ABOUT THE ELECTRONIC DISSECTION MANUAL: This electronic laboratory manual is a modified version of Grant's Dissector by Patrick Tank. This manual should not be regarded as U.C.'s comprehensive textbook of gross anatomy. The primary purpose of this manual is to provide you with the instructions for performing dissections. Additional information such as nerve supply, actions, insertions of muscles, etc., are sprinkled in for your convenience but these should not be regarded as particular points of emphasis.

For this and all subsequent courses, you must be prepared to identify and define all boldfaced structures, spaces, and openings. In addition, you must be prepared to answer questions about these anatomical features. Integration of information from multiple sources is an important skill that every physician must obtain. As you undertake your study of anatomy, you should integrate information from all assigned sources including, but not limited to, this lab manual, other lab manuals, your assigned readings, lectures, etc. You will be asked what we refer to as "secondary questions." For example, we may tag a boldfaced muscle and ask for its nerve supply, blood supply (in cases such that it is a specific vessel), origin, insertion, or function. We may also ask clinical questions about these features.

• Introduction
• Osteology
• Surface Anatomy
• Superficial and Intermediate Back
  • Skin and Superficial Fascia
  • Superficial Back Muscles
    • Trapezius
    • Latissimus dorsi
    • Rhomboids
    • Levator scapulae
• Intermediate Back Muscles
• Focus questions - Superficial and Intermediate Back
• Deep Back and Vertebral Canal
  • Deep back muscles
  • Laminectomy
  • Meninges and spinal cord
• Focus questions - Deep Back and Vertebral Canal

OSTEOLOGY

Learning Objectives: Upon completion of the osteology assignment, the student will be able to:

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.

Refer to a skeleton. Identify:

• Clavicle
• Scapula
• Acromion
• Spine of the scapula
• Superior angle of the scapula
• Medial (vertebral) border of the scapula
• Inferior angle of the scapula
• Ilium
• Iliac crest
• Posterior superior iliac spine
• Occipital bone
• External occipital protuberance (inion)
• Superior nuchal line
• Foramen magnum
• Temporal bone
• 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 on the pelvic girdle. A typical thoracic vertebra will be reviewed, and the cervical and lumbar vertebrae will be compared with it.

Refer to a disarticulated thoracic vertebra and identify:

• Body
• Pedicle
• Lamina
• Vertebral arch - formed by the combination of pedicles and laminae.
• Vertebral foramen
• Transverse process
• Transverse costal facet
• Spinous process
• Superior and inferior articular processes
• Superior and inferior articular facets
• Superior and inferior vertebral notches
• Superior and inferior costal demifacets

The bodies of thoracic vertebrae tend to be heart-shaped. The spinous process of a thoracic vertebra is long, slender, and directed inferiorly over the spinous process of the vertebra 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 (e.g., the tubercle of rib 5 articulates with the transverse costal facet of vertebra T5). Generally, the head of a rib articulates with the bodies of two adjacent vertebrae, more specifically, it articulates with the superior costal demifacet of the vertebral body of the same number and the inferior costal demifacet of the vertebral body superior to it (there are exceptions at either end of the rib cage with which you need not be concerned).

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 (see figure below showing a comparison of typical cervical, thoracic, and lumbar vertebrae) in that typical cervical vertebrae (by typical, we are referring to C3-C6) have smaller bodies; larger, triangular-shaped 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 (transverse foramen)
• Spinous process

The C1 and C2 vertebrae are atypical. Using disarticulated C1 and C2 vertebrae (see figure), and on an articulated skeleton, observe the following features:

• the Atlas (C1) does not have a body. It consists of anterior and posterior arches, fused together by left and right lateral masses to form a bony ring. The anterior arch features an anterior tubercle; the posterior arch has a posterior tubercle. On the superior surface of the posterior arch there is a pair of grooves for the vertebral artery.
• the Axis (C2) has the dens, which had been the body of C1 that become fused to C2 during development.

The vertebra prominens (C7) has the most prominent spinous process in the cervical region, hence its name.

Lumbar vertebrae differ from thoracic vertebrae in that lumbar vertebrae have larger, kidney-shaped 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, identify:

• 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.

SURFACE ANATOMY

Learning Objectives: Upon completion of the surface anatomy assignment, the student will be able to:

1. Identify surface anatomy landmarks
2. Extrapolate the position of surface anatomy landmarks relative to internal structures.

Palpate the following subcutaneous structures on a volunteer or on your cadaver (refer to the figure and to your lab guide The Anatomical Basis of the Physical Exam: Osteology and Surface Anatomy Laboratory Exercises:

• External occipital protuberance
• Superior nuchal Line
• Mastoid process
• 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
• Rib cage
• Vertebral line
• Paravertebral line
• Scapular line
• Erector spinae muscles (most noticeable in the lumbar region on either side of the Median Furrow)
• Lateral border of the latissimus dorsi muscle (in the posterior axillary fold)
• Iliac crest (at vertebral level L4)
• Posterior superior iliac spine (PSIS)
• Sacrum
• Median sacral crest

Identify the positions of the following:

• Intercristal Line (Supracristal plane)
• Dermatomes of the back (C7-L4)

THE SUPERFICIAL AND INTERMEDIATE BACK

Learning Objectives:
Upon completion of this assignment, the student will be able to:

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 describe the general functions of the superficial and intermediate back muscles.
6. Identify and name the nerve and blood supply to the superficial and intermediate back muscles.
7. Identify the bony prominences of the back and spine that may be palpated and used for reference to underlying structures.

Dissection Instructions - Skin and Superficial Fascia

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 and removed. Posterior cutaneous nerves will be studied. The superficial fascia will then be removed.

1. 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.
2. 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.
3. Make a transverse skin incision from the external occipital protuberance (X) laterally to the base of the mastoid process (M).
4. Make a transverse skin incision from R to B superior to the scapula and superior to the acromion.
5. 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 6 inches wide to make skinning easier.
6. 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.

Dissection Instructions - Superficial Fascia

1. Using a prosection specimen provided by your instructor, locate the greater occipital nerve and the occipital artery. Do not attempt to dissect the greater occipital nerve or occipital artery in your cadaver.
2. The greater occipital nerve is the dorsal ramus of spinal nerve C2. The greater occipital nerve pierces the semispinalis capitis muscles (we will study this muscle later) and the trapezius muscle about 3 cm inferolateral to the external occipital protuberance on its way to the skin. The greater occipital nerve supplies general sensation from the skin of the scalp on the back of the head. The occipital artery passes lateral to the greater occipital nerve to supply the posterior scalp.
3. 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 when the time comes, you will have enough experience to differentiate these in later blocks." 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: 1) the nerve is medial to the artery, and 2) nerve pierces 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.

4. Spinal nerves give rise to dorsal rami which supply the deep (true, intrinsic) muscles of the back with motor fibers. Each dorsal ramus gives rise to posterior cutaneous branches that pierce the trapezius muscle or latissimus dorsi muscle to enter the superficial fascia and skin. The posterior cutaneous branches supply sensory receptors and glands of the skin.
5. 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), dissect in the superficial fascia about 1-2 cm lateral to the midline to find at least one representative of a posterior cutaneous branch of a dorsal ramus. The posterior cutaneous branches provide sensory innervation of the skin. Once you find at least one posterior cutaneous branch, go ahead and remove the remaining superficial fascia from the muscle. Try to leave examples of cutaneous nerves that you locate. As you dissect and reflect muscles, pull the cutaneous nerves through the muscles to preserve them.
6. 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 the dermatome chart below and become familiar with the concept of segmental innervation.

Dissection Instructions - Superficial Muscles of the Back

As described earlier, the superficial muscles of the back are not true back muscles in that they don't act primarily on the vertebral column. The superficial muscles of the back are appendicular muscles that act on the upper limb or its pectoral girdle (clavicle and scapula).

The superficial muscles of the back are the trapezius, latissimus dorsi, rhomboid major, rhomboid minor, and levator scapulae. 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.

Trapezius Muscle

1. Clean the surface of the trapezius muscle (L. Trapezoides, an irregular four-sided figure).
2. Do not disturb the superior border of the trapezius muscle. Observe the proximal attachment of the trapezius muscle on the external occipital protuberance, the medial part of the superior nuchal line, the nuchal ligament, and the spinous processes of thoracic vertebrae (specifically, vertebrae C7 to T12). The nuchal ligament is a thick band of connective tissue found in the midline of cervical region where the vertebral column is posteriorly concave (you will see this more clearly on a skeleton).
3. 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.
• When all three parts of the trapezius muscle contract simultaneously, the scapula rotates in the coronal plane, causing the shoulder joint to angle upward. This action of the trapezius muscle is essential for abducting the upper limb to above shoulder height (we will see later that the serratus anterior muscle assists the trapezius muscle with this action).

4. 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 gently break the plane of loose connective tissue that lies between the trapezius muscle and the deeper muscles of the back. The trapezius, as is the case with many muscles, can be thin enough to tear when not handled gently. At each step, insert your fingers only far enough to clear the fascia deep to a portion of the muscle, detach the muscle as described in the next step, then repeat.
5. 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). Do this for both the right and left trapezius muscles (dissections should always be performed bilaterally unless otherwise indicated). Start inferiorly and continue the cut superiorly as far as the external occipital protuberance. Note that the posterior neck muscles may already be partially dissected.
6. 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.
7. If necessary, 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). Make this cut very close to the bone. Leave the trapezius muscle attached to the clavicle and cervical fascia.
8. Reflect the trapezius muscles superolaterally. Leave the cervical fascia attached along the superior border of the trapezius muscle to act as a hinge.
9. Study the deep surface of the reflected trapezius muscle. Find and clean the plexus of nerves (subtrapezial plexus) 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 (specifically proprioception; proprio- fr. L, "one's own", provides the central nervous system with feedback about the extent of muscle contraction and tension). The trapezius muscle is supplied by the superficial branch of the transverse cervical artery. Dissect the superficial branch of the transverse cervical artery that accompanies the nerves. You may remove the transverse cervical vein to clear the dissection field.
10. A note about the cranial nerves, there are 12 pairs of them. The accessory nerve (Cranial Nerve XI) is the first you've encountered. As you learn the cranial nerves, you will be responsible for knowing them by name and by number, so it's a good idea to make the mental linkage between accessory nerve and CN XI second nature now. If you indicate a cranial nerve by number, you must use roman numerals, as opposed to arabic numerals. For example, in this case, CN XI is correct, CN 11 is not.
11. To get to the trapezius muscle, the accessory nerve (Cranial Nerve XI) passes through the posterior triangle of the neck. Do not follow the nerve proximally into the posterior triangle at this time. The posterior triangle will be studied with the neck during the Brain, Mind, and Behavior Block.
12. 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

1. Clean the surface and define the borders of the latissimus dorsi muscle (L. Latissimus, widest).
2. The proximal attachments of the latissimus dorsi muscle are the spinous processes of lower thoracic vertebrae (T7 to T12), the thoracolumbar fascia (see description below), and the iliac crest (the latissimus dorsi muscle also has a proximal attachment lateral to the angles of ribs 9 to 12).
3. 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 but we do expect you to know, at this time, the names of the nerve and artery that supply the latissimus dorsi muscle). The distal attachment of the latissimus dorsi muscle, its nerve, and its artery will be dissected with the upper limb.
4. The latissimus dorsi muscle extends, adducts, and medially rotates the humerus (i.e., it spins the anterior surface of the humerus around its long axis toward the midline) at the shoulder joint. You are responsible for knowing the actions of muscles. If the actions of the latissimus dorsi muscle, or any other muscle, aren't clear, please ask an instructor during the laboratory session. When the arms are raised to grasp an object above one's head (such as in climbing or doing chin-ups), extension of the arms, mediated by the latissimus dorsi muscle, raises the body toward the hands.
5. 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 (see dashed line on right side in image above). Do not cut too close to the lumbar spinous processes.
6. Reflect the latissimus dorsi muscles laterally. Do not disturb their attachments to the ribs. They may also have attachments to the inferior angles of the scapula. If so, do not disturb these attachments.
7. Be certain that only the latissiumus dorsi muscle and not the serratus anterior or serratus posterior inferior muscle is reflected.

The thoracolumbar fascia (lumbodorsal fascia) is a deep investing membrane, which covers the deep muscles of the back. 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, which you will study in a later course. The erector spinae muscles are enclosed between the middle and posterior layers. The erector spinae muscles will be dissected during the next laboratory session.

Above, it passes anterior to 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 thoracolumbar fascia is a thin fibrous lamina that 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 and laterally to the angles of the ribs.

Rhomboid Major and Minor Muscles

1. 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 its spine.
2. 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 of the scapula.
3. The rhomboid muscles retract the scapula, rotate the scapula to depress the glenoid cavity (the glenoid fossa is a shallow depression on the scapula that articulates with the head of the humerus), and hold the scapula close to the thoracic wall.
4. To reflect the rhomboid muscles, insert your fingers deep to the inferior border of the rhomboid major muscle and separate it from deeper muscles.
5. 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.
6. The rhomboid muscles are supplied by the dorsal scapular nerve, artery and vein. 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.
7. A note about the variations in the origin of the dorsal scapular artery. It may branch directly from the subclavian artery in the root of the neck, in which case it is properly called the dorsal scapular artery. Alternatively, it may arise from the transverse cervical artery, in which case it should properly be called the deep branch of the transverse cervical artery. Furthermore, it is possible for your cadaver to have a dorsal scapular artery on one side and a deep branch of the transverse cervical artery on the other (never assume that anatomy must be symmetric). In any case, there is no way you can know at this time.

Levator Scapulae

1. 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.
2. 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.
3. The distal attachment of the levator scapulae muscle is the superior angle of the scapula.
4. As its name implies, the levator scapulae muscle elevates the scapula and rotates the scapula to depress the glenoid cavity.
5. In addition to the rhomboid muscles, the dorsal scapular nerve and artery supply the levator scapulae muscle as well.

Dissection Review

1. Replace the superficial muscles of the back in their correct anatomical positions.
2. Use the dissected specimen to review the proximal attachment, distal attachment, action, innervation, and blood supply of each muscle that you have dissected.
3. Review the movements that occur between the scapula and the thoracic wall.
4. Use an illustration to observe the origin of the transverse cervical artery and the origin of the dorsal scapular artery.

Dissection Instructions - Intermediate Muscles of the Back

Like the superficial muscles, the intermediate muscles of the back are not true back muscles in that they don't act primarily on the vertebral column. The intermediate muscles of the back are accessory respiratory muscles that assist in lifting the ribs during deep inspiration or assist in depressing the ribs during deep expiration.

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 that 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.

1. The proximal attachments of the serratus posterior superior muscle are the nuchal ligament and the spinous processes of the upper thoracic vertebrae (C7 to T3). Its distal attachments are the superior borders of the upper ribs (ribs 2 to 5), lateral to their angles.
2. The proximal attachments of the serratus posterior inferior muscle are the spinous processes of the thoracolumbar vertebrae (T11 to L2). Its distal attachments are the inferior borders of the lower ribs (ribs 9 to 12), lateral to their angles.
3. The serratus posterior muscles are innervated by intercostal nerves (ventral rami of thoracic spinal nerves).

THE DEEP BACK AND VERTEBRAL CANAL

The deep muscles of the back act on the vertebral column. There are many deep muscles of the back and only a few will be dissected: splenius capitis muscle, splenius cervicis muscle, semispinalis capitis muscle, and the erector spinae muscles (spinalis, longissimus, iliocostalis). 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 three columns of muscle that compose the erector spinae muscles will be identified.

Learning Objectives: Upon completion of this assignment, the student will be able to:

1. Identify the deep back muscles that you are required to dissect during this lab exercise, describe their general nerve supply, and their actions.
2. 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.
3. Identify the terminal specialties of the cord, their relation to lumbar puncture, the nerve rootlets, and blood supply.
4. Describe the anatomy of the cord and vertebrae as related to fractures, dislocations, and possible cord injury.
5. 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).
6. Describe a typical spinal nerve, the somatic motor and sensory components found in any portion, and their distribution.
7. Describe conceptually how any region of the thoracic wall gets its blood supply and innervation.
8. Define and explain the significance of dermatomes.
9. Explain the difference between superficial and deep (true) back muscles.

Dissection Instructions - Deep Muscles of the Back

Splenius Muscle

1. Identify the splenius muscle (Gr. splenion, bandage).
2. 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 the upper thoracic vertebrae (C7 to T6).
3. 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 the upper cervical vertebrae (C1 to C4).
4. 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.
5. Reflect the muscles laterally, leaving their distal attachments undisturbed.

Erector Spinae

1. The erector spinae muscles (L. erector, one who erects) lie deep to the serratus posterior muscles, which were reflected in the previous laboratory session.
2. Reflect both serratus posterior muscles laterally from their proximal attachments on the spinous processes. Leave them attached to their distal attachments on the ribs.
3. The erector spinae muscles are composed of three columns of muscle: spinalis, longissimus, and iliocostalis. The intent of this dissection is to identify these three columns of muscle.
4. Use a scalpel to incise the posterior surface of the thoracolumbar fascia. Use blunt dissection to remove the thoracolumbar fascia ON ONE SIDE ONLY from the posterior surface of the erector spinae muscles.
5. Use your fingers to separate the three columns of muscle at midthoracic levels. The columns of the erector spinae muscles are fused to each other at the level of their inferior attachments to the sacrum and ilium.
6. Identify:
• Spinalis muscle - the medial column of the erector spinae muscles. 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 muscles. 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 (you will not be asked to identify this muscle), attaches to the mastoid process of the temporal bone.
• Iliocostalis muscle - the lateral column of the erector spinae muscles. Its inferior attachment is the ilium (iliac crest) and its superior attachments are on ribs (L. costa, rib).
7. All three columns of the erector spinae muscles extend the vertebral column when both sides work together. If only one side of the erector spinae muscles is active, it bends the vertebral column laterally toward the side that is active.

You are not required to dissect the transversospinal group of muscles (see figure if you're interested in these muscles) located deep to the erector spinae muscles. 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 compose this group: semispinalis, multifidus, and most deeply, rotatores.

The semispinalis capitis muscle is the most superficial member of the transversospinal group of muscles. Because the semispinalis capitis muscle has an important clinical relationship to the greater occipital nerve, it will be studied:

1. The semispinalis fibers in the cervical region attach to the base of the skull, and thus are specifically identified as the semispinalis capitis muscle.
2. Identify the semispinalis capitis muscle (L. semi, half; L. spinalis, Spine). Note that the semispinalis capitis muscle may have been already partially dissected.
3. The semispinalis capitis muscle lies deep to the splenius muscles and its fibers course vertically, parallel to the long axis of the neck.
4. The inferior attachments of the semispinalis capitis muscle are the transverse processes of the upper thoracic vertebrae.
5. 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.
6. Do not dissect the semispinalis capitis muscle further at this time.

The following table is provided as a convenient summary. You need not memorize its contents but concentrate on the concepts outlined in your dissection instructions.

Laminectomy

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 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 L1/L2 intervertebral disc. 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 that corresponds to spinal cord segments C5 to T1 and a lumbosacral (lumbar) enlargement that corresponds to spinal cord segments L1 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 (e.g., 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. The C8 spinal nerve, which does not have a correspondingly numbered vertebra, exits between vertebra C7 and T1.

Dissection Instructions - Laminectomy

The order of dissection will be as follows: The spinalis muscles will be pulled laterally to expose the transversospinal group of muscles. The transversospinal group of muscles will be removed 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.

1. Be sure that you have protection for your eyes while chiseling or sawing!
2. Using a scalpel, cut down along the right side of the spinous processes to detach spinalis muscles from these processes.
3. Use your fingers to pull the spinalis muscles laterally to expose the transversospinal group of muscles.
4. Repeat the previous 2 steps on the left side.
5. To prepare for the laminectomy, you should have the vertebral lamina (area between the transverse and spinous processes) devoid of muscle tissue, i.e., completely scoop out the transversospinal group of muscles, from C3 to the sacrum.
6. Cut and scrape the vertebral laminae clean of muscle tissue because the autopsy saw works well on bones, not soft tissue.
7. Use bone clippers (or a mallet and chisel) to remove the spinous processes from their vertebrae (observe the strong supraspinous ligaments that interconnect the tips of the spinous processes along the length of the vertebral column).
8. 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.
9. 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.
10. 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). Continue your cut inferiorly through the lamina of successive vertebra, down to and including L5. Try to maintain a straight saw cut as you go from lamina to lamina.
11. Repeat for the other side of the vertebral column.
12. At the cranial end of the laminectomy, use a scalpel to cut the soft tissue (ligamentum flavum) between the C2 and C3 vertebrae. Be careful not to cut the underlying dura mater.
13. Repeat the previous step at the caudal end of the laminectomy, between the L5 vertebra and the sacrum.
14. Remove the cut portion of the vertebral column from the cadaver, freeing any remaining attachments with a mallet and chisel. 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.
15. Be careful around sharp edges of the remaining parts of the vertebrae; you can use the mallet to hit (blunt) the sharp bony edges.
16. Wipe tissue debris from your construction equipment (mallets, chisels) with paper towels and return them to the proper storage drawer.
17. Unplug the autopsy saw and wipe the blade clean of tissue debris with paper towels and return it to the blue bin.
18. 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

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

Meninges and Spinal Cord

1. Observe the epidural (extradural) space. Use blunt dissection to remove the epidural fat and the posterior internal vertebral venous plexus from the epidural space.
2. Identify the dural sac, which ends inferiorly at vertebral level S2. 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 L5. Attempt to do this without damaging the underlying arachnoid mater. Retract the dura mater and pin it open.
3. Identify the arachnoid mater. 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.


4. 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.
5. Identify the following features of the spinal cord (Refer to figure):
• Cervical enlargement (spinal cord segments C5 to T1) provides nerves to the upper limb.
• Lumbosacral (lumbar) enlargement (spinal cord segments L1 to S3) provides nerves to the lower limb. The lumbosacral 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.
6. Identify the cauda equina (L., tail of horse), a collection of ventral and dorsal roots in the lower vertebral canal.
7. Identify the filum terminale internum, a delicate filament that is 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.
8. The 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.

9. The pia mater forms two denticulate ligaments, one on each side of the spinal cord. Each denticulate ligament has 21 teeth and each tooth is attached to the inner surface of the dura mater, anchoring the spinal cord.
10. Use a probe to follow dorsal roots and ventral roots to the point where they pierce the dura mater and enter the intervertebral foramen. 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.
11. 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.

12. 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). 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.