Pre-lab Exercise: Surface Anatomy of the Back
Primary Lab Objectives and Goals:
- Prepare the body (make sure you have watched the informational video referenced below BEFORE lab).
- Examine the surface anatomy of the back and compare it with landmarks on the skeletons in the lab. Understand the relationships between body surfaces and bones with special reference to injury and pathology. Please make sure you have read the pre-lab material and consulted a textbook or other source for information about bones.
- Reflect the skin of the back from the neck to the lumbar region. Note the thickness of of the skin in each region and consider the functional consequences of this variation.
- Identify and reflect the superficial hypaxial muscles (limb muscles located on the superficial back). Recognize that hypaxial muscles generally (with a couple of exceptions) attach to and move limbs.
1. PREPARATION: The procedures for preparing the cadaver are described in detail in this video. Instructors will be in the lab to help you with this procedure and answer any questions you have.
- Remove and carefully set aside the plastic wrap and sheet.
- Wrap the hands, feet, and head in the cloth provided at each table.
- Soak the cloth in appropriate wetting solution, which can be found in spray bottles in the tool room.
- Cover the wrapped hands, feet, and head with a plastic bag and tie with string to prevent drying out.
- Turn the cadaver face-down (prone) on the table.
At the end of each lab sesssion you will spray down your cadaver and cloth sheet and then cover the cadaver with the sheet and plastic cover. More details are provided on the video and by your instructor.
2. IN-LAB SURFACE ANATOMY EXERCISE: Bony landmarks on the back
Examine the back of the cadaver. Attempt to locate the following bony landmarks underlying the skin of the back (refer to accompanying diagram). Mark those landmarks with pen provided at your table. Not all landmarks will be easy to palpate or identify. The accessibilty of each landmark will vary by the stiffness, position, and amount of body fat of the cadaver. These landmarks will help guide your incisions when you begin dissection.
- External occipital protuberance
- Spinous process of the 7th cervical vertebra
- Spine and medial border of scapula
- Last two ribs (floating ribs 11 and 12)
- Iliac crests
- Spinous process of 4th lumbar vertebra (in midline along line connecting iliac crests)
If you are interested in the surface anatomy in relation to the underlying organs (e.g., lungs, heart) to better understand needle biopsies, auscultation, and/or radiological imaging please click HERE for an optional exercise.
If you wish to see radiological images of the bones of the back as well as organs of the thorax, refer to the thumbnail gallery below:
3. SKIN REMOVAL. Begin to remove skin from the back as shown in the figure below. Please take note here and throughout the dissector of the useful HINTS that will help with your dissection.
- Make a (~10 inches) midline incision on the back in the thoracic region between the two scapulae.
- Identify the subcutaneous layer of superficial fascia.
- Separate the skin and attached superficial fascia from the underlying deep fascia covering the muscles
It is very difficult to cut through the skin and fat on the back. Make sure that you have a supply of fresh scalpels that you can use and change blades often. The fat can be thick and dense, in some places mimicking muscle (i.e. it looks red instead of yellow, very dense, can look striated). The fat is especially thick in the region of the lower back. You will ONLY cut through the skin and superficial fascia, so you will have to use your fingers to probe the space between the hypodermis and the fascia surrounding muscles. You may need to remove a lot of fat to find the plane. Don't take too much time with that but do try to clear the space so you can see muscle fibers.
HINT: It is very difficult to cut through the skin and fat on the back. Replace scalpel blades often as they become dull. Fat throughout the back can be thick and dense, in some places mimicking muscle (i.e. fat may be very firm, appear striated, and may be reddish rather than yellow in color). Fat of the lower back tends to be especially thick. Take care to ONLY cut through the skin and superficial fascia then use your fingers to probe the space between the hypodermis and muscular fascia. You may need blunt dissect out a large amount of fat to find the plane. Do not linger on this task but do try to clear the space so you can see muscle fibers.
NOTE: The first muscle you will encounter is the trapezius. Be sure to have your atlas out and open to a picture of this muscle. Imagine what this muscle will look like under the skin, including size, shape, and fiber direction.
- Once you find the correct plane, extend the midline incision up to the external occipital protuberance.
- Next, extend the incision down to the lumbar region, followed by three horizontal cuts through the skin and the superficial fascia layer:
- One across the shoulders at the level of the scapular spine before it becomes the acromion.
- A second across the back below the inferior angle of the scapula.
- A third above the iliac crest.
HINT: When skinning the back, you will encounter a number of cutaneous nerves. It is not necessary to examine each one of these. Removing the skin from the back and exposing the underlying muscles is a time-intensive and labor-intensive job. Don’t be sidetracked by these details.
- Reflect these flaps laterally, along the plane between the superficial and deep fascial layers
When skinning the back, you will encounter a number of cutaneous nerves. It is not necessary to examine each one of these. Removing the skin from the back and exposing the underlying muscles is a time-intensive and labor-intensive job. Don’t be sidetracked by these details.
Note that working along the superficial fascial plane involves severing the cutaneous nerves and vessels (i.e. those that innervate/supply the skin). The segmentation of the trunk will be evident in the pattern of cutaneous innervation via specific spinal nerves.
Muscles that attach to the scapula or humerus and thus move the upper limb are hypaxial (not expaxial) muscles. Therefore, even though we see them on the back, due to their embryonic origins they are innervated by ventral rami of spinal nerves.
The trapezius is innervated by a cranial nerve (CN XI, or spinal accessory nerve) because of its embryonic origin as a so-called gill arch muscle. Other hypaxial muscles are innervated by ventral rami of spinal nerves, whereas epaxial muscles are innervated by dorsal rami. Which of these three muscle types (gill arch, hypaxial, and epaxial) will be influenced by spinal cord injury at a level below C4?
4. SUPERFICIAL BACK:
- Find the lateral and superior borders of trapezius
- Near the vertebral column, where muscles attach to the spinous process of the vertebrae, cut (transect) the two sides of trapezius along the sagittal axis of the body and reflect them laterally by folding them away from the vertebral column. For now, leave this muscle attached to the scapula and humerus.
- Find the attachment of the trapezius to the occipital bone and carefully cut this border to fully reflect the trapezius.
- Observe the rhomboids deep to the trapezius, running from the spinous processes to the medial border of the scapulae. Cut the rhomboids near their attachment to the spinous processes, and reflect these muscles laterally together with the attached scapulae.
- Find the superior and inferior borders of latissimus dorsi. Carefully transect latissimus dorsi along the edge where it attaches to the thoracolumbar fascia, leaving the posterior layer of thoracolumbar fascia in place. Reflect latissimus dorsi laterally.
Note that the medial (vertebral) border of the scapula remains attached to the ribs anteriorly by serratus anterior and to the cervical transverse processes by levator scapulae.
- Run your hand over the ventral surface of the scapula and around toward the ventral chest to feel the extent and attachment of serratus anterior.
Readiness Assessment. Please answer these questions prior to attending lab.
1. To begin we wrap specific structures with cloth and a plastic bag. Which structures get wrapped up?
2. Where do you make your first incisions? What will be the first muscle you see in this region? Is it epaxial or hypaxial?
3. When you cut the trapezius, what muscle will you find below it also attaching to the scapula?
1. Neurological Testing: Sensory level
Dermatomes correspond to the cutaneous distribution of spinal nerves. In order to localize a spinal cord lesion, it is useful to determine where cutaneous sensation is normal and where it is abnormal. This is done by using a pin prick to test sensation along successive dermatomes. Some useful dermatomes to remember include:
- C5 - shoulder
- T4-T5 - nipple line
- T10 - the umbilicus
- L4 – patella
- S1, S2 - heel.
The chicken pox virus may hibernate in dorsal root ganglia and may be reactivated by stress to cause shingles. The clinical manifestation of such a flare-up is a line of blisters following the cutaneous distribution of one or more spinal nerves.
- A patient presents in the clinic with a band of blisters running around the chest at the level of the nipple. What spinal nerve is involved?
- A band running across the back at shoulder level, continuing down the lateral surface of the arm and forearm, ending along the thumb indicates which spinal nerve?
- A band runs around the lower back and mid-abdomen at the level of the umbilicus indicates which spinal nerve is involved?
- A band found across the upper buttocks, wrapping around the lateral thigh, crossing the patella, traveling down the leg obliquely towards the medial surface to the medial ankle, and ending along the medial big toe indicates which spinal nerve?