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Lab 1 - skin and epaxial muscle

Suggested readings:
Gray's Ch. 1 (all)
Gray's Ch. 2 (all)

Primary Lab Objectives and Goals:

  1. Prepare the body (make sure you have watched the informational video referenced below BEFORE lab).

  2. 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.

  3. 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.

  4. 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.

  5. Expose and identify the epaxial muscles (deep muscles that lie along the vertebral column).  Understand that muscles act on the back and there are multiple layers (with different names) from deep to superifical.
  1. Expose the muscles and nerves of the suboccipital triangle (region of the posterior neck below the head).  This is a complicated dissection, the main goal of which is to show the underlying structure of deep neck muscles and nerves.

 

Procedure:

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 address any concerns you have.

  • Remove and carefully set aside the plastic wrap and sheet. 
  • Wrap the hands, feet, and head in the cloth provide at each table. 
  • Soak the cloth in appropriate “wetting solution”.
  • Cover wrapped areas with a plastic bag and tie with string. 
  • Turn the cadaver face-down (prone) on the table.

At the end of the lab sesssions you will wet the sheet 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. Locate the following landmarks on the skin of the back (refer to accompanying diagram). You may choose to mark those landmarks with pen provided at your table. Not all landmarks will be easy to palpate or identify. This varies by the stiffness, position, and body fat of the cadaver:

  1. External occipital protuberance
  2. Spine of the 7th cervical vertebra
  3. Spine and medial border of scapula
  4. Last two ribs (floating ribs 11 and 12)
  5. Sacrum
  6. Iliac crests
  7. Spine of 4th lumbar vertebra (in midline along line connecting iliac crests)

step2 figure

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 in expandable boxes that will help with your dissection.

  1. Make a short (~6 inches) midline incision on the back in the thoracic region between the two scapulae.
  2. Identify the subcutaneous layer of superficial fascia.
  3. Separate the skin and attached superficial fascia from the underlying deep fascia covering the muscles (HINT-1)

HINT-1

  • 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.
  • 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.

    1. Once you find the correct plane, extend the midline incision up to the external occipital protuberance.
    2. 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.
    3. Reflect these flaps laterally, along the plane between the superficial and deep fascial layers (HINT-2)

    HINT-2

    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.

     

    Atlas Images:

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    FUNCTIONAL ANATOMY:
    Note that working along the superficial fascial plane involves severing the nerves and vessels that reach 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, even though we see them on the back.  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 border of trapezius and the superior and inferior borders of latissimus dorsi,
    • Near the vertebral column, where muscles attach to the spinous process of the vertebrae, cut (transect) the trapezius and latissimus dorsi along the long axis of the body and reflect them laterally by folding them away from the vertebral colum. They will remain attached to the scapula and humerus.
    • Cut the rhomboids near their attachment to the spinous processes, and reflect these muscles laterally together with the attached scapulae.

    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.
    step5 figure

    Atlas Images:

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    5. THORAX TO LOWER BACK: 

    You are now looking at the posterior layer of the thoracolumbar fascia. It is overlaid by a separate fascial layer containing two trivial slips of hypaxial muscle mostly reduced to several slips of dense fasciae (serratus posterior superior and serratus posterior inferior). Deep to the thoracolumbar fascia lie the erector spinae muscles.

    • Expose the erector spinae:
      • If they have not already been reflected with the rhomboids and latissimus dorsi, reflect the fascial slips of serratus posterior superior and inferior
      • Remove the thoracolumbar fascia to expose the erector spinae

    Atlas Images:

    thumb thumb

    FUNCTIONAL ANATOMY:
    The epaxial muscles of this part of the back are arranged in three groups:

    1. The superficial group consists of the erector spinae, which is composed of three columns from medial to lateral: spinalis, longissimus, and iliocostalis.
    2. The intermediate group consists of semispinalis, multifidus and rotatores.
    3. The deep group consists of levatores costarum, interspinales, and intertransversarii.

    These muscle groups also differ in how many vertebral spaces they cross and to what extent they connect to spinous processes, transverse processes and ribs.  Examine the images of the superficial group.  Although all three can extend the back, which muscle plays the largest role in also creating lateral back flexion and axial rotation of the spine when recruited on one side?

     

     

     

    6. EPAXIAL MUSCLES:

    • Identify spinalis, longissimus, and iliocostalis, noting their fiber directions and attachments.
    • Reflect them to expose the successively deeper and shorter underlying layers of epaxial muscles.

    The deeper epaxial layers are difficult to dissect and distinguish, but you should at least get some idea of their variety and general location:

    • Identify if possible multifidus (most well-developed in the lumbar region running medially from transverse to spinous processes) and semispinalis (best developed in the upper thoracic and cervical regions).
    step 8 figure

    Atlas Images:

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    7. UPPER BACK/NECK: 

    • Identify the large cutaneous branch of C.2's dorsal ramus (the greater occipital nerve) running up over the scalp. This is a large nerve and the most easily seen dorsal ramus.  This may have been severed during skinning so you may be searching for its cut end.
    • Define the borders of the splenius muscle. Note that its lateral edge is crossed by a ventral-ramus cutaneous nerve, the lesser occipital nerve (ventral ramus C.2).
    • Cut the Splenius near the midline and reflect it laterally to its insertions on the skull (splenius capitis) and cervical vertebrae (splenius cervicis), exposing the semispinalis muscles.
    • Isolate the greater occipital nerve and occipital artery (the nerve passes through semispinalis capitis while the artery runs over the muscle).
    step 6 figure

    Atlas Images:

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    8. SUBOCCIPITAL TRIANGLE:

    The muscles comprising the suboccipital triangle connect the atlas and axis to the skull. They are small and deeply situated. You will need to flex the neck over a block to facilitate your approach.

    The goal is to locate the attachment sites of the 4 suboccipital muscles: obliquus capitis inferior (transverse process of atlas to spine of axis); obliquus capitis superior (transverse process of atlas to skull); rectus capitis posterior major (spine of axis to skull); and rectus capitis posterior minor (spine of atlas to skull). This needs to be done carefully and with patience.  (see demonstration VIDEO - courtesy of Loyola U.)

    The first step is to expose the deep muscles in this region.  Splenius capitis and cervicis were reflected laterally earlier (see STEP 5 above), thus exposing the semispinalis muscles. It may also help to transect the posterior portion of the sternocleidomastoid muscle and reflect it laterally to better expose the semispinalis muscles.
    • Cut semispinalis capitis near its insertion at the external occipital protuberance.
    • Reflect it postero-laterally exposing the superior most attachment of semispinalis cervicis to the spinous process of C2. Use this attachment to the spinous process of C2 as a landmark to locate the suboccipital triangle.

    Atlas Images:

    thumb thumb

    The suboccipital triangle and the 4 suboccipital muscles lie between the spinous process of C2 and the base of the skull.

    • Identify the four suboccipital muscles: rectus capitus posterior, major and minor, and the superior and inferior oblique capitus.
    • Note the triangle formed by three of these four muscles and look for the dorsal ramus of C.1 (the suboccipital nerve) passing through the triangle to innervate these muscles.

     

     

     Clinical Notes

    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:

    1. C5 - shoulder
    2. T4-T5 - nipple line
    3. T10 - the umbilicus
    4. L4 – patella
    5. S1, S2 - heel.

    lab1-clinicalNote

    2. Shingles

    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.

    Questions:

    1. 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?

    2. 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?

    3. A band runs around the lower back and mid-abdomen at the level of the umbilicus indicates which spinal nerve is involved?

    4. 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?

     

     

     

     

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