The Back

THE BACK

TABLE OF CONTENTS

Click hyperlinks to go to the assigned dissection.

SURFACE ANATOMY ON THE CADAVER

OSTEOLOGY EXERCISE

DISSECTION ASSIGNMENT: MONDAY, SEPTEMBER 14 – BACK I

DISSECTION ASSIGNMENT: WEDNESDAY, SEPTEMBER 16 -BACK II

DISSECTION ASSIGNMENT: MONDAY, SEPTEMBER 21 – BACK III

DISSECTION ASSIGNMENT: TUESDAY, SEPTEMBER 22 – PECTORAL REGION I

DISSECTION ASSIGNMENT: WEDNESAY, SEPTEMBER 23 – PECTORAL REGION II

The back region contains the superficial muscles of the back, the intermediate muscles of the back, and the deep muscles of the back. All of these muscles attach to the vertebral column. The vertebral column serves the dual purpose of forming the axis of the body and providing a protective bony covering for the spinal cord.

Surface Anatomy on the Cadaver

Surface anatomy is studied on a living subject as part of your assigned SURFACE ANATOMY exercises. You will be participating in surface anatomy exercises on living subjects with your lab partners at another time. In the cadaver, fixation may make it difficult to distinguish bone from well-preserved soft tissues. However, before beginning your dissection, you should make an effort to palpate, or at least appreciate the general location of, specific anatomic landmarks. Turn the cadaver to the prone position (face down) and attempt to palpate the following structures SURFACE ANATOMY OF THE BACK

External occipital protuberance
Superior border of the trapezius muscle
Spinous process of the seventh cervical vertebra (vertebra prominens)
Spine of the scapula (at vertebral level T3)
Acromion of the scapula
Medial (vertebral) border of the scapula
Inferior angle of the scapula (at vertebral level T7)
Spinous processes of thoracic vertebrae
Erector spinae muscle (most noticeable in the lumbar region)
Median furrow
Lateral border of the latissimus dorsi muscle (posterior axillary fold)
Iliac crest (at vertebral level L4)
Posterior superior iliac spine

This image shows musculature that are palpable structures in the back: PALPABLE STRUCTURES IN THE BACK - MUSCULATURE

This image shows bony prominences that are palpable structures in the back: PALPABLE STRUCTURES IN THE BACK - BONY PROMINENCES

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Osteology

Refer to a skeleton. On the scapula, identify SKELETON OF THE BACK AND VERTEBRAL COLUMN

Acromion
Spine
Superior angle
Medial (vertebral) border
Inferior angle

On the ilium, identify:
Iliac crest
Posterior superior iliac spine

On the occipital bone, identify:

External occipital protuberance (inion)
Superior nuchal line

On the temporal bone, identify:

Mastoid process

The vertebral column consists of 33 vertebrae: 7 cervical (C), 12 thoracic (T), 5 lumbar (L), 5 sacral (S), and 4 coccygeal (Co). The upper 24 vertebrae (cervical, thoracic, and lumbar) allow flexibility and movement of the vertebral column, whereas the sacral vertebrae are fused to provide rigid support of the pelvic girdle. A typical thoracic vertebra will be described, and the cervical and lumbar vertebrae will be compared to it.

Refer to a disarticulated thoracic vertebra and identify: TYPICAL THORACIC VERTEBRA IN LATERAL AND SUPERIOR VIEWS

Body

Vertebral arch – formed by the combination of pedicles and laminae
Pedicle (2 – the two here indicates that there are two of them, right an left, per vertebra)
Lamina (2)
Vertebral foramen
Transverse process (2)
Transverse costal facet (2)
Spinous process
Articular processes – superior and inferior (2 each)
Articular facets – superior and inferior (2 each)
Vertebral notches – superior and inferior (2 each)
Costal facets – superior and inferior (2 each)

The spinous process of a thoracic vertebra is long, slender, and directed inferiorly over the spinous process of the vertebra that is inferior to it. Articulation with ribs is a characteristic of thoracic vertebrae. The tubercle of a rib articulates with the transverse costal facet of the thoracic vertebra of the same number (i.e., the tubercle of rib 5 articulates with the transverse costal facet of vertebra T5). The head of a rib articulates with the bodies of two adjacent vertebrae, (i.e. the head of the rib articulates with the superior costal facet of the vertebra of the same number and the inferior costal facet of the vertebra superior to it). PART OF THE THORACIC VERTEBRAL COLUMN

An intervertebral disc and the articular processes unite two adjacent vertebrae. The vertebral notches of two adjacent vertebrae combine to form an intervertebral foramen. A spinal nerve passes through the intervertebral foramen.

Cervical vertebrae differ from thoracic vertebrae in the following ways: COMPARISON OF CERVICAL, THORACIC, AND LUMBAR VERTEBRAE

Cervical vertebrae have smaller bodies; larger vertebral foramina; shorter spinous processes, which bifurcate at the tip; and transverse processes that contain a foramen transversarium. On a disarticulated cervical vertebra and on an articulated skeleton, identify the following features common to all cervical vertebrae:

Transverse process
Foramen transversarium
Spinous process

Using a disarticulated atlas and axis, and on an articulated skeleton, observe the following features of individual cervical vertebrae:

Atlas (C1) does not have a body.
Axis (C2) has the dens, which is the body of C1 that has become fused to C2 during development.
Vertebra prominens (C7) has the most prominent spinous process in the cervical region, hence its name.

Lumbar vertebrae differ from thoracic vertebrae in the following ways: COMPARISON OF CERVICAL, THORACIC, AND LUMBAR VERTEBRAE

Lumbar vertebrae have larger bodies, have broad spinous processes that project posteriorly, and do not have transverse costal facets for ribs. On a skeleton, observe the lumbar vertebrae and notice that their spines do not overlap like the spines of thoracic vertebrae.

The sacrum is formed by five fused vertebrae and it does not have identifiable spines or transverse processes. On the dorsal surface of the sacrum, identifySACRUM AND COCCYX

Median sacral crest

Posterior (dorsal) sacral foramina
Sacral hiatus

The coccyx is a small triangular bone formed by four rudimentary coccygeal vertebrae that are fused together.

TABLE: REGIONAL CHARACTERISTICS OF THE VERTEBRAE LINK

TABLE: LIGAMENTS OF THE VERTEBRAL COLUMN LINK

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MONDAY, SEPTEMBER 14 – BACK I

Skin and Superficial Fascia

LEARNING OBJECTIVES:

1. Define the "anatomical position". Using the conventional anatomical terms, describe the body and the spatial relationships of its parts, for example dorsal/ventral, medial/lateral, proximal/distal, and superficial/deep.

2. Recognize and define the standard planes and sections used to describe parts of the body and the relationships of the various planes and sections to one another.

3. Describe the general structural plan of the body and the relationships of the layers, partitions and compartments one encounters when dissecting from superficial to deep in any particular region.

4. Demonstrate a cutaneous nerve and describe the pattern of cutaneous nerves on the back.

5. Identify, and give the general attachments of, nerve and blood supply to, and the general functions of the superficial back muscles.

6. Identify the bony prominences of the back and spine that may be palpated and used for reference to underlying structures.

7. NOTE: FOR THIS AND ALL SUBSEQUENT DISSECTIONS you must be able to identify any of the structures listed in the official course BODY PARTS LIST.

Dissection Overview

The order of dissection will be as follows: The skin will be reflected from the back, posterior surface of the neck, and posterior surface of the proximal upper limb. Posterior cutaneous nerves will be studied. The superficial fascia will then be removed.

Dissection Instructions

Skin Incisions

Refer to the following figure: SKIN INCISIONS
Use a scalpel to make a skin incision in the midline from the external occipital protuberance (X) to the tip of the coccyx (S). The skin is approximately 6 mm thick in this region.
Make an incision from the tip of the coccyx (S) to the midaxillary line (T). This incision should pass approximately 3 cm inferior to the iliac crest.
Make a transverse skin incision from the external occipital protuberance (X) laterally to the base of the mastoid process (M).
Make a transverse skin incision from R to B superior to the scapula and superior to the acromion.
At the level of the inferior angle of the scapula, make a transverse skin incision from the midline (U) to the midaxillary line (V).
To facilitate skinning, make several parallel transverse incisions above and below the one described in the previous step. The strips of skin that result should be about 7.5 cm wide to make skinning easier.
Reflect the skin from medial to lateral, leaving the skin attached laterally. On one side, reflect only the skin, leaving the superficial fascia covering the muscles. On the other side, reflect the superficial fascia with the skin. Note that, after the dissection, the skin flaps can be placed back over the deeper structures, serving to protect these underlying structures from dessication.

Superficial Fascia

In the superficial fascia, locate the greater occipital nerve and the occipital artery. GREATER OCCIPITAL NERVE AND OCCIPITAL ARTERY

The greater occipital nerve is the dorsal ramus of spinal nerve C2. The greater occipital nerve pierces the trapezius muscle about 3 cm inferolateral to the external occipital protuberance. The deep fascia in this area is very dense and tough. It may be difficult to find the greater occipital nerve by dissecting in this deep fascia, even though it is a large nerve. Therefore, it may be useful to cut the trapezius from the cervical spinous process and nuchal ligament near the midline, enough to lift it slightly away from the underlying semispinalis capitus muscle. You may have to cut the trapezius from the superior nuchal line as well. In this way, you should see the greater occipital nerve passing through the semispanilis capitus and the trapezius on its way to the skin. The occipital artery may be found lateral to the greater occipital nerve near the external occipital protuberance. Note that the occipital artery takes a superior-medial course as it passes from the external carotid artery laterally toward the greater occipital protuberance.
A note about identifying arteries, nerves, and veins: Nerves are flat, solid, white, and strong. Arteries are round, hollow, and elastic. Veins are flat, hollow, thin walled, and are often blue-black because they are filled with coagulated blood. Larger structures are readily distinguished in the dissection laboratory, and you will have enough experience before the first exam that you can readily differentiate these. Smaller structures make it more difficult to differentiate these structures. However, note that, in many cases in which the structures are too small to distinguish based on these physical characteristics, you should still be able to make distinctions based on anatomical relations that you are responsible for learning as part of the dissection. In this case, if the greater occipital nerve and occipital artery on your cadaver are too small to see clearly which is which, you should easily distinguish between them because the nerve is medial to the artery, and the nerve passes through the back muscles as it makes its way to the skin over the skull, while the artery arises from the lateral side of the neck and travels in a superior-medial direction.

Read a description of the dorsal ramus of a spinal nerve. The posterior cutaneous branches of the dorsal rami pierce the trapezius muscle or latissimus dorsi muscle to enter the superficial fascia. BRANCHES OF A TYPICAL SPINAL NERVE

On the side where you did NOT reflect the superficial fascia with the skin (i.e. the side where you left the superficial fascia covering the back muscles), dissecting in the superficial fascia about 1-2 cm lateral to the midline to find at least one representative of a dorsal ramus. Once you find at least one dorsal ramus, go ahead and remove the remaining superficial fascia from the muscle.
In the neck, reflect the superficial fascia only as far laterally as the superior border of the trapezius muscle. Do not cut the deep fascia along the superior border of the trapezius muscle. The accessory nerve is superficial at this location and it is in danger of being cut.

Dissection Review

Review the branching pattern of a typical spinal nerve and understand that cutaneous branches of the dorsal rami innervate the skin of the back.

Study a dermatome chart and become familiar with the concept of segmental innervation. SCHEMATIC DEMARCATION OF DERMATOMES

Superficial Muscles of the Back

Dissection Overview

The superficial muscles of the back are the trapezius, latissimus dorsi, rhomboid major, rhomboid minor, and levator scapulae. SUPERFICIAL (EXTRINSIC) MUSCLES OF THE BACK

The order of dissection will be as follows: The superficial surface of the trapezius muscle will be cleaned. The trapezius muscle will be examined and reflected. The latissimus dorsi muscle will be studied and reflected. The rhomboid major muscle, rhomboid minor muscle, and levator scapulae muscle will be studied. Dissection of the superficial back muscles should be performed bilaterally.

Dissection Instructions

Trapezius Muscle

Clean the surface of the trapezius muscle (L. Trapezoides, an irregular four-sided figure). HOW TO REFLECT THE MUSCLES OF THE BACK

Do not disturb the superior border of the trapezius muscle. Observe the proximal attachment of the trapezius muscle on the external occipital protuberance, the nuchal ligament, and the spinous processes of vertebrae C7 to T12.
Observe the three parts of the trapezius muscle, each of which has a distinctly different action:

Superior part of the trapezius muscle attaches to the lateral one-third of the clavicle; this portion elevates the scapula.
Middle part of the trapezius muscle attaches to the acromion and spine of the scapula; this portion retracts the scapula.
Inferior part of the trapezius muscle attaches near the medial end of the spine of the scapula; this portion depresses the scapula.

To reflect the trapezius muscle, insert your fingers deep to the inferolateral border of the muscle (medial to the inferior angle of the scapula). Use your fingers to break the plane of loose connective tissue that lies between the trapezius muscle and the deeper muscles of the back.

Use scissors to detach the trapezius muscle from its proximal attachment on the spinous processes and the nuchal ligament (dashed line on left side, near midline) HOW TO REFLECT THE MUSCLES OF THE BACK Start inferiorly and continue the cut superiorly as far as the external occipital protuberance. Note that you may already have completed this cut superiorly when you searched for the greater occipital nerve.

Use scissors to make a short transverse cut (2.5 cm) across the superior end of the trapezius muscle to detach it from the superior nuchal line. Spare the greater occipital nerve, and do not extend the transverse cut beyond the border of the trapezius muscle.

Use scissors to cut the trapezius muscle from its distal attachments on the spine and acromion of the scapula (dashed line on left side, near spine of scapula) HOW TO REFLECT THE MUSCLES OF THE BACK Make this cut very close to the bone. Leave the trapezius muscle attached to the clavicle and cervical fascia.

Reflect the trapezius muscle superolaterally. Leave the cervical fascia attached along the superior border of the trapezius muscle to act as a hinge.
Study the deep surface of the reflected trapezius muscle. Find and clean the plexus of nerves formed by the accessory nerve (cranial nerve XI) and branches of the ventral rami of spinal nerves C3 and C4. The accessory nerve provides motor innervation to the trapezius muscle; the branches of nerves C3 and C4 are sensory (proprioception). Dissect the superficial branch of the transverse cervical artery that accompanies the nerves. Remove the transverse cervical vein to clear the dissection field.
The accessory nerve passes through the posterior triangle of the neck. Do not follow the nerve into the posterior triangle at this time. The posterior triangle will be dissected with the neck.
Continuing our discussion about identifying arteries, nerves, and veins: Your cadaver may be large enough that the accessory nerve and superficial branch of the transverse cervical artery are readily distinguished. However, in some cadavers, they might be too small to differentiate with certainty. In addition, they both take a similar course anatomically (at least the anatomy you are seeing right now; when we get to the neck, you will see that they do have a different origin), For practical exams, in cases where the structures are too small to distinguish and there is no anatomical basis to differentiate them, we will indicate in the question to “Identify the tagged nerve.” or “Identify the tagged artery.” to prompt you toward the correct answer.

Latissimus Dorsi Muscle

Clean the surface and define the borders of the latissimus dorsi muscle (L. Latissimus, widest). HOW TO REFLECT THE MUSCLES OF THE BACK

· The proximal attachments of the latissimus dorsi muscle are the spines of vertebrae T7 to T12, the thoracolumbar fascia, and the iliac crest. The latissimus dorsi muscle also has a proximal attachment to ribs 9 to 12, lateral to their angles.

· Note that the distal attachment of the latissimus dorsi muscle is the floor of the intertubercular sulcus on the anterior side of the humerus, but do not dissect this attachment. The latissimus dorsi muscle receives the thoracodorsal nerve and artery on its anterior surface near its distal attachment; do not look for these structures now. The distal attachment of the latissimus dorsi muscle, its nerve, and its artery will be dissected with the upper limb.

To reflect the latissimus dorsi muscle, insert your fingers deep to the superior border of the muscle (medial to the inferior angle of the scapula), and break the plane of loose connective tissue that lies between it and deeper structures. Raise the latissimus dorsi muscle enough to insert scissors and cut through its proximal attachment on the thoracolumbar fascia HOW TO REFLECT THE MUSCLES OF THE BACK (dashed line on right side). Do not cut too close to the lumbar spinous processes.
Reflect the latissimus dorsi muscle laterally. Do not disturb its attachment to the ribs. It may also have an attachment to the inferior angle of the scapula. If so, do not disturb its attachment to the inferior angle of the scapula.

The thoracolumbar fascia (lumbodorsal fascia) THE THORACOLUMBAR FASCIA is a deep investing membrane which covers the deep muscles of the back of the trunk. It is made up of three layers, anterior, middle, and posterior. The anterior layer is the thinnest and the posterior layer is the thickest. Two spaces are formed between these three layers of the fascia. Between the anterior and middle layer lies the quadratus lumborum muscle. The erector spinae muscle is enclosed between the middle and posterior layers.

Above, it passes in front of the serratus posterior superior muscle and is continuous with a similar investing layer on the back of the neck—the nuchal fascia.

In the thoracic region the lumbodorsal fascia is a thin fibrous lamina which serves to bind down the Extensor muscles of the vertebral column and to separate them from the muscles connecting the vertebral column to the upper extremity. It contains both longitudinal and transverse fibers, and is attached, medially, to the spinous processes of the thoracic vertebrae: laterally to the angles of the ribs.

Rhomboid Major and Rhomboid Minor Muscles

Clean the surface and borders of the rhomboid (rhomboideus) minor muscle and the rhomboid major muscle (Gr. Rhombos, shaped like a kite). Typically, the separation between the rhomboid muscles is not very obvious and the two muscles must be distinguished from each other by using their distal attachments.
The proximal attachments of the rhomboid minor muscle are the nuchal ligament and the spinous processes of vertebrae C7 and T1. The distal attachment of the rhomboid minor muscle is the medial border of the scapula at the level of the spine.
The proximal attachments of the rhomboid major muscle are the spinous processes of vertebrae T2 to T5. The distal attachment of the rhomboid major muscle is the medial border of the scapula inferior to the spine.
The rhomboid muscles retract the scapula, rotate the scapula to depress the glenoid cavity, and hold the scapula close to the thoracic wall.
To reflect the rhomboid muscles, insert your fingers deep to the inferior border of the rhomboid major muscle and separate it from deeper muscles.
Working from inferior to superior, use scissors to detach the rhomboid major muscle from its proximal attachments on the spinous processes. Continue the cut superiorly and detach the rhomboid minor muscle from its proximal attachments on the spinous processes. Reflect these two muscles laterally.
Examine the deep surface of the two rhomboid muscles near their distal attachments on the medial border of the scapula. Use blunt dissection to find the dorsal scapular nerve and dorsal scapular vessels. Remove the dorsal scapular vein to clear the dissection field. The dorsal scapular nerve and artery course parallel to the medial border of the scapula.
The dorsal scapular artery may branch directly from the subclavian artery, or it may arise from the transverse cervical artery, in which case it is also called the deep branch of the transverse cervical artery.

Levator Scapulae Muscle

Identify the levator scapulae muscle (L. levare, to raise). At this stage of the dissection, the levator scapulae muscle can be seen only near its distal attachment on the scapula.
Note that the proximal attachments of the levator scapulae muscle are the transverse processes of the upper four cervical vertebrae. Do not dissect its proximal attachments.
The distal attachment of the levator scapulae muscle is the superior angle of the scapula.
The dorsal scapular nerve and artery supply the levator scapulae muscle. The levator scapulae muscle elevates the scapula and rotates the scapula to depress the glenoid cavity.

Dissection Review

Replace the superficial muscles of the back in their correct anatomical positions.
Use the dissected specimen to review the proximal attachment, distal attachment, action, innervation, and blood supply of each muscle that you have dissected.
Review the movements that occur between the scapula and the thoracic wall.

Use an illustration to observe the origin of the transverse cervical artery and the origin of the dorsal scapular artery. Observe two triangles associated with the latissimus dorsi muscle: the triangle of auscultation and the lumbar triangle. THE LUMBAR TRIANGLE AND THE TRIANGLE OF AUSCULTATION

IN THE CLINIC TRIANGLES OF THE BACK

Triangles of the Back

The triangle of auscultation is bounded by the latissimus dorsi muscle, the trapezius muscle, and the rhomboid major muscle. Within the triangle of auscultation, intercostal space 6 has no overlying muscles. This area is particularly well suited for auscultation (listening to sounds produced by thoracic organs, particularly the lungs).

The lumbar triangle is bounded by the latissimus dorsi muscle, the external oblique muscle, and the iliac crest. The floor of the lumbar triangle is the internal oblique muscle of the abdomen. On rare occasions, the lumbar triangle is the site of a lumbar hernia.

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WEDNESDAY, SEPTEMBER 16 -BACK II

Intermediate Muscles of the Back

SUPERFICIAL AND INTERMEDIATE MUSCLES OF THE BACK

The intermediate muscles of the back are the serratus posterior superior muscle and the serratus posterior inferior muscle. The serratus posterior superior and inferior muscles are very thin muscles, which may have been accidentally reflected with the rhomboid muscles or the latissimus dorsi muscle. If you do not see the serratus posterior muscles, look for them on the deep surface of the reflected rhomboid muscles or the reflected latissimus dorsi muscle.

The proximal attachments of the serratus posterior superior muscle are the nuchal ligament and the spinous processes of vertebrae C7 to T3. Its distal attachments are the superior borders of ribs 2 to 5, lateral to their angles.
The proximal attachments of the serratus posterior inferior muscle are the spinous processes of vertebrae T11 to L2. Its distal attachments are the inferior borders of ribs 9 to 12, lateral to their angles.
The serratus posterior muscles are respiratory muscles, and they are innervated by intercostal nerves.

FOCUS QUESTIONS ANSWER

1. What are the names of the nerves that innervate the subcutaneous tissue and skin?

2. What is the function of the investing fascia?

3. Between some muscles there is loose fascia, fat and connective tissue. What is the function of this fascia?

4. Why do nerves, arteries, and veins branch in different manners?

5. Where do nerves C3 and C4 (branching off of anterior primary rami of spinal nerves) combine to form the subtrapezial plexus?

6. What is the thoracolumbar fascia (aponeurosis)?

7. In reference to latissimus dorsi: How are the costal attachments of the muscle related to adjacent muscles? Is there an attachment to the scapula?

Deep Muscles of the Back and Spinal Cord

LEARNING OBJECTIVES:

1. Identify and give the function of the significant parts of a typical vertebra and associated ligaments.

2. Identify the specialized vertebrae.

3. Describe the spine, its curvatures, and vertebral column movements.

4. Identify the coverings and the supporting structures of the spinal cord. Give the point of the termination of the spinal cord and the dural sac.

5. Identify the terminal specialties of the cord, their relation to lumbar puncture, the nerve rootlets, and blood supply.

6. Describe the anatomy of the cord and vertebrae as related to fractures, dislocations, and possible cord injury.

7. Describe or illustrate the location and function of the basic somatic motor and sensory neurons on a cross section of the spinal cord (see Atlas+ or textbook).

8. Describe a typical spinal nerve, the somatic motor and sensory components found in any portion, and their distribution.

9. Describe conceptually how any region of the thoracic wall gets its blood supply and innervation.

10. Define and explain the significance of dermatomes.

11. Explain the difference between superficial and deep (true) back muscles.

Dissection Overview

The deep muscles of the back DEEP MUSCLES OF THE BACK act on the vertebral column. There are many deep muscles of the back INTERMEDIATE AND DEEP INTRINSIC BACK MUSCLES and only a few will be dissected: splenius capitis muscle, splenius cervicis muscle, semispinalis capitis muscle, and erector spinae muscle. All of the deep muscles of the back are innervated by dorsal rami of spinal nerves.

The order of dissection will be as follows: The deep muscles of the posterior neck (splenius capitis and cervicis) will be studied and reflected. The semispinalis capitis muscle will be studied. The erector spinae muscle will be dissected and the three columns of muscle that comprise its component parts will be identified.

Dissection Instructions

Splenius Muscle

Identify the splenius muscle (Gr. splenion, bandage).
The splenius muscle lies deep to the trapezius muscle. The fibers of the splenius muscle course obliquely across the neck. The proximal attachment of the splenius muscle is the nuchal ligament and the spinous processes of vertebrae C7 to T6.
The splenius muscle has two parts that are named according to their distal attachments:

Splenius capitis muscle (L. caput, head) is attached to the mastoid process of the temporal bone and the superior nuchal line of the occipital bone.
Splenius cervicis muscle (L. cervix, neck) is attached to the transverse processes of vertebrae C1 to C4.

The two parts of the splenius muscle are not easily distinguished at this stage of the dissection. Detach both parts of the splenius muscle from the nuchal ligament and the spinous processes of vertebrae C7 to T6.
Reflect the muscles laterally, leaving their distal attachments undisturbed.

Erector Spinae Muscle

The erector spinae muscle (L. erector, one who erects) lies deep to the serratus posterior muscles. DEEP MUSCLES OF THE BACK
Detach both serratus posterior muscles from their proximal attachments on the spinous processes. Reflect the muscles laterally, leaving them attached to the ribs.
The erector spinae muscle is composed of three columns of muscle: spinalis, longissimus, and iliocostalis. The intent of this dissection is to identify these three columns of muscle.
Use a scalpel to incise the posterior surface of the thoracolumbar fascia. Use blunt dissection to remove it from the posterior surface of the erector spinae muscle.
Use your fingers to separate the three columns of muscle at midthoracic levels. The columns of the erector spinae muscle are fused to each other at the level of their inferior attachments to the sacrum and ilium.

Identify: DEEP MUSCLES OF THE BACK

Spinalis muscle – the medial column of the erector spinae muscle. The inferior attachments of the spinalis muscle are on spinous processes. Its superior attachments are also on spinous processes. The spinalis muscle is present at lumbar, thoracic, and cervical vertebral levels.
Longissimus muscle (L. longissimus, the longest) – the intermediate column of the erector spinae muscle. Its inferior attachment is on the sacrum and its superior attachments are the transverse processes of the thoracic and cervical vertebrae. Note that its most superior portion, the longissimus capitis muscle, attaches to the mastoid process of the temporal bone.
Iliocostalis muscle – the lateral column of the erector spinae muscle. Its inferior attachment is the ilium (iliac crest) and its superior attachments are on ribs (L. costa, rib).

All three columns of the erector spinae muscle extend the vertebral column when both sides work together. If only one side of the erector spinae muscle is active, it bends the vertebral column laterally toward the side that is active.

Transversospinal Group of Muscles THE TRANSVERSOSPINAL GROUP OF DEEP BACK MUSCLES

The transversospinal group of muscles is located deep to the erector spinae muscle. The muscles in the transversospinal group attach to transverse processes and spinous processes. The muscles of the transversospinal group cause rotational and lateral bending movements between adjacent vertebrae and act to stabilize the vertebral column. A number of muscles comprise this group: semispinalis, multifidus, and more deeply, rotatores. On one side, in the thoracic region, remove some of the fibers of the spinalis muscle to expose muscles of the transversospinal group. You do not need to distinguish the semispanilis, multifidus, and rotators specifically, just recognize them as the transversospinal group of muscles.

Semispinalis Capitis Muscle

The semispinalis fibers in the cervical region attach to the base of the skull, and thus are specifically identified as the semispinalis capitis muscle.

Identify the semispinalis capitis muscle (L. semi, half; L. spinalis, spine). THE SEMISPINALIS MUSCLES

The semispinalis capitis muscle is the most superficial member of the transversospinal group of muscles. The semispinalis capitis muscle lies deep to the splenius muscles and its fibers course vertically, parallel to the long axis of the neck.
The inferior attachments of the semispinalis capitis muscle are the transverse processes of the upper thoracic vertebrae.
The superior attachment of the semispinalis capitis muscle is the occipital bone between the superior and inferior nuchal lines. Note that the greater occipital nerve passes through the semispinalis capitis muscle.
Do not dissect the semispinalis capitis muscle further at this time.

IN THE CLINIC

Back pain is a nonspecific term for pain from a variety of sources. It is experienced by 80% of people during their lifetime and results from an upright stance and a bipedal gait. A few of the many factors that can cause back pain are dehydration and stiffening of the intervertebral disc with age, osteoporosis of the zygapophysial joints between the veretebrae, and weakened back and abdominal musculature, leading to poor posture.

TABLE: INTRINSIC MUSCLES OF THE BACK (origin, insertions, actions) LINK

Dissection Review

Use the dissected specimen to review the location, innervation, and action of each muscle or column of muscles in the deep group of back muscles.

Suboccipital Region

Dissection Overview

First, review the relevant osteology:

On a skull, identify: SKELETON OF THE BACK, VERTEBRAL COLUMN, AND SKULL

Superior nuchal line

Inferior nuchal line
External occipital protuberance
Foramen magnum

On the atlas (C1 vertebra), identify: POSTERIOR VIEW OF ATLAS (C1) AND AXIS (c2)

Posterior tubercle

Posterior arch
Groove for the vertebral artery
Transverse process
Foramen transversarium

On the axis (C2 vertebra), identify: POSTERIOR VIEW OF ATLAS (C1) AND AXIX (C2)

Spinous process
Transverse process
Foramen transversarium

The order of dissection will be as follows: The greater occipital nerve will be identified and followed deeply. The semispinalis capitis muscle will be reflected. The muscles that bound the suboccipital triangle will be identified. The contents of the suboccipital region (vertebral artery and suboccipital nerve) will be studied. All dissections will be done bilaterally, with the exception of the vertebral artery, whick will be dissected on one side only.

Dissection Instructions

Identify the semispinalis capitis muscle. THE SUBOCCIPITAL REGION

Once again, find the greater occipital nerve. Use blunt dissection to follow the greater occipital nerve deeply, through the semispinalis capitis muscle. Detach the semispinalis capitis muscle close to the occipital bone and reflect it inferiorly. Preserve the greater occipital nerve.
Deep to the semispinalis capitis muscle, follow the greater occipital nerve proximally to the lower border of the obliquus capitis inferior muscle. Note that the greater occipital nerve (dorsal ramus of C2) emerges between vertebrae C1 and C2.

Identify and clean the three muscles that form the boundaries of the suboccipital triangle: CONTENTS OF THE SUBOCCIPITAL TRIANGLE

Obliquus capitis inferior muscle (inferior oblique muscle) forms the inferior boundary of the suboccipital triangle. Verify that the proximal attachment of the obliquus capitis inferior muscle is the spinous process of the axis (C2). Its distal attachment is on the transverse process of the atlas (C1).
Rectus capitis posterior major muscle (rectus major muscle) forms the medial boundary of the suboccipital triangle. Confirm that the proximal attachment of the rectus capitis posterior major muscle is the spinous process of the axis. Its distal attachment is the inferior nuchal line of the occipital bone.
Obliquus capitis superior muscle (superior oblique muscle) forms the lateral boundary of the suboccipital triangle. Confirm that the inferior attachment of the obliquus capitis superior muscle is the transverse process of the atlas. Its superior attachment is the occipital bone between the superior and inferior nuchal lines.

The muscles that bound the suboccipital triangle produce extension and lateral bending of the head at the atlanto-occipital joints and rotation of the head at the atlantoaxial joints.

The contents of the suboccipital triangle are the suboccipital nerve and the vertebral artery. CONTENTS OF THE SUBOCCIPITAL TRIANGLE Note that the suboccipital nerve (dorsal ramus of C1) emerges between the occipital bone and vertebra C1. The suboccipital nerve supplies motor innervation to the muscles of the suboccipital region. The suboccipital nerve is the only dorsal ramus that has no cutaneous distribution. Dissect the fascia within the borders of the suboccipital triangle to find the main trunk and branches of the suboccipital nerve.

Determine which side has the better dissection of the muscles of the suboccipital triangle and the suboccipital nerve, and identify the vertebral artery on the other side. To do so, use the muscles of the suboccipital triangle and finger palpation to identify the posterior arch of the atlas. Then, remove the muscles of the suboccipital triangle, taking care not to cut deep to the posterior arch of the atlas. Use blunt dissection to identify the vertebral artery coursing along the superior border of the posterior arch of the atlas. Remove the vertebral vein if necessary. Once the artery is identified, trace it laterally to the transverse foramen of the atlas. Use blunt dissection to find the artery as it courses between the transverse foramina of the atlas and axis.
Use an illustration to study the course of the vertebral artery through the neck and into the skull.

TABLE: MUSCLES OF THE SUBOCCIPITAL REGION (origin, insertion, action) LINK

Dissection Review

Review the actions of the suboccipital muscles. THE SHORT SUBOCCIPITAL (NUCHAL) MUSCLES
Review the distribution of the branches of a thoracic dorsal ramus and compare the thoracic pattern to the distribution of the dorsal rami of spinal nerves C1 to C3.

FOCUS QUESTIONS ANSWER

1. Describe the anatomy of the cord and vertebrae as related to fractures, dislocations, and possible cord injury.

2. Describe or illustrate the location and function of the basic somatic motor and sensory neurons on a cross section of the spinal cord.

3. Describe a typical spinal nerve, the somatic motor and sensory components found in any portion, and their distribution.

4. Describe conceptually how any region of the thoracic wall gets its blood supply and innervation.

5. Define and explain the significance of dermatomes.

6. Explain the difference between superficial and true back muscles.

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MONDAY, SEPTEMBER 21 – BACK III

Vertebral Canal, Spinal Cord, and Meninges

Dissection Overview

The vertebral canal is a bony tube formed by the stacked vertebral foramina of the cervical vertebrae, thoracic vertebrae, lumbar vertebrae, and sacral canal. THE SPINAL CORD WITHIN THE VERTEBRAL CANAL The vertebral canal encloses and protects the spinal cord, its membranes (spinal meninges), and blood vessels. The spinal cord begins at the foramen magnum of the occipital bone and usually terminates in the adult at the level of the second lumbar vertebra. Because the spinal cord is shorter than the vertebral canal, the spinal cord segments are found at higher vertebral levels than their names would suggest.

The spinal cord is not uniform in diameter throughout its length. It has a cervical enlargement THE CERVICAL AND LUMBAR ENLARGEMENTS that corresponds to spinal cord segments C4 to T1 and a lumbar enlargement that corresponds to spinal cord segments L2 to S3. There are 31 pairs of spinal nerves (8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal), which emerge between adjacent vertebrae. Most spinal nerves are numbered according to the vertebra above them as they pass through the intervertebral foramen (i.e., spinal nerve T1 exits the vertebral canal below vertebra T1). However, in the cervical region, spinal nerves are numbered differently—they are numbered according to the vertebra below. For example, spinal nerve C1 exits the vertebral canal above vertebra C1 and the C8 spinal nerve, which does not have a correspondingly numbered vertebra exits between vertebra C7 and T1.

The order of dissection will be as follows: The erector spinae muscles will be removed from the lower back to expose the laminae of the vertebrae. The laminae will then be cut and removed (laminectomy) to expose the spinal meninges. The spinal meninges will be examined and will be opened to expose the spinal cord. The spinal cord will then be studied.

Dissection Instructions

Be sure that you have protection for your eyes while chiseling or sawing!
To prepare for the laminectomy, you should have the vertebral lamina (area between the transverse and spinous processes) devoid of muscle tissue from C3 to the sacrum (PRESERVE THE SUBOCCIPITAL TRIANGLE STRUCTURES).
Cut and scrape the vertebral laminae clean of muscle tissue because the autopsy saw works well on bones, not soft tissue.
Use bone clippers (or a mallet and chisel) to remove the spinous processes from the vertebra (observe the strong supraspinous ligaments that interconnect the tips of the spinous processes along the length of the vertebral column).
Obtain an autopsy saw from the blue bin. Be sure that the switch is in the “off” position before you plug it in; use care with the autopsy saw; keep your soft tissues and bone away from the blade; when finished, wipe the autopsy saw clean of tissue debris with paper towels and return it to the blue bin.
Turn on the autopsy saw and cut into the lamina of the C3 vertebra. Refer to the accompanying images for a demonstration of the correct angle of the saw blade.

The dashed lines indicate the cuts made by the autopsy saw. Be sure that you make the cuts at about a 45 degree angle. Begin cutting the laminae at C3 and proceed down to the sacrum. Panel C shows the portion of the neural arch that will be removed

As the saw blade cuts through the lamina, you will feel the blade drop (Be careful; avoid cutting the underlying dura mater; cut only as deep as the bone).
Repeat for the other side of the vertebral column; try to maintain a straight saw line.
Remove the cut portion of the vertebral column from the cadaver, freeing any remaining attachments with a mallet and chisel.
Be careful around sharp edges of the remaining parts of the vertebrae; you can use the mallet to hit (blunt) the sharp bony edges.
Wipe tissue debris from your construction equipment (mallets, chisels) with paper towels and return it to the proper storage drawer.

REMEMBER TO LEAVE ALL BONE PIECES WITH THE CADAVER. NEVER PLACE BONE IN THE CADAVERIC WASTE CONTAINERS.

Attempt to keep as much of the lamina intact as possible during removal. The dura mater will remain with the spinal cord and will be undamaged.
On the deep surface of the removed spinous specimen, gently remove any adherent epidural fat to expose the ligamenta flava. The ligamenta flava connect the laminae of adjacent vertebrae.

Spinal Meninges

Observe the epidural (extradural) space. Use blunt dissection to remove the epidural fat and the posterior internal vertebral venous plexus from the epidural space.
Identify the dural sac, which ends inferiorly at vertebral level S2LOWER PORTION OF THE VERTEBRAL CANAL AND SPINAL CORD. A. LATERAL VIEW. B. POSTERIOR VIEW In the thoracic region, lift a fold of dura mater with forceps and use scissors to cut a small opening in its dorsal midline. Use scissors to extend the cut inferiorly to vertebral level S2. Attempt to do this without damaging the underlying arachnoid mater. Retract the dura mater and pin it open.
Identify the arachnoid mater. RELATIONSHIPS OF THE MENINGES TO THE SPINAL CORD AND NERVE ROOTS. A. TRANSVERSE SECTION. B. POSTERIOR VIEW. It is very delicate. Incise the arachnoid mater in the dorsal midline and observe the subarachnoid space. The subarachnoid space contains cerebrospinal fluid in the living person but not in the cadaver.
Retract the arachnoid mater and observe the spinal cord. The spinal cord is completely invested by pia mater, which is on the surface of the spinal cord and cannot be dissected from it.
Identify the following features of the spinal cord:

Cervical enlargement (spinal cord segments C4 to T1) provides nerves to the upper limb.
Lumbar enlargement (spinal cord segments L2 to S3) provides nerves to the lower limb. The lumbar enlargement is located at lower thoracic vertebral levels.
Conus medullaris (medullary cone) is the end of the spinal cord located between vertebral levels L1 and L2.

Cauda equina (L., tail of horse) is a collection of ventral and dorsal roots in the lower vertebral canal. LOWER PORTION OF THE VERTEBRAL CANAL AND SPINAL CORD
Filum terminale internum is a delicate filament continuous with the pia mater. It arises from the inferior tip of the conus medullaris and ends at S2, where it is encircled by the lower end of the dural sac.

Filum terminale externum (coccygeal ligament) is the continuation of the filum terminale internum below vertebral level S2. The filum terminale externum passes through the sacral hiatus and ends by attaching to the coccyx. You will not see the filum terminal externum on your cadaver.

The pia mater forms two denticulate ligaments, one on each side of the spinal cord. RELATIONSHIP OF THE MENINGES TO THE SPINAL CORD AND NERVE ROOTS Each denticulate ligament has 21 teeth and each tooth is attached to the inner surface of the dura mater, anchoring the spinal cord.
Use a probe to follow dorsal roots and ventral roots to the point where they pierce the dura mater and enter the intervertebral foramen. DISSECTION OF A DORSAL ROOT GANGLION The dorsal roots are on the dorsal side of the denticulate ligament and the ventral roots are on the ventral side of the denticulate ligament. The spinal nerve will be formed outside of the vertebral canal at the point where the dorsal and ventral roots join each other.
Observe small blood vessels that course along the ventral and dorsal roots. These are branches of posterior intercostal, lumbar, or vertebral arteries, (depending upon vertebral level) that pass into the vertebral canal through the intervertebral foramen and supply the spinal cord.
In the thoracic region, expose one spinal nerve. Place a probe into an intervertebral foramen to protect the nerve within it (i.e. place the probe between the bone and exiting nerve roots). Use bone cutters to remove the posterior wall of the intervertebral foramen, and then use blunt dissection to expose the spinal ganglion (dorsal root ganglion). DISSECTION SHOWING DORSAL AND VENTRAL ROOTS OF A SPINAL NERVE Distal to the spinal ganglion, identify the spinal nerve and follow it distally to the point where it divides into a dorsal ramus and a ventral ramus.

IN THE CLINIC

Vertebral Venous Plexuses

The veins of the vertebral venous plexuses are valveless, permitting blood to flow superiorly or inferiorly, depending on blood pressure gradients. The vertebral venous plexuses can serve as routes for metastasis of cancer from the pelvis to the vertebrae, vertebral canal, and cranial cavity.

IN THE CLINIC

Lumbar Puncture

Cerebrospinal fluid (CSF) can be obtained from the subarachnoid space inferior to the conus medullaris. LINK At this level, there is no danger of penetrating the spinal cord with the puncture needle.

Dissection Review

Review the formation and branches of a typical spinal nerve.
Describe the way that the deep back muscles receive their innervation.
Review the coverings and parts of the spinal cord and study an illustration that shows the blood supply to the spinal cord.
Consult a dermatome chart and relate this pattern of cutaneous innervation to the spinal cord segments.

FOCUS QUESTIONS ANSWER

1. What is the coccygeal ligament?

2. What do the lateral continuities of the dura mater cover at and distal to the intervertebral foramen?

3. What does the subarachnoid space usually contain?

4. With what is the subarachnoid space continuous?

5. Where does the filum terminale lie?

6. Where does the filum terminale attach?

8.What are the relations of the denticulations and the denticulate ligament to the roots of spinal nerves?

8. How many denticulations are there?

9. Where do the denticulations attach?

10. At what vertebral level is the conus medullaris?

11. Where is the dura mater in relation to the intervertebral foramen?

12. Examine dorsal and ventral rootlets as they emerge from the cord. How is the segmental pattern created?

13. What is a root?

14. What is the functional difference between dorsal and ventral roots?

15. Trace the roots to the point of union to form the spinal nerve. Are they in separate sheaths before uniting?

16. Where is the dorsal root ganglion?

17. Note posterior and anterior spinal arteries. What are their sources?

18. What reinforces the spinal arteries?

19. What are radicular arteries?

20. Are all radicular arteries the same?

21. What is the source for radicular arteries?

22. Observe arrangement of gray matter and white matter of the cord. What is their significance?

23. After exposing the posterior longitudinal ligament in cervical and lumbar regions, can you see the intervertebral discs?

24. What is the significance of the attachment of the posterior longitudinal ligament to the discs?

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TUESDAY, SEPTEMBER 22 – PECTORAL REGION I

THE PECTORAL REGION

Dissection Overview

The pectoral region (L., pectus, chest) covers the anterior thoracic wall and part of the lateral thoracic wall. The order of dissection will be as follows: The breast will be dissected in female cadavers. The superficial fascia will be removed in cadavers of both sexes. The superficial fascia of the thorax contains the usual elements that are common to superficial fascia in all body regions: blood vessels, lymph vessels, cutaneous nerves, and sweat glands. In addition, the superficial fascia of the anterior thoracic wall in the female contains the mammary glands, which are highly specialized organs unique to the superficial fascia of the thorax.

Surface Anatomy and Osteology

The surface anatomy of the pectoral region will be studied on a living subject as part of your SURFACE ANATOMY assigned exercises. You can also observe some of the anatomy on the cadaver. At this point we will only consider anatomy relevant to our dissection of the pectoral region. A more detailed study of the osteology of the thorax will be undertaken during our dissection of the thorax in Block II. Turn the cadaver to the supine position and palpate the following structures: SURFACE ANATOMY OF THE ANTERIOR THORACIC WALL

Clavicle
Acromion of the scapula
Manubrium
Body of the sternum
Xiphoid process
Seventh costal cartilage
Costal margin

Skeleton of the Thorax

Refer to a skeleton. Examine a rib from the midthorax level and identify: TYPICAL LEFT RIB, POSTERIOR VIEW

Head
Neck
Tubercle
Costal angle
Shaft (body)
Costal groove

Examine the sternum and identify: SURFACE ANATOMY OF THE ANTERIOR THORACIC WALL

Manubrium (L. manubrium, handle)
Body
Xiphoid process (Gr. xiphos, sword)

Examine a scapula and identify: SCAPULA, ANTERIOR VIEW

Acromion
Coracoid process

Observe that the medial end of the clavicle articulates with the manubrium of the sternum (sternoclavicular joint) and the lateral end of the clavicle articulates with the acromion of the scapula (acromioclavicular joint): ARTICULATIONS OF THE SCAPULA

Dissection Instructions

Skin Incisions

Refer to Figure SKIN INCISIONS FOR THE ANTERIOR THORACIC REGION
Use a sharp scalpel to make a midline incision from the jugular notch (A) to the xiphosternal junction (C).
Make transverse incision from the jugular notch (A) along the clavicle to the acromion (B). Extend this cut to a point just beyond the shoulder (F).
Make a transverse incision from the xiphosternal junction (C) along the costal margin, to the midaxillary line (V).
Make a transverse incision through the nipple, from the sternum to the midaxillary line. An additional transverse incision from the sternum to the midaxillary line may be useful to aid in reflection of the skin.
DO NOT make a vertical lateral incision (G-V), or the circular incision around the arm (G-F), because these would result in removal of the skin.
Starting near the midline, reflect each flap of skin and superficial fascia from medial to lateral; the plane of dissection should be between the superficial fascia of the skin and the deep fascia of the pectoralis major muscle. Just lateral to the sternum, take note of any anterior cutaneous branches of the intercostal nerves and anterior intercostal vessels as they are ligated.
Reflect the skin just a little ways into the upper limb. We will dissect the upper limb in the winter quarter. Stay away from the deep structures of the axilla, especially the brachial plexus.
Near the midaxillary line, pull the skin and superficial fascia laterally, teasing apart the connective tissue with your fingers until you find at least one representative of a lateral cutaneous branch of an intercostal nerve, and accompanying branches of the posterior intercostal vessels, as they leave the intercostal space to supply the skin of the lateral wall of the thorax.
Between the lateral thoracic wall and the axilla, find one named lateral cutaneous branch of an intercostal nerve, the intercostobrachial nerve, which is the lateral cutaneous branch of T2 and, therefore, will exit between ribs 2 and 3 on its way to the skin of the axilla and medial side of the arm. Thus the nerve has clinical importance because of its association with referred pain of a heart attack.

Look in the superficial fascia near the clavicle for fibers of the platysma muscle. This very thin muscle of facial expression will be dissected in more detail when we study the head and neck in the spring. Any fibers of the platysma muscle may be reflected superiorly to accommodate further dissection of the thorax

Breast

The breast is dissected in female cadavers only. Students with male cadavers must observe at another dissection table. Because of the advanced age of some cadavers, it may be difficult to dissect and identify all of the structures listed. Expect the lobes of the gland to be replaced by fat with advanced age.

The breast extends from the lateral border of the sternum to the midaxillary line, and from rib 2 to rib 6. The mammary gland is a modified sweat gland that is contained within the superficial fascia of the breast. THE BREAST IN SAGITTAL SECTION The breast is positioned anterior to the pectoral fascia (the deep fascia of the pectoralis major muscle). The pectoral fascia is attached to the overlying skin by the suspensory ligaments of the breast that pass between the lobes of the mammary gland.

Identify the areola and the nipple. THE BREAST IN SAGITTAL SECTION
If not already done so during the reflection of the skin flaps, use a sharp scalpel blade to divide the mammary gland into two halves. The cut can be made in any orientation, but should pass through the nipple. Do not extend your cut deep into the underlying pectoral fascia and pectoralis major muscle.
Examine the cut surface of the breast, and use the handle of a forceps to scoop the fat out of several compartments between suspensory ligaments. These areas between suspensory ligaments once contained lobes of functional glandular tissue.
Find and clean one of the 15 to 20 lactiferous ducts that converge on the nipple. Identify the lactiferous sinus, which is an expanded part of the lactiferous duct located deep to the nipple.
Note that in most cadavers, it will be difficult to distinguish the lactiferous ducts from the suspensory ligaments based on their individual physical appearance. The easiest way to distinguish lactiferous ducts from suspensory ligaments is that the lactiferous ducts converge on the nipple, while most of the suspensory ligaments connect to other areas of the skin of the breast.
Trace one lactiferous duct to the nipple and attempt to identify its opening.
Use an illustration to study the lymphatic drainage of the mammary gland.
Insert your fingers behind the breast and open the retromammary space. This is the plane you used for reflection of the skin. Note that the normal breast can be easily separated from the underlying deep fascia of the pectoralis major muscle.

IN THE CLINIC

Breast

For descriptive purposes, clinicians divide the breast into four quadrants. The superolateral (upper outer) quadrant contains a large amount of glandular tissue and is a common site for breast cancers to develop. From this quadrant, an “axillary tail” of breast tissue often extends into the axilla.

In advanced stages of breast cancer, the tumor may invade the underlying pectoralis major muscle and its fascia. When this happens, the tumor and breast become fused to the chest wall, a condition that can be detected by palpation during a physical examination. As the breast tumor enlarges, it places traction on the suspensory ligaments, resulting in dimpling of the skin overlying the tumor.

Dissection Review

Review the location and parts of the breast.
Use an illustration to review the vascular supply to the breast.
Discuss the pattern of lymphatic drainage of the breast and identify by name the lymph node groups that are involved.
Use an illustration of the branching pattern of a typical spinal nerve to review the innervation of the anterior thoracic wall and breast. DISTRIBUTION OF A THORACIC SPINAL NERVE

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WEDNESDAY, SEPTEMBER 23 – PECTORAL REGION II

Muscles of the Pectoral Region

Dissection Overview

The muscles of the pectoral region are the pectoralis major, pectoralis minor, and subclavius muscles. The muscles of the pectoral region attach the upper limb to the thoracic skeleton. The pectoral muscles are positioned immediately deep to the superficial fascia (deep to the breast).

The dissection will proceed as follows: The pectoralis major muscle will be studied and reflected. The pectoralis minor muscle and clavipectoral fascia will be studied. The subclavius muscle will be identified. The pectoralis minor muscle will be reflected, and the branches of the thoracoacromial artery will be dissected.

Dissection Instructions

Clean the superficial surface of the pectoralis major muscle, using your fingers to define its borders CUTS FOR REFLECTION OF THE PECTORALIS MAJOR AND PECTORALIS MINOR MUSCLES Study an illustration and note that the deep fascia on the surface of the pectoralis major muscle is called pectoral fascia and that it is continuous with the axillary fascia that forms the floor of the axilla.
Identify the two heads of the pectoralis major muscle: clavicular head and sternocostal head. Observe that the juncture of these two heads is at the sternoclavicular joint.
Use your fingers to trace the tendon of the pectoralis major muscle to its distal attachment on the humerus. The pectoralis major muscle flexes, adducts, and medially rotates the humerus.
Between the clavicular head of the pectoralis major muscle and the adjacent deltoid muscle, use blunt dissection to define the borders of the deltopectoral triangle and find the cephalic vein. Preserve the cephalic vein in subsequent steps of this dissection. Note that variation in venous structures is quite common; therefore the cephalic veins on your cadaver may be large or small, and are often not bilaterally symmetrical.
Relax the sternal head of the pectoralis major muscle by flexing and adducting the arm. Gently insert your fingers posterior to the inferior border of the pectoralis major muscle. Create a space between the posterior surface of the pectoralis major and the clavipectoral fascia. Push your fingers superiorly to open this space.
Use scissors to detach the sternoscostal head of the pectoralis major muscle from its attachment to the sternum (CUTS FOR REFLECTION OF THE PECTORALIS MAJOR AND PECTORALIS MINOR MUSCLES, dashed line on right side).
Palpate the deep surface of the pectoralis major muscle to locate the medial and lateral pectoral nerves and vessels. Preserve these nerves and vessels.
Use scissors to cut the clavicular head of the pectoralis major muscle close to the clavicle.
Preserve the cephalic vein. Gently reflect the pectoralis major muscle laterally, noting that the lateral pectoral nerve and the pectoral branch of the thoracoacromial artery enter the deep surface of the clavicular head. Do NOT damage these structures.
Deep to the pectoralis major muscle are the clavipectoral fascia, pectoralis minor muscle, and subclavius muscle.
Identify the pectoralis minor muscle. The proximal attachment of the pectoralis minor muscle is ribs 3 to 5 near their costal cartilages. Its distal attachment is the coracoid process of the scapula. The pectoralis minor muscle draws the glenoid cavity of the scapula anteriorly and inferiorly.
Note that the medial pectoral nerve pierces the pectoralis minor muscle and then enters the pectoralis major muscle, innervating both, while the lateral pectoral nerve (and the thoracoacromial artery) passes through the clavipectoral fascia superior to the pectoralis minor muscle, and thus innervates only the pectoralis major muscle.
Identify the subclavius muscle, which is located inferior to the clavicle MUSCLES IN THE PECTORAL REGION. The subclavius muscle, which is attached to the clavicle and the first rib, depresses the clavicle.
Read a description of the clavipectoral fascia and understand that it is immediately deep to the pectoralis major muscle. The clavipectoral fascia is attached to the clavicle. It passes both superficial and deep to the subclavius muscle and the pectoralis minor muscle. The clavipectoral fascia is attached to the axillary fascia inferiorly.
Clean the cephalic vein where it crosses the anterior surface of the pectoralis minor tendon. The cephalic vein passes through the clavipectoral fascia medial to the pectoralis minor tendon.
Use scissors to detach the pectoralis minor muscle from its proximal attachments on ribs 3 to 5 (CUTS FOR REFLECTION OF THE PECTORALIS MAJOR AND PECTORALIS MINOR MUSCLES, dashed line on left side).
Reflect the pectoralis minor muscle superiorly. Leave the muscle attached to the coracoid process of the scapula.
Clean and define the branches of the thoracoacromial artery BLOOD SUPPLY TO THE PECTORAL REGION :

Acromial branch passes laterally across the coracoid process toward the acromion.
Deltoid branch courses laterally in the deltopectoral groove between the deltoid muscle and pectoralis major muscle. The deltoid branch accompanies the cephalic vein.
Pectoral branch passes between the pectoralis major muscle and the pectoralis minor muscle and supplies both.
Clavicular branch courses superiorly and medially to supply the subclavius muscle.

Note that you will NOT be required to name these branches specifically, just be able to identify the main thoracoacromial artery.
Also note that the thoracoacromial artery, and the lateral and medial and pectoral nerves, may be difficult to differentiate, especially if they are small. To make the distinction clear, dissect the thoracoacromial artery posteriorly until you see it branching from the large axillary artery. Clean out a small region of the axillary artery to positively identify it, removing any small veins that obscure the dissection field. Note that the lateral pectoral nerve arises from the brachial plexus of nerves (to be dissected later), and here is seen passing superior-lateral to the axillary artery. The medial pectoral nerve passes medial-inferior to the axillary artery.
Along the lateral border of the pectoralis minor muscle, identify the lateral thoracic artery. This vessel arises from the axillary artery near the origin and at right angles to the thoracoacromial artery, and passes inferiorly toward the thoracic cage.
Identify the serratus anterior muscle MUSCLES OF THE PECTORAL REGION. Note its extensive proximal attachment on the upper eight ribs. The distal attachment of the serratus anterior muscle is the deep surface of the scapula along the entire length of its medial border. You cannot see the distal attachment at this time. Find the innervation of the serratus anterior muscle, the long thoracic nerve, which arises from the brachial plexus and runs inferiorly on the lateral surface this muscle.

Dissection Review

Replace the pectoral muscles into their correct anatomical positions.
Review the attachments of the pectoralis major, pectoralis minor, and subclavius muscles. Review their actions, innervations, and blood supply.
Review the relationship of the clavipectoral fascia to the muscles, vessels, and nerves of this region.
Be sure that you understand the role played by the clavipectoral fascia in supporting the floor of the axilla.

FOCUS QUESTIONS ANSWER

1. From where do the lateral cutaneous branches (anterior and posterior) of intercostal nerves arise and to where do they distribute?

2. Is any pinkish mammary glandular tissue visible?

3. What are suspensory ligaments of the mammary glands?

4. Consider blood supply, nerve supply, and lymphatic drainage of the breasts.

5. Why are lateral and medial pectoral nerves reversed (with respect to their names and relative locations) from what you would expect? 6. Where does the cephalic vein terminate?

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