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S1: Gross Anatomy : 9:00-10:00
Scribe: Andrew Treece
Friday, January 23, 2009
Proof: Dylan Vaught
Dr. Zehren
Posterior Triangle of Neck
Page 1 of 6
***NOTE: Dr. Zehren’s lectures have notes below the slides in the electronic files which I assume sum up his
main points for each slide. SCM = sternocleidomastoid muscle
I.
Posterior Triangle of Neck [S1]
II. Boundaries of Posterior Triangle [S2]
III. Side view of neck [S3]
a. Basically the posterior triangle is bounded by the SCM anteriorly, trapezius posteriorly, and clavicle inferiorly.
IV. Sterno Cleidomastoid Muscle [S4]
a. The SCM has two heads of origin, there’s a sternal head from the menubrium of the sternum and a clavicular
head from the medial third of the clavicle and those heads unite and the muscle passes up to insert into the
mastoid process of the skull and the lateral portion of the superior nuchal line.
b. Sometimes it’s called just the sternomastoid
V. Anterolateral view [S5]
a. This muscle will do a number of things. When there is unilateral contraction, it tilts the head toward the shoulder
on the same side that’s called lateral flexion.
b. The other thing is that it rotates the head so the face turns to the opposite side.
VI. Lateral View [S6]
a. When the muscles act bilaterally they flex the cervical spine.
b. This muscle also can act in reverse, if the head and neck are fixed in position, by virtue of the fact that it
attaches to the clavicle and sternum it can elevate the clavicle and sternum and by elevating the sternum the
ribcage is elevated so it can be used as a muscle of inspiration during forced labor inspiration.
c. During deep breathing it can help elevate the ribs by elevating the sternum.
VII. Torticollis [S7]
a. Now, there’s a condition called torticollis which in Latin means “twisted neck”.
b. Due to injury to SCM and sometimes occurs at birth during delivery when a child is born, and when the OB is
pulling on the infants head and if it’s pulled too far to one side it can tear some of the fibers of the SCM and over
time that tear is replaced by fibrous connective tissue and cause a permanent contraction of the muscle.
c. See how the neck is lateral flexed toward the same side and the face toward the opposite side.
d. In severe cases a surgeon can go in and divide the muscle from the clavicle so the person can move his head
properly.
VIII. Trapezius Muscle[S8]
a. Very large muscle on the back with origin that’s quite extensive.
b. Trapezoid shaped and comes from the medial part of the superior nuchal line and from the INEON or External
Occipital Protuberance.
c. It also arises from the midline of the vertebral column called the Nuchal ligament that is a fibrous connective
tissue structure that is in the midline of the back of the neck and from all of the spines.
d. Notice how the fibers sweep down from the skull and laterally from the middle portion and upward from the
lower portion
e. The upper fibers insert into the lateral third of the clavicle (elevate shoulders), the middle fibers to the scapula
(pull shoulder blades back), and lower fibers to spine of scapula (depress scapula).
f. Important function: It also rotates scapula so glenoid cavity which is the socket where the head of humerus
articulates, so when you elevate your arm above your head, the entire scapula rotates on the chest wall.
IX. Spinal Accessory Nerve [S9]
a. Both of these muscles are innervated by the spinal accessory nerve (CN 11).
b. It runs across the posterior triangle.
X. Roof of Posterior Triangle[S10]
XI. Muscles of Facial Expression [S11]
a. The platysma muscle is a very, very thin flat muscle lying just deep to the skin and extends across the lower part
of the posterior triangle and helps form the roof.
S1: Gross Anatomy : 9:00-10:00
Scribe: Andrew Treece
Friday, January 23, 2009
Proof: Dylan Vaught
Dr. Zehren
Posterior Triangle of Neck
Page 2 of 6
b. It is a muscle of facial expression, and much of it is in the neck
c. The other thing that forms the roof is the investing fascia which forms a complete tube around the neck.
XII. Floor of Posterior Triangle[S12]
XIII. Muscles of Floor of Posterior Triangle [S13]
a. Like the roof it’s full of both muscular and fascial tissue.
b. There is the Splenius capitis, and just below that is the levator scapulae, and then three scalene muscles
(posterior, anterior, and middle).
c. These muscles form a floor to the triangle.
XIV.
Fascial floor of posterior Triangle S[14]
a. To see these you have to remove the roof and the fascia that covers them directly, which is the prevertebral
layer of fascia, a different one than from before.
XV. Splenius Capitis Muscle [S15]
a. Now the splenius capitis is really a back muscle in the back of the head, but it’s considered along with the
intrinsic muscles in the back.
b. You can see it arises from the lower part of the nuchal ligament and from the upper thoracic spines.
c. The direction of the fibers is superio-lateral, so it inserts into the mastoid process and the adjacent area of the
skull.
d. When this contracts, if bilaterally, will extend the head and pull it back. If only one muscle contracts, it will pull
the head toward the same side.
e. Dorsal rami of cervical nerves innervate the splenius capitis.
XVI.
Levator Scapulae Muscle [S16]
a. Now the levator scapulae arises from the upper four transverse processes of the cervical vertebra and inserts
into the upper part of the medial border of the scapula.
b. Named because of function (pulls up), and helps the trapezius in shrugging the shoulders.
XVII.
Innervation of Levator Scapulae [S17]
a. It’s innervated by this nerve called the dorsal scapular nerve.
b. It runs deep to the muscle and runs along the medial border of the scapula and innervates some other muscles
as well.
XVIII.
Scalene Muscles[S18]
a. Scalene muscles, there are three, and they all arise from the transverse processes of various cervical vertebrae.
b. They insert, in the case of the anterior middle scalenes, they run down to insert to the first rib.
c. The posterior scalene is blended at its origin with the middle scalene and can be difficult to separate, but if you
follow it down you’ll find that it inserts into the second rib.
XIX.
Muscles of Cervical Intervertebral Joints [S19]
a. So three scalene muscles on each side that bilaterally flex the cervical spine, along with muscles like the SCM.
b. They can also, if the head and neck are fixed by other muscles, they can elevate the first two ribs and can be
used as muscles of inspiration under certain conditions.
c. Unilaterally they are going to bend the cervical spine to the same side, and that’s called lateral flexion.
XX. Innervation of Prevertebral and Scalene Muscles[S20]
a. Innervation of these scalenes you will not see in lab, but you can appreciate they will be innervated by ventral
rami of cervical nerves because they are more on the front of the vertebral column.
b. These represent the ventral rami of the first four cervical nerves and there are little tiny twigs coming off these
ventral rami, and some of these go to the scalene muscles and other prevertebral muscles.
XXI.
XXII.
XXIII.
Contents of Posterior Triangle [S21]
Omohyoid Muscle[S22]
a. Well, there’s one muscle that is between the roof and floor that’s called the omohyoid muscle.
Omohyoid Muscle[S23]
S1: Gross Anatomy : 9:00-10:00
Scribe: Andrew Treece
Friday, January 23, 2009
Proof: Dylan Vaught
Dr. Zehren
Posterior Triangle of Neck
Page 3 of 6
a. This is a rather strange muscle, it has two bellies, an inferior and superior belly.
b. The inferior only is in the posterior triangle and the superior is in the anterior triangle.
c. This is best considered with other muscles in the front of the neck that we will get to later.
d. The inferior belly does however cross the confines of the posterior triangle and it’s used descriptively to divide
the posterior triangle into two smaller triangles.
XXIV.
XXV.
XXVI.
Occipital and Omoclavicular [S24]
a. We have a large superior triangle called the occipital triangle.
b. Also there is a much smaller triangle below the omohyoid that goes by any of these three names:
Omoclavicular, supraclavicular, or often subclavian because the third part of the subclavian artery is found in
this area.
c. It’s a very thin muscle that can be so thin it doesn’t look like muscle.
Nerves [S25]
Carefree and Careful zones [S26]
a. There are some very important nerves in the posterior triangle.
b. One of the most important is the accessory nerve. It crosses the posterior triangle and this can divide the
posterior triangle into a CAREFUL zone and a CAREFREE zone.
c. This means that the carefree zone that is superior to the accessory nerve, you can dissect carefree because
there is not much there to worry about.
d. Down below the accessory nerve in the careful zone there are a lot of structures that you need to be careful not
to injure.
XXVII. Carefree and Careful Zones 2 [S27]
a. Down below the nerve you see the subclavian artery, brachial plexus, and a lot going on that can easily be cut,
so be careful.
XXVIII. Accessory Nerve [S28]
a. This is a strange nerve in that it is the 11th cranial nerve but it doesn’t arise from the brain.
b. It arises from the upper five segments of the cervical spinal cord.
c. This is a coronal section through the skull, and the unusual thing about the accessory nerve is that the efferent or
motor fibers cell bodies lie in the ventral horn of the upper five segments of the cervical spinal cord.
d. Their axons leave the cord and join together to form a single accessory nerve that ascends within the vertebral
canal, enters the skull through the foramen magnum, and no sooner gets in the skull that is leaves through the
jugular foramen which is why it’s considered a cranial nerve.
e. Then it runs inferiorly across the posterior triangle giving off branches to the SCM then disappearing deep to the
edge of the trapezius and innervating it on its deep surface.
f. The accessory nerve also has afferent fibers in it according to Netter. The SCM and trapezius are also
described as being innervated by cervical nerves 2, 3, and 4. They send fibers into these muscles, and these
cervical fibers are both efferent and afferent and are supplemental to the accessory nerve and provide motor
innervation but also have afferent or proprioceptive fibers from these muscles going back to the spinal cord.
g. Not all people agree with this, but just be aware that the SCM and trapezius do have innervation from cervical
nerves 2, 3, and 4.
XXIX.
Brachial Plexus [S29]
a. Another major structure in the posterior triangle is the brachial plexus which is the nerve network that
innervates the entire upper limb. A plexus means a network, and in the case of nerve plexus it usually means
ventral rami that form the plexus.
b. The brachial plexus has five major parts, and can be remembered by the pneumonic Roger Travis Drinks Cold
Beer.
c. The entire brachial plexus is formed by ventral rami of C5, 6, 7, 8 and T1. The ventral rami of those nerves
form the roots of the plexus. They are referred to as the roots because they are the very beginning, but in fact
they are not the roots of these spinal nerves, they are ventral rami.
d. Then these roots, or rami, will unite to form superior, middle, and inferior trunks. C5 and 6 form the superior
trunk, C7 continues by itself to form the middle trunk, and C8 and T1 form the inferior trunk.
e. Each trunk then divides into the anterior and posterior division for a total of 6 divisions.
S1: Gross Anatomy : 9:00-10:00
Scribe: Andrew Treece
Friday, January 23, 2009
Proof: Dylan Vaught
Dr. Zehren
Posterior Triangle of Neck
Page 4 of 6
f. The three posterior divisions all unite to form a single structure called the posterior cord. The anterior divisions
of the superior and middle trunks unite to form the lateral cord. And the anterior division of the inferior trunk
continues as the middle cord. So we have three cords as well (3 anterior divisions and 3 posterior divisions)
g. The cords divide into five terminal branches, and these are the chief branches of the brachial plexus.
h. They are the radial nerve, which is the largest of all the terminal branches and comes off the posterior cord.
There is an axillary nerve that also comes off the posterior cord. The lateral cord gives rise to the
musculocutaneous nerve. The medial cord gives rise to the ulnar nerve. And both the lateral and medial cords
give rise to the median nerve. These five terminal branches descend into the upper limb to innervate the
muscles and skin of the upper limb, both motor and sensory.
XXX.
Brachial Plexus in Relationship to Clavicle [S30]
a. The divisions of the plexus occur behind the clavicle, so in lab we only saw the upper parts of the plexus
XXXI.
Five components to cervical plexus[S31]
a. Another plexus we have to consider is the cervical plexus, which is formed by the ventral rami of the first four
cervical nerves.
b. Notice how the ventral rami of the first four cervical nerves communicate with each other to form a plexus.
c. There are descriptively five parts of the cervical plexus. There are twigs going to the scalenes. The C2, 3, 4 go
to the SCM and trapezius often by communicating with the accessory nerve. We want to concentrate on the
cutaneous branches of the cervical plexus because these are the ones you can see in lab.
XXXII. Cutaneous Branches of Cervical Plexus[S32]
a. This is a good picture to look at when you’re dissecting these nerves.
b. You see there are four cutaneous nerves, which means they are sensory to skin, eminating from the posterior
edge of the SCM muscle just above the midpoint. This is called the nerve point of the neck, or Erbs point, and
that is where you look for the beginning of these nerves in the posterior triangle.
c. They are identified by the course they take. The great auricular nerve is an easy one to find and it runs
vertically across the SCM and the name tells you that it goes to the ear. This nerve accompanies the external
jugular vein.
d. The lesser occipital is difficult to dig out but it runs along the posterior edge of the SCM and goes up behind the
ear and innervates the scalp behind the ear and a little of the ear itself.
e. There is a transverse cervical nerve that runs transversely across the SCM and branches to innervate the skin
of the anterior triangle of the neck.
f. Finally, we have the supraclavicular nerves that arise as a trunk then divides into medial, intermediate, and
lateral supraclavicular nerves. These innervate the skin of the upper chest and shoulder region, and they pierce
the platysma but don’t innervate it. All of these are strictly sensory. If you cut too deeply you will remove these
nerves.
g. Notice that most of these nerves have fibers from the second and third cervical nerves in them, except for the
supraclaviculars which have fibers from C3 and 4.
XXXIII. Vessels[S33]
XXXIV. Arteries in Posterior Triangle[S34]
a. The subclavian artery is the chief artery supplying the upper limb.
b. This artery on each side is divided into three parts based on its relationship to the anterior scalene.
c. The first part of the artery is the part that’s medial to the anterior scalene muscle, the second part is posterior to
the anterior scalene, and the third part which is the part that actually lies in the posterior triangle is lateral to the
anterior scalene.
d. As soon as this artery crosses the outer margin of rib one, it changes its name. It now enters the armpit and
now is called the axillary artery. (The subclavian and axillary are the same artery in different locations).
e. So that is the chief artery, but there are a couple of smaller arteries to look out for: transverse cervical and
suprascapular arteries which cross in front of the anterior scalene muscle
XXXV. Arteries in Posterior Triangle 2 [S35]
a. The transverse cervical and suprascapular arteries emerge as branches of the thyrocervical trunk, which is a
direct branch of the subclavian.
b. As they run across the posterior triangle they are headed toward the scapular region and that’s the area they
supply.
S1: Gross Anatomy : 9:00-10:00
Scribe: Andrew Treece
Friday, January 23, 2009
Proof: Dylan Vaught
Dr. Zehren
Posterior Triangle of Neck
Page 5 of 6
XXXVI. External Jugular Vein [S36]
a. The chief vein is the external jugular vein.
b. This vein is in the posterior triangle even though it begins up near the angle of the mandible.
c. It is formed by the union of the tiny vein behind the ear, the posterior auricular vein, and the posterior division of
the retromandibular vein.
d. The external jugular vein descends superficial to the SCM muscle, and then pierces the fascia of the roof of the
triangle, the investing fascia, enters the posterior triangle, descends, and ultimately drains into the subclavian
vein.
e. You may see small veins running into it which accompany the similarly named arteries, the transverse cervical
and suprascapular veins which run with the arteries are tributaries to the external jugular vein.
f. The vein can vary in size, it can be quite conspicuous or may be very small, but it clinically is important because
if the pressure gets outside of normal range if there is congestion, it will become swollen, and during a physical
exam, if a physician notices an engorged external jugular he might be suspicious to know what is causing it
such as heart disease, obstruction of the superior vena cava by a tumor, or lymph nodes that might be enlarged
due to the spread of cancer, so it’s almost like a barometer to measure venous pressure.
XXXVII. Interscalene Triangle [S37]
XXXVIII.
Boundaries of Interscalene Triangle [S38]
a. Want to talk about the relationships of the interscalene muscle. Here you see the anterior and middle scalene
muscles inserting into rib one, and those two muscles along with rib one form a scalene triangle.
b. Two important muscles emerge through that interscalene triangle, the subclavian artery and the brachial plexus.
c. They pass between the anterior and middle scalenes within the triangle, and is important because if the
interscalene triangle is narrowed for whatever reason such as presence of a seventh cervical rib.
XXXIX. Cervical Ribs and Related anomalies [S39]
a. Something that narrows the interscalene triangle could potentially compress the subclavian artery and/or the
brachial plexus.
b. Here is a well developed cervical rib impinging on the subclavian artery. The result will be ischemia, reduced
blood supply, to the upper limb.
XL. Cervical Ribs and Related anomalies 2[S40]
a. On the other hand, a cervical rib could compress part of the brachial plexus.
b. Here you can see how the lowest trunk of the brachial plexus is elevated by this extra rib, and that compression
that the rib puts on the plexus is going to cause nerve disturbances in the lower trunk that can lead to pain or
parasthesia (abnormal sensations like tingling or burning) in the upper limb, it could result in muscle weakness if
motor fibers are affected.
c. The bottom line is that compression of the subclavian artery or brachial plexus is called anterior scalene
syndrome, and in this case they would have to go in and remove the cervical rib, which is not a common feature.
Anywhere from 0.5 to 1% are said to have cervical ribs.
XLI.
Anterior Relationships of Anterior Scalene Muscle [S41]
a. The anterior scalene also has some anterior relationships.
b. We have already mentioned that the transverse cervical artery and suprascapular artery cross in front of the
anterior scalene and if they do that they also run in front of the phrenic nerve. This is a nerve you can find
descending directly in front of the anterior scalene muscle and clamped down to the muscle by these two arteries.
c. If a surgeon wants to access the phrenic nerve in the neck he can reflect the SCM muscle and find the phrenic
nerve right on the anterior scalene. He may wish to block that nerve, anesthetize it, or crush it. The phrenic nerve
innervates the diaphragm, the most important muscle of inspiration.
d. You have two hemidiaphragms, one on each side, so you may want to paralyze half of the diaphragm if you
want to prevent one side from contracting if there is some lung or respiratory problem and you don’t want the
diaphragm on one side functioning for a period of time. Gradually it will regenerate.
e. The subclavian vein is also important, and it’s not in the interscalene triangle but it’s anterior to the muscle. This
is the vein that drains the upper limb.
XLII.
Subclavian Vein Puncture [S42]
a. Now this vein is very important clinically because it’s frequently used for vein puncture. Why might you want to
put a catheter in the subclavian vein? Possibly to administer drugs, or to administer nutritional fluids, or maybe
to measure venous pressure. So catheterization of the subclavian vein is a common procedure.
S1: Gross Anatomy : 9:00-10:00
Scribe: Andrew Treece
Friday, January 23, 2009
Proof: Dylan Vaught
Dr. Zehren
Posterior Triangle of Neck
Page 6 of 6
b. A needle is introduced just below the clavicle. When this is done, it is usually done on the right side if possible
because you have the thoracic lymphatic duct on the left side.
XLIII.
XLIV.
XLV.
XLVI.
Cervical Pleura [S43]
a. There is some danger with this procedure. If the needle goes to deeply and pierces the vein it could go and also
pierce the cervical pleura which is a membrane extending up from the thoracic cavity and lining the pleural
cavity, so if you penetrate that membrane and penetrate that lung underneath, you’ll get air leaking out of the
lung into the cavity underneath.
b. This is called a pneumothorax, air in the pleural cavity, so the lung collapses and that is an extremely serious
situation.
c. There is also potential for piercing the subclavian artery, and that can lead to bleeding into the pleural cavity
which is called a hemothorax.
Deep Fasciae of Neck [S44]
Fascial Layers of Neck [S45]
a. This is a nice slide from the Netter Atlas that shows you the deep fasciae of the neck. This is a transverse
section through the neck anteriorly. You can see the cervical viscera, trachea, esophagus, thyroid gland here in
front, and the vertebral column.
b. There are three basic layers of deep cervical fascia.
c. The first is called the investing fascia shown in red. This forms a complete tubular investment around the neck,
just deep to the superficial fascia and skin. The superficial fascia is the stuff directly beneath the skin that has
fat and cutaneous nerves and vessels, but the deep fascia here is more membranous and defined. You’ll notice
the investing fascia splits to enclose the SCM muscle and then those two layers come together as a single layer,
then they split again to enclose the Trapezius muscle. In between those two muscles is a single layer that’s the
roof of the posterior triangle on the side of the neck.
d. The second layer of deep cervical fascia shown in orange is the prevertebral fascia, which is misleading
because it not only covers the prevertebral muscles but continues laterally across the floor of the posterior
triangle to cover the scalene muscles, and then continues around the back of the neck. So it too forms a tube,
although a smaller one, enclosing prevertebral and some of the muscles of the back of the neck.
e. The third layer is the visceral fascia that surrounds the cervical viscera. It has two components: a pretracheal
layer in blue that forms a sheath around the thyroid gland, and another part called the buccopharyngeal fascia in
green that connects one lobe of the thyroid to the other lobe and passes posterior to either the pharynx or at this
level the esophagus. So it’s behind the gut tube, and together they make up the visceral layer of the deep
cervical fascia.
f. Now, there is one other fascial structure in the neck that is important. This is the carotid sheath, a fascia around
this neurovascular bundle in the neck and some books say the three layers of the deep cervical fascia contribute
to the carotid sheath. It is a tube of fascia surrounding three structures as they descend in the neck: the
internal common carotid artery, internal jugular vein, and the vagus tenth cranial nerve which is behind and
between those two vessels.
g. An important SPACE lies behind the pharynx and esophagus, and is called the retropharyngeal space. From
this figure you can see this space is bounded (ignore orange middle line), and the retropharyngeal space is
defined as the space in the neck behind the buccopharyngeal fascia but in front of the prevertebral fascia, and
then on either side bounded by the carotid sheath. In that space is some loose tissue which allows the pharynx
and esophagus to expand when you swallow food. So you need mobility, but if an infection occurs in this space
it can spread throughout the space.
Fascial Layers of Neck (side view) [S46]
a. This slide shows you the vertical extent of the retropharyngeal space to the skull above and down into the
chest below behind the heart, the area called the mediastinum.
b. Again, the buccopharyngeal fascia in green is the anterior boundary, the prevertebral fascia in orange is the
posterior boundary, and in between is the retropharyngeal space.
c. An infection may arise from a tooth abscess that can spread all the way to the chest, swallowing a chicken
bone that pierces the wall of the pharynx can lead to infection also.
(end time 00:46:00)