Dissector Answers - Superficial Back

Learning Objectives:

Upon completion of this session, 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 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 and define any of the underlined, boldfaced terms or structures.

Learning Objectives and Explanations:

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. (W&B 3-4)

All anatomical descriptions are used in relation to the anatomical position, which refers to the body as standing erect with:

• head, eyes and toes pointed anteriorly (forward)
• upper limbs by the sides with palms facing anteriorly
• lower limbs together with toes directed anteriorly.

Terms of relationship in the anatomical position:

• Superficial, intermediate, deep: used in dissection to describe the position of one structure with respect to another (The investing fascia is superficial to the muscle.)
• Medial: indicates that a structure is nearer to the median plane (midpoint) of the body. Lateral: indicates that a structure is farther away from the median plane. (The eyes are lateral to the nose.)
• Posterior (dorsal): denotes the back surface of the body, or a structure nearer to the back. Anterior (ventral): denotes the front surface of the body, or a structure nearer to the front. (The sternum is anterior to the vertebral or spinal column.)
• Proximal, distal: relative terms that are used when contrasting positions with respect to the attachment/origin of the limb/structure. Proximal is nearer and distal is farther from the attachment/origin. (The wrist is distal to the elbow.)

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. (W&B 3-4)

Anatomical descriptions are based on four imaginary planes that pass through the body in the anatomical position:

• Median/sagittal/midsagittal plane: the vertical plane passing longitudinally through the body, dividing it into right and left halves.
• Parasagittal planes: vertical planes passing through the body parallel to and on the right or left side of the median plane.
• Coronal/frontal planes: vertical planes passing through the body perpendicular to the median plane, dividing it into anterior and posterior sections.
• Horizontal (transverse) planes: planes passing through the body at right angles to the median and coronal planes. It divides the body into superior and inferior parts. Views of this type are commonly called cross-sections.

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. (W&B 4-6)

Structural layers, from superficial to deep:

• Skin: made of epidermis and dermis. The epidermis is the thin, outer, impervious cellular layer. The dermis is the thicker, leather-like layer that gives skin its strength. In it are hair follicles, sweat glands, sebaceous glands (innervation and blood supply is through the subcutaneous tissue deep to skin). (Latin/Greek, epi = upon, attached to)
• Subcutaneous tissue or fat: also known as superficial fascia or hypodermis. A fascia is a covering, a sheet of connective tissue. (Latin, fascia = band)
• Deep or investing fascia: a denser connective tissue layer that "invests" the area like a stocking or sleeve. It binds groups of muscles together as a unit and has partitions between the muscles called intermuscular septa. The investing fascia is usually named regionally (e.g. thoracolumbar fascia). (Latin, septum = partition, wall)
• Muscular fascia: surrounds individual muscles directly
• Neurovascular bundle: veins, arteries, lymphatic vessels, and nerves often travel together in these bundles. They vary in size and do not always have all four elements. They often have branches that begin deep and travel superficially - supplying muscles, then subcutaneous tissue, and finally terminating in the skin.
• The deepest layer in limbs is the bone, while in the trunk the body cavities will be the deepest.

4. Demonstrate a cutaneous nerve and describe patterns of cutaneous nerves on the back. (W&B 88-89, N 170, 177, 180, 192, 258, TG 1-12, 1-17)

The sensory nerves of the back all arise from the dorsal primary ramus of the associated spinal nerve. Above the mid-thorax, these terminal cutaneous branches can be found piercing the trapezius near the mid-line and traveling laterally from there to innervate the skin. Below the mid-thorax, the cutaneous branches pierce the muscles in an increasingly lateral position as you move inferiorly. (Latin, ramus = branch)

The cutaneous branches of dorsal primary rami only innervate skin from the back of the head to the upper buttocks, and as far laterally as the rib angles, approximately. The rest of the trunk, and all of the skin of the limbs, is supplied by branches of ventral primary rami of spinal nerves. The cutaneous nerves of the trunk are distributed segmentally to supply strips of skin, roughly following the direction of the ribs, called dermatomes.

5. Identify, and give the general attachments of, nerve and blood supply to, and the general functions of the superficial back muscles. (N 174, 424, TG 1-13)

Note: Muscles are typically described by their origin and insertion. The origin of a muscle is the point from which it acts - usually, but not always, this is more proximal in the limbs. The insertion of a muscle is the more moveable point - the part that is acted upon - usually, this is more distal in the limbs.

Muscle	Origin	Insertion	Action	Innervation	Artery	Notes
trapezius	medial third of the superior nuchal line, external occipital protuberance, ligamentum nuchae, spinous processes of vertebrae C7-T12	lateral third of the clavicle, medial side of the acromion and the upper crest of the scapular spine, tubercle of the scapular spine	elevates and depresses the scapula (depending on which part of the muscle contracts); rotates the scapula superiorly; retracts scapula	motor: spinal accessory (XI), proprioception: C3-C4	transverse cervical a.	named for its shape; trapezius is an example of a muscle that migrates during development from its level of origin (cervical) to its final position, pulling its nerve and artery along behind
latissimus dorsi	vertebral spines from T7 to the sacrum, posterior third of the iliac crest, lower 3 or 4 ribs, sometimes from the inferior angle of the scapula	floor of the intertubercular groove	extends the arm and rotates the arm medially	thoracodorsal nerve (C7,8) from the posterior cord of the brachial plexus	thoracodorsal a.	the inserting tendon twists so that fibers originating highest insert lowest
levator scapulae	transverse processes of C1-C4 vertebrae	medial border of the scapula from the superior angle to the spine	elevates the scapula	dorsal scapular nerve (C5); the upper part of the muscle receives branches of C3 & C4	dorsal scapular a.	levator scapulae is named for its action
rhomboideus major	spines of vertebrae T2-T5	medial border of the scapula inferior to the spine of the scapula	retracts, elevates and rotates the scapula inferiorly	dorsal scapular nerve (C5)	dorsal scapular a.	named for its shape
rhomboideus minor	inferior end of the ligamentum nuchae, spines of vertebrae C7 and T1	medial border of the scapula at the root of the spine of the scapula	retracts, elevates and rotates the scapula inferiorly	dorsal scapular nerve (C5)	dorsal scapular a	named for its shape

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

• Triangle of auscultation (W&B 90, N 152, TG 1-13): A triangular gap formed by the superior horizontal border of the latissimus dorsi, the medial border of the scapula, and the inferolateral border of the trapezius. This is a good place to examine posterior segments of the lungs with a stethoscope.
• Lumbar triangle (W&B 90, N 152, TG 1-13): Bordered medially by the latissimus dorsi, laterally by the external abdominal oblique, and inferiorly by the iliac crest. This point is vulnerable to abdominal hernia.
• Vertebra (spina) prominens (W&B 88, N 21, TG 1-01): This prominent vertebra at C7 is characterized by a long spinous process. It can be palpated and marks the location of the last cervical vertebra.
• Scapula (W&B 88, N 420, 421, TG 1-01): The spine of the scapula can be palpated medially at level T3.
• Iliac crest (crest of ilium) (W&B 88, N 486, TG 1-01): This important landmark can be palpated at level L4. It denotes superior border of the greater pelvic region.
• External occipital protuberance (inion) (W&B 88, N 4): A median projection located a little inferior to the bulging part of the posterior aspect of the head at the junction of the head and neck. Directly below this the spinous process of C2 can be palpated. C1 has no spinous process, and its small posterior tubercle is neither visible nor palpable. (Latin, occiput = against head)
• Nuchal groove (N 152, TG 1-01): The median area between C3 and C5 denotes the short bifid spinous processes of C3 through C5 vertebrae. (Latin, nucha = nape)
• Mastoid processes (N 4, TG 7-05, 7-06): These are prominences of the temporal bone posterior to the ear and the angle of the jaw on either side of the head. The transverse process of C1 may be felt by deep palpation between the mastoid processes. (Latin/Greek, mastoides = resembling a nipple)

Cultural enrichment: Check out these sections from the 1918 version of Gray's Anatomy of the Human Body! Some of the terms are (of course) out-of-date, but the illustrations are timeless. Surface Anatomy of the Back - Surface Markings of the Back - The Muscles Connecting the Upper Extremity to the Vertebral Column - The Accessory Nerve

Questions and Answers:

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

The cutaneous branches of the dorsal rami innervate the subcutaneous tissue and skin of the back. Their origin are the spinal nerves running from C3 to the coccyx. These innervate the "intrinsic" (deep) muscles of the back and the overlying skin, but not the muscles in between. It is important to remember that the extrinsic or superficial muscles of the back (latissimus dorsi, trapezius, etc.) are not innervated by the dorsal rami but by branches off of the ventral primary rami of spinal nerves. (Do not confuse rami with roots here... while branches of the dorsal primary rami supply motor function to the deep back muscles and sensory function to the skin, sensory nerves arise from the dorsal root and motor nerves arise from the ventral root.) (N 180, 258, TG 1-12, 1-17)

8. What is the function of the investing fascia?

The investing fascia (deep fascia) invests the muscles and associated tendons up to their attachment to bone and is itself attached to the periosteum of the bones. Parts of it are named for the muscle being invested. For example, the thoracolumbar fascia encloses the deep muscles of the back in the thoracic and lumbar regions. Furthermore, groups of muscles with similar functions may be contained in a common compartment formed by the investing fascia. These fascial compartments often contain or direct the spread of an infection or a tumor.

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

Loose fascia and fat are located throughout the body at sites which require padding or mobility, where friction is a concern, or where blood vessels and/or nerves require padding.

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

Nerves, arteries, and veins all have different embryological origins, different anatomical sources, different targets, and obviously different functions. But, just like different people travel on the same interstate highway to go to different places, nerves, arteries, and veins can start somewhere, jump into a neurovascular bundle in order to travel a distance, then jump off to reach their destination.

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

C3, C4, and the accessory nerve (CN XI) combine at the C5 level to form the subtrapezial plexus. Together they innervate the trapezius muscle. The accessory nerve carries the motor fibers, while C3 and C4 carry the proprioceptive sensory fibers. (N 127, 177, TG 1-13)

12. What is the thoracolumbar fascia (aponeurosis)?

The thoracolumbar fascia is an extensive fascial sheet that splits into anterior and posterior layers, thereby enclosing the deep back muscles. It is thin and transparent where it covers the thoracic parts of the deep muscles but is thick and strong in the lumbar region. The lumbar part of the thoracolumbar fascia, extending between the 12th rib superiorly and the iliac crest inferiorly, is a point of origin for the internal oblique and transverse abdominal muscles. (N 174, 179, TG 1-13)

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

The serratus posterior inferior and serratus anterior muscles lie deep to the latissimus dorsi, attaching to the costal surfaces. At the mid-scapular plane, however, between the 9th and 12th rib, there is also attachment of the latissimus dorsi to the ribs. Additionally, the latissimus dorsi often has a small attachment to the inferior angle of the scapula. (N 174, TG 1-13)