deltoid muscle can contract independently, producing other arm movements.
When the spinal part contracts alone, it produces extension and lateral rotation of the arm. When the acromial part of the deltoid contracts independently, it causes the arm to abduct, while the contraction of the clavicular part causes the arm to flex and medially rotate.
The clavicular and spinal heads, also known as the anterior and posterior head, are used to swing the arms during walking.

Teres major15:53–16:38

Next, there's the teres major muscle, which is a thick rounded muscle attached proximally to the posterior surface of the inferior angle of the scapula.
Distally, the teres major muscle attaches to the medial lip of the intratubercular groove of the humerus. As teres major travels from the scapula to the humerus, its inferior border forms the inferior border of the lateral part of the posterior wall of the axilla.
This muscle is innervated by the lower subscapular nerve, which receives fibers from the anterior rami of C5 and C6. When Terry's major contracts, it abducts and medially rotates the arm.
All right, it's time to have a look at the legendary rotator cuff muscles, or simply the sits muscles. They form a cuff that surrounds the head of the humerus and holds it in the glenoid cavity, thereby stabilizing the glenohumeral joint.

Rotator cuff muscles (SITS)16:38–18:56

OK, let's take a closer look at the supraspinatus first. This muscle arises in the supraspinous fossa of the scapula and then extends distally to attach to the superior facet of the greater tubercle of the humerus.
It's innervated by the suprascapular nerve, which receives fibers from the anterior rami of C5 and C6. Instead of rotating the arm like the rest of the group, the contraction of the supraspinatus initiates abduction, which is moving the arms laterally away from the body.
Next, there's the infraspinatus, which is a thick triangular muscle that attaches proximally to the infraspinous fossa of the scapula and extends distally to attach to the middle facet of the greater tubercle of the humerus.
This muscle is also innervated by the suprascapular nerve. And when the muscle contracts, it rotates the arm laterally.
Next up is the teres minor. This is a narrow elongated muscle that attaches to the middle part of the lateral border of the scapula proximally and to the inferior facet of the greater tubercle of the humerus distally.
The teres minor is innervated by the axillary nerve, which receives fibers from the C5 and C6 spinal segments. Just like the infraspinatus, the teres minor muscle rotates the arm laterally.
Finally comes the subscapularis, a thick triangular muscle that proximally attaches to and fills the subscapular fossa. It then extends distally, forming the posterior wall of the axilla before attaching to the lesser tubercle of the humerus.
The subscapularis is innervated by the upper and lower subscapular nerves which receive fibers from the anterior rami of C5, C6, and C7.
As a member of the rotator cuff club, the subscapularis rotates the arm. Unlike infraspinatus and teres minor, which are lateral rotators, the subscapularis performs medial rotation.
OK, I understand that this was too big of a bite to chew on at once, so let's take a pause and see if the sits muscles sit well in your memory by trying to remember their names and their origins and insertions.

Quiz18:56–19:15

All right, now I know what you're thinking. The shoulder and the pectoral regions are tightly packed with muscles and other various structures.

Anatomical spaces19:15–21:23

Partly true. But between these large muscles, there are also gaps called anatomical spaces.
Anatomical spaces serve as passageways for nerves or vessels. You can think of these like freeway on and off ramps that closely intersect with each other.
One such anatomical space in the scapular region is called the quadrangular space. From a posterior view, the quadrangular space is bounded superiorly by the inferior border of the teres minor muscle.
The inferior border is bounded by the superior border of teres major. The medial border by the lateral side of the long head of the triceps brachii muscle, and the humeral shaft constitutes the lateral border.
The quadrangular space provides passage to the axillary nerve and the posterior circumflex humeral artery as they move through the posterior wall of the axilla to enter the scapular region.
As we've discussed, the axillary nerve innervates the teres minor and deltoid muscles, and the posterior circumflex humeral artery supplies the deltoid, teres minor and major, and the long head of the triceps brachii.
These structures also innervate and supply the glenohumeral joint. Another important space to look at is the triangular space, which is also located in the axilla medial to the quadrangular space.
When viewed posteriorly, its superior border consists of the inferior border of teres minor. The lateral border of this space is bound by the medial border of the long head of the triceps brachii muscle, while the inferior border of this triangular space is bound by the superior border of the teres major muscle.
The triangular space provides the passageway to the circumflex scapular artery as it branches off from the subscapular artery and curves around the lateral border of the scapula.
It passes posterior between the subscapularis and teres major to supply muscles on the back of the scapula. Then it joins the arterial anastomosis around the scapula.

Suprascapular nerve and artery21:23–24:06

OK, now let's shift gears again and talk about the suprascapular nerve and artery which innervate and supply blood to different local muscles.
The suprascapular nerve arises from the upper trunk of the brachial plexus, receiving fibers from the anterior rami of C5, C6, and sometimes C4.
It courses laterally across the posterior triangle of the neck, superior to the brachial plexus and then through an area called the suprascapular notch, which is inferior to the superior transverse scapular ligament to innervate the supraspinatus and infraspinatus muscles.
Running with the suprascapular nerve, we have the suprascapular artery which arises from the thyrocervical trunk in the neck.
However, instead of traveling below the superior transverse scapular ligament in the suprascapular notch, the artery passes over top of this ligament to supply the supraspinatus and eventually descends behind the neck of the scapula to supply the infraspinatus.
A good way to remember what travels above and below the superior transverse scapular ligament is to think of it as a bridge and remember the phrase army over navy.
The A in army represents the artery going over top of the ligament or bridge, and the N in navy represents the nerve. And we can remember this because the navy travels on the water which goes under bridges.
The suprascapular artery is important because it's involved in a network of anastomosing arteries located around the scapula.
There are several arteries in this network, including the dorsal scapular artery, branching from the transverse cervical artery traveling along the medial border of the scapula.
the suprascapular artery as it branches in the supraspinous fossa and infraspinous fossa, and the circumflex scapular artery as it travels along the posterolateral border of the scapula from its origin from the subscapular artery, which arises from the axillary artery.
Other areas of anastomosis are the anterior and posterior circumflex humeral arteries. These arterial anastomoses are very important, as they form an interconnected channel of vessels through which blood can still flow to the muscles even in case of a crisis like a stenosis, occlusion, or damage to one of the contributing arteries.
Let's take an example. In scenarios where the axillary artery is ligated or occluded between the first rib and the subscapular artery, such as an atherosclerosis, this would result in a loss of blood flow distal to this area.
However, the anastomosis between the subscapular artery, the suprascapular artery, and the dorsal scapular artery provide an anastomosis which would allow blood to reverse its flow through the subscapular artery and reach the third part of the axillary artery.
That was a lot, so press pause and stretch a little bit and see if you can answer the following question. Does the posterior circumflex humeral artery pass through the quadrangular space or the triangular space?

Quiz24:06–24:23

And now let's review the movements of the scapula and discuss the muscles that contribute to each movement. First, scapular elevation, which is an upward translational movement, is done mainly by the descending part of the trapezius muscle with contribution from the levator scapulae and the rhohomboids.

Movements of the scapula24:23–26:16

Depression, which is a downward translational movement of the scapula, is primarily a passive movement done by gravity. Active depression of the scapulae is done by the sternocostal head of the pectoralis major muscle, the latissimus dorsi.
The ascending part of the trapezius muscle, the inferior part of the serratus anterior, and the pectoralis minor. The scapulae also undergo protraction, which is when the scapulae are abducted away from the spine or pulled forward.
This movement is mainly done by the serratus anterior as well as by the pectoralis major and pectoralis minor. Then there's retraction, which is squeezing the scapulae together towards the midline of your back.
This is also known as abducting the scapulae towards the spine. This movement is principally made by the middle part of the trapezius muscle with help from the rhomboids and the latissimus dorsi.
Next, there's upward rotation of the scapula that results in an upward tilt of the glenoid cavity in the coronal plane. This movement is done by the inferior part of the serratus anterior and the descending and ascending parts of the trapezius.
Finally, there's downward rotation of the scapula which results in a downward tilt of the glenoid cavity. This one is made possible mainly by the latissimus dorsi with contributions of other muscles that include the levator scapulae, rhomboids, the pectoralis minor.
The sternalcoal head of the pectoralis major muscle as well as gravity. Finally, let's have a look at the muscles that abduct the arm, starting with the supraspinatus.

Arm abduction26:16–27:20

This muscle abducts the arm from 0 to 15 degrees. From here, the deltoid muscle takes over to abduct the arm the rest of the way.
However, as the arm is abducted, the scapula also upwardly rotates. This is known as scapulohumeral rhythm.
The movement occurs in a 2:1 ratio. For every 3 degrees of elevation, approximately 2 degrees occurs at the glenohumeral joint and 1 degree at the scapulothoracic joint.
Hence, when the arm has moved 180 degrees so that it's vertical at the side of the head, 120 degrees occurred at the glenohumeral joint, and 60 degrees occurred due to upward rotation at the scapulothoracic joint.
Recall that the upward rotation is done by the trapezius and serratus anterior muscles, which along with the supraspinatus and deltoid, work together to bring your arm from full abduction.
To full abduction All right, as a quick recap, there are 4 anterior axial appendicular muscles which are the pectoralis major.

Review27:20–29:30

pectoralis minor, the subclavius, and the serratus anterior, as well as the four posterior axial appendicular muscles which are the trapezius, latissimus dorsi.
The levator scapulae and the rhomboids. Next are 6 scapulohumeral muscles, which are the deltoid muscle, teres major.
Plus the four rotator cuff or sits muscles, these are the supraspinatus, infraspinatus, teres minor, and subscapularis, all of which support and act on the glenohumeral joint.
There's also the deltopectoral groove situated between the pectoralis major and deltoid muscle where the cephalic vein is found running superficially.
In the axilla, there are two important anatomical spaces, the quadrangular space. And the triangular space.
Next, there are the suprascapular nerve and artery which travel together, downwards and laterally to the superior transverse scapular ligament.
This is where the suprascapular nerve passes below the ligament and the suprascapular artery passes above it to supply the supraspinatus and infraspinatus.
There's also an important network of arterial anastomoses around the scapula, which include the suprascapular artery, the subscapular artery, scapular circumflex artery.
And the dorsal scapular artery. The movements of the scapula include elevation.
Depression Protraction Retraction Upward rotation And downward rotation. Finally, regarding abduction of the arm, the supraspinatus abducts from 0 to 15 degrees, and then the deltoid muscle takes over to keep abducting the arm.
As the arm is abducted, the scapula must be rotated upwards by the trapezius and serratus anterior.