Superficial & Deep Back
Tabulae sceleti et musculorum corporis humani. Albinus, Bernhard Seigfried (1697-1770).
When you ask a student who has taken a course in human gross anatomy
what their favorite part of the body was to dissect, the answer is never the
superficial and deep back. While the back may not have the glamour of the head
or the uncanny quality of the digestive organs, the function of the back is
interesting if you think about it. Let me illustrate. On those friday evenings you
spend at your favorite bar looking for that special someone there is a deep
evolutionary drive that is at work here weeding out the inappropriate
candidates for procreation. Now you may be thinking, “I’m just looking for some
fun, who said anything about making babies?” I did. In the following example, tell
me which of the following individuals is going to appear more attractive to you.
First, a person who sits or stands upright at the bar with their shoulders
squared, but still appearing relaxed. Second, a person who is either standing, but
leaning on the bar or sitting somewhat slumped on the barstool with their
shoulders rounded. Of course you picked the first person after reflecting on the
scenario. The muscles found on the superficial and deep back are the ones
working to achieve this affect. Well these muscles and a persons self esteem.
I would also suggest that the surface of the back has a sexual allure to it as
well. As evidence for this statement, consider the one of the many awards shows
next time they are on television and count the number of backless gowns worn
by the actresses / female singers. These ladies are not wearing gowns like this
because it is hot outside. The back also functions as a large palate for the tattoo
artist allowing space for a detailed horimono design. Finally, no discussion about
the back would be complete without mention of the vertebral column. The
vertebral column is interesting to any person who has suffered a herniated disc
or knows a person with scoliosis. Chiropractors naturally love spinal anatomy, if
not why choose that profession? Inside of the vertebral column you will find the
spinal cord and emerging through the intervertebral foramen, individual spinal
nerves.
 Palpation Features: The external occipital protuberance (inion) is in the center
of the superior nuchal line and that is the superior attachment of the trapezius
muscle. The vertebra prominens (C7 spinous process) is not only palpable, but
sometimes visible on a person when viewed from a lateral perspective. The
acromion of the scapula is the bony landmark found lateral to the clavicle and is
sometimes visible where the trapezius meets the deltoid at the superior aspect of
the shoulder. If you follow the acromion in a posterior direction, it continues as
the spine of scapula. The inferior angle of scapula is occasionally obscured by the
upper border of the latissimus dorsi, but when you find it, it should be in the same
horizontal line as the 7th thoracic vertebrae and in the same vertical line as the
angle of the ribs. The iliac crests are inferior points of attachment for the
latissimus dorsi and are often obscured by the fat of the region commonly called
“The muffin top”. If you locate the most superior edge of the left and right iliac
crest and imagine a horizontal line running between the two points then that line
normally will pass through the space between the 3rd and 4th lumbar vertebrae.
The clinical relevance of this is that this is the location a needle is typically
inserted to collect cerebrospinal fluid (spinal tap).
 Note: Keep in mind that even though we don’t dissect the vertebral column you
will be responsible for these bones and the ligaments that secure them to one
another.
 The Back: Anatomical borders of “the back” are from the inferior portion of the
neck to the buttocks in the vertical plane, and from the angle of the ribs on the left
to the angle of the ribs on the right. “The back” is not to be confused with the
definition of the back in the Sir Mix-a-lot song “Baby Got Back”, which is clearly
a reference to the buttocks. Even a person who is not an anatomist knows this.
 Neurovascular bundles: What you are seeing here are the branches of dorsal
rami along with an artery and a vein. They can be found at regular intervals
(superior to inferior) emerging through trapezius and latissimus dorsi just lateral
to the vertebral column. If you see branches closer to the midaxillary line, what
you are seeing are lateral cutaneous branches of intercostals nerves. And as you
may already know, intercostal nerves are branches of ventral rami. Most of the
neurovascular bundles are cut when the superficial fascia is reflected, so I add this
feature for the occasional times these bundles escape the swipe of the scalpel.
 Trapezius: One of the superficial back muscles that you may hear being
described in a clinical setting in three parts, upper fibers, middle fibers and lower
fibers. Although it looks like one muscle when you look at the entire back, keep
in mind that the left and right sides have independent innervation, so it is possible
to come across a patient with only one side functioning. How might this happen
you ask? Lets say we have a patient with cancer that spread into the lymph nodes
of the cervical region. If a good but overworked surgeon accidentally cuts the
spinal accessory nerve while removing one of these lymph nodes that would be
one cause of such a presentation.
Figure 1: Left side spinal accessory nerve lesion
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Spinal Accessory Nerve (CN XI): In this dissection you can find this nerve on
the deep surface of trapezius. It emerges from an opening in the inferior portion of
the skull called the jugular foramen. It is the motor nerve to this muscle as well as
to the sternocleidomastoid, a muscle of the neck.
Transverse Cervical Artery: The source of this artery is the thyrocervical trunk.
The transverse cervical artery has two parts, the superficial branch (which we see
on the deep surface of trapezius) and a deep branch that is also called the dorsal
scapular artery. Note that occasionally trapezius receives some of its blood supply
from a branch of the suprascapular artery. If such a branch exists in your cadaver
it will be cut when you reflect trapezius.
Occipital Artery: The source of this artery is the external carotid artery. Like the
transverse cervical artery, we can’t see it’s source in this dissection, but should
you be wondering where it is coming from, now you know. This artery will be
emerging from the area of the mastoid process then ascending the posterior
cranium deep to the splenius capitis and superficial to the semispinalis capitis, and
piercing trapezius just lateral to the greater occipital nerve. The connective tissue
in this area tends to be rather dense and most students will cut the artery along
with the greater occipital nerve. So, if you find that you can expose these
structures with a minimal amount of damage you should feel pretty good about
yourself for today. (You will only be exposing the most superior portion of this
artery today, don’t remove musculature inferior to the superior nuchal line except
trapezius)
Greater Occipital Nerve: Find this nerve piercing the trapezius muscle
approximately 3 cm inferolateral to the inion. Recall from lecture the difference
between a ventral ramus and dorsal ramus of a spinal nerve. The greater occipital
nerve is part of the dorsal ramus of spinal nerve C2. Just as with the occipital
artery, please don’t try to dig this nerve out from deep to the muscle today.
Hmmm, was it the roots or the rami that allows you to determine the function of a
group of neurons?
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Latissimus Dorsi: While the origin of this muscle should be clearly observable in
this dissection, its insertion may not be as obvious at this point. Once you can
appreciate the path this muscle follows, its actions on the upper extremity become
obvious. So, take note of how the superior portion of this muscle intervenes
between the thorax and the arm on its way to the floor of the intertubercular
sulcus of the humerus. In today’s dissection you may find that latissimus
interdigitates with the external oblique and possibly serratus anterior so be
mindful of this when reflecting the muscle. The innervation of the latissimus dorsi
is the thoracodorsal nerve.
Thoracodorsal nerve: This nerve comes from the posterior cord of the brachial
plexus, specifically spinal levels C6-C8. That last sentence may not mean much to
you now, but it will in a few weeks. It can be identified as it emerges from the
area of the axilla and enters the deep surface of latissimus dorsi. You will see
more proximal portions of this nerve when you do the pectoral/axilla dissection.
Thoracodorsal artery: This artery is the primary blood supply for latissimus
dorsi as you may have guessed. This artery one of the terminal branches of the
subscapular artery. What I mean by “terminal branch” is that the subscapular
artery bifurcates, meaning instead of giving off any more branches it simply splits
into two arteries, like reaching a fork in the road. The other terminal branch
(circumflex scapular artery) is discussed in the scapulo-deltoid dissection.
Rhomboid Minor & Major: You find these muscles deep to the trapezius
securing the scapula to the vertebral column. Note that rhomboid minor attaches
at the root of the scapular spine while rhomboid major attaches more inferiorly on
the vertebral border of the scapula. Use this information to differentiate one from
the other. Both muscles are innervated by the dorsal scapular nerve (C4,C5)
which may be seen in this dissection if you are careful not to hack it apart. The
dorsal scapular nerve can be found close to the medial border of the scapula
traveling from the deep surface of levator scapulae to rhomboid minor then to
rhomboid major. The dorsal scapular nerve should lie in close proximity to the
artery of the same name.
Levator Scapulae: After the rhomboids have been cut from their vertebral
attachments, pull the scapula laterally and note how levator scapulae splits into
four slips as it approaches its superior attachments. The superior attachments I
speak of are the posterior tubercles of the transverse processes of cervical
vertebrae 1 – 4.
Serratus Posterior Superior & Inferior: These muscles will be tricky to dissect
out for two reasons. (1) They are thin flat muscles (2) their fiber orientation
matches the muscles superficial to them. Have fun with that. The serratus
posterior superior lies deep to the rhomboids while the serratus posterior inferior
lies deep to latissimus dorsi. Both are innervated by intercostals nerves. Recall
that intercostals nerves are NOT dorsal primary rami. These two serratus muscles
are considered to be the intermediate layer of back muscles and you should not
include the serratus anterior muscle in this group.
Thoracolumbar Fascia: This tissue separates the intrinsic (true) back muscles
from the extrinsic back muscles. What you see here is the superficial portion. The
deep portion gives attachment to the transversus abdominus and the internal
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obliques, which will be discussed in the anterior abdominal wall dissection. Cut
the superficial portion close to the vertebral column in the lumbar region and
reflect it laterally when you want to begin exposing the true (deep) back muscles.
Erector Spinae: These muscles can be divided into three columns from lateral to
medial they are iliocostalis, longissimus, and spinalis. The columns can be
separated and differentiated by running the fingertips between them.
Multifidus Muscles: These muscles help to stabilize the spine and are part of the
transversospinalis group. Note the “up and in” orientation of the muscle fibers
making them contralateral rotators when contracting unilaterally. Multifidus gets
its name from the Latin terms multus, many + findere, to split, which gives us
divided into many sections. An appropriate name for a muscle that travels up the
entire vertebral column (Sacrum to C2) with the longest fibers spanning only 4
vertebrae. You will have to move the erector spinae muscles laterally to view the
multifidus.
Anterior Longitudinal Ligament: Attaches superiorly at the basilar part of the
occipital bone and extends inferiorly, superficial to the vertebral bodies, to the
anterior portion of the upper sacrum. This ligament is most adherent to the
intervertebral discs, hyaline cartilage and vertebral end plates. It is more loosly
attached (by comparison) to the vertebral bodies themselves.
Posterior Longitudinal Ligament: Found on the posterior surface of the
vertebral bodies C2 (superiorly) to the sacrum (inferiorly). Between C2 and the
occipital bone this ligament blends with the cranial dura to form the tectorial
membrane. This ligament is broad and more uniform in width through the cervical
and thoracic regions while in the lumbar spine it narrows as it crosses the
vertebral bodies and widens as it crosses the IV discs. This makes sense as this
allows the ligament to help prevent posterior herniation of the nucleus pulposis,
which would compress the spinal cord or cauda equina.
Apical ligament: Spans the distance between the superior aspect of the dens and
the anterior aspect of the foramen magnum. This is separate / distinct from both
anterior and posterior longitudinal ligaments.
Alar Ligaments: This pair of ligaments attach to the postero-lateral aspects of the
dens to the medial sides of the occipital condyles. (One on the left and one on the
right, just so that we are clear). Functionally, they are thought to limit the amount
of rotation between the atlas and the axis. Then why don’t they attach to the atlas
you may be thinking? Well, the atlas and the cranium rotate more as a single unit
on the dens than as two separate parts.
Anterior and posterior atlanto-occipital membranes: While the anterior
atlanto-occipital membrane somewhat blends with the anterior longitudinal
ligament, the posterior atlanto-occipital membrane does not blend with the
posterior longitudinal ligament. The posterior atlanto-occipital membrane will be
removed to expose the vertebral artery in dissection 2, suboccipital region.
Some Clinical Curiosities…
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Rhomboid of Michaelis: This anatomical region on females can be used to get a
rough estimate of the dimensions of the birth canal. Named for gynecologist G.A.
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Michaelis, the boundaries for this landmark are the left and right PSIS, the
spinous process of L4 vertebrae, and the upper part of the natal cleft. In a normal
female, the vertical and horizontal dimensions of this rhomboid are roughly equal.
While I can find several references for this landmark, its obstetric utility is
virtually non-existent in the literature. The Rhomboid of Michaelis will not be on
the exam, I include it because sometimes the history of medicine can be
interesting. (Fig. 4.2)
Triangle of Auscultation: Found between the medial border of the scapula,
upper border of latissimus dorsi, and the inferior/lateral border of trapezius. This
puts you superficial to the sixth intercostals space. In this triangle there is a
minimal amount of superficial tissue and therefore an ideal place to position a
stethoscope to listen to the lungs. It is common to hear this in most clinical
settings. On a fit individual, you should be able to find this landmark with some
ease. In a more sedentary person you just have to ballpark the area. To increase
the size of the sixth intercostal space, have the patient flex forward with both arms
folded across the chest.
Lumbar Triangle (of Petit): Inferiorly bordered by the iliac crest, medially by
the latissimus dorsi and laterally by the external abdominal oblique. While it is a
rare occurrence, this may be the site of a hernia (lumbar hernia).
Jefferson (Burst) Fracture: Classically described as a fracture of 4 parts of the
atlas resulting from a compression force delivered through the axial skeleton by
way of the head. Example, diving head first into the shallow end of a swimming
pool. If you look at the coronal section of the craniovertebral and atlantoaxial
joints you can see how a compression force delivered through the head would
effectively drive the two lateral masses of the atlas in opposite directions. This of
course a result of the orientation of the facet (zygopophyseal) joints. One of the
real dangers here is a rupture of the transverse ligament that secures the dens to
the atlas. If this ligament is ruptured there is a possibility of compressing the
spinal cord within the vertebral canal. (illustration below)
A
B
C
A: inferior portion of occipital bone
B: atlas (C1)
C: axis (C2)
Figure 2: MRI Jefferson Burst Fracture