#3 - Back and Vertebral Canal
Demonstration

Your laboratory instructor has prepared a prosection in which the muscles of the back will be demonstrated as well as the contents of the lower end of the vertebral canal.

  1. Reflection of the muscles of the upper limb on the back, which previously was carried out, exposes the thoracolumbar aponeurosis and fascia (467/N167). The aponeurosis is a flat sheetlike tendinous expansion that allows for attachment of the latissimus dorsi to lumbar spinous process. Fascia separates true back muscles from appendicular muscles. True back muscles, developed from the epaxial division of the myotome, are segmentally innnervated by dorsal rami of spinal nerves.


  2. In the neck, the splenius muscle (467/N167) is the most superficial of the true back muscles (just deep to trapezius). When it is reflected the powerful extensor of the head, the semispinalis capitis (473/N167), is revealed. Note that the greater occipital nerve traverses the muscle belly.


  3. In the lower part of the back arising from the ilium, sacrum and lumbar spines is the erector spinae muscle (467/N168). It has been divided and subdivided by anatomists, but functionally is one muscle mass causing extension of the vertebral column. The subdivisions (spinalis, longissimus, iliocostalis) should be ignored.


  4. Deep to the erector spinae in the groove between the spinous processes and transverse processes of the vertebrae are 4 different small muscles individually named because they span varying numbers of vertebrae between attachments. Collectively they are designated transversospinal muscles (471/N169). They help extend the vertebral column and rotate the trunk to the opposite side, because they pull the vertebral spine toward the ipsilateral transverse process.


  5. To demonstrate the meninges and spinal cord, the laminae and spines (N147) of vertebrae have been removed from the mid thoracic region to the upper part of the sacrum. This exposes the pedicles and bodies of the vertebrae (N147) which together surround the vertebral foramen (N147) and opens the vertebral canal (N147) that contains the spinal cord and its coverings.

  6. Ligaments connecting these bony parts which will be demonstrated include (457/N151):

    1. Supraspinous (N151).
    2. Interspinous (N151).
    3. Ligamentum flavum (N151) - means the yellow ligament. The color is because of elastic fibers.
    4. Posterior longitudinal ligament (451/N151) (dura must be displaced to display this structure).
  7. Anterior to the posterior longitudinal ligament is the vertebral body and intervertebral disc. The intervertebral disc is composed of a fibrous ring, the anulus fibrosus, and a central gel-like material, the nucleus pulposus. You will not be dissecting out these structures, but specimens and models are available in the laboratory.

  8. Some remnants of the internal vertebral venous plexus (466/N166) may be demonstrable in the epidural space. It is a valveless venous channel, that communicates with venous structures in the skull and at each vertebral level. Since upward flow in the plexus has been demonstrated during valsalva maneuvers (holding breath while bearing down as in defecation), it has been implicated in the movement of cancer cells from the pelvic cavity to distant sites including the brain.


  9. The meninges will be shown (479/N162):

    1. The dura (N162) - tough, dense layer extends to S2.
    2. The arachnoid (N155) - filmy, in life approximated to dura. In the cadaver it has collapsed onto the pia because cerebrospinal fluid is no longer inflating the subarachnoid space.
    3. The pia (N162) - can't be separated from nervous tissue. It is the vascular layer.
      1. Filum terminale (N153) - extends from conus medullaris to S2. Anchors the cord.
      2. Denticulate ligament (482/N162)-attaches laterally to dura. Stabilizes the cord. Lies midway between dorsal and ventral roots.

  10. All the meningeal layers combine at S2 to form the coccygeal ligament (N153)(some authors refer to this as the filum terminale externum).


  11. The lumbar enlargement of the spinal cord will be demonstrated and the dorsal rootlets (477/N162) emerging from the cord. Ventral rootlets (N162) can only be seen if the cord is lifted out of the vertebral canal. The rootlets combine to form dorsal and ventral roots for each spinal nerve. Since differential growth has resulted in a longer dural sac than spinal cord, the roots of lumbar and sacral nerves are stretched into the cauda equina (N153), which lies in the lumbar subarachnoid space. The tapering inferior end of the spinal cord, the conus medullaris (N153), is usually located in adults between vertebrae L1 and L2.


  12. Dorsal and ventral roots leave the vertebral canal in a meningeal sheath through the intervertebral foramen (437/N152). A dorsal root ganglion (N163), site for cell bodies of sensory neurons, is typically found in or close to an intervertebral foramen. Just distal to the dorsal root ganglion the meningeal sheaths with each other and are continuous with the connective tissue sheath of the nerve, the epineurium. The subarachnoid space thus extends to the lateral aspect of the dorsal root ganglion. Dorsal and ventral roots join to form a mixed spinal nerve (477/N163) just beyond the dorsal root ganglion. Almost immediately the spinal nerve divides into a dorsal ramus (N163) - to supply true back muscles and skin of the back, structures derived from the epaxial myotome - and a ventral ramus (N163) - to supply muscles of the limb and trunk, structures derived from the hypaxial myotome (the text uses the terms dorsal and ventral primary rami). All these structures will be demonstrated.

 

True Muscles of Back and
Vertebral Canal Alternative Dissection

Should a prosection and demonstration not be available, the following dissection directions may be used to display the anatomy of the back.

  1. Reflection of the muscles of the limb (trapezius, rhomboids and latissimus dorsi) will reveal the insignificant serratus posterior superior and inferior muscles and the thoracolumbar aponeurosis and fascia (N167) which separates the true muscles of the back from the overlying limb muscles. The fascia is an embryological remnant indicating that the true muscles of the back have a different embryological origin from the muscles of the limb.


  2. In the lower part of the back observe the erector spinae (N167) attached below to the ilium, sacrum and lumbar spinous processes and above to ribs, and transverse or spinous processes of vertebrae. The muscle has been divided into three columns; spinalis, longissiumus and iliocostalis and each column subdivided by region. It is functionally one muscle causing extension of the vertebral column. The subdivisions should be ignored.


  3. The most superficial muscle in the neck is the splenius capitis and cervicis (N167). Cut the splenius one-half inch from its attachment to the vertebral spines and reflect laterally. The semispinalis capitis (N167) muscle, a powerful extensor of the head, will be revealed. Close to the skull note the greater occipital nerve (N167) as it penetrates the semispinalis capitis to gain access to the posterior aspect of the scalp.


  4. Deep to the erector spinae in the groove between the transverse and spinous processes of the vertebrae identify the transversospinal muscles (N169). These muscles extend the vertebral column and rotate the trunk to the opposite side. There are four separate muscles in the transversospinal category. Each spans different numbers of vertebrae as it proceeds from transverse to spinous processes. The most superficial is the semispinalis which spans 5-6 vertebrae. Next the multifidus spans 3-4 vertebrae. The long rotator spans 2 vertebrae and the short rotator attaches to the spinous process of the vertebra immediately above. Because of the obliquity of the spines in the thoracic region the short rotator is virtually horizontal in configuration. Rotator muscles are not prominent in the lumbar region. The individual muscles may be demonstrated by carefully removing thin layers of muscle and noting the changes in angulation of the fibers. Functionally speaking, there is little reason to distinguish the individual components of the transversospinal group.


  5. If the suboccipital region is to be explored, cut the thick belly of the semispinalis capitis one-half inch from the skull and reflect this muscle inferiorly and laterally. Preserve the greater occipital nerve (N171) which traverses the muscle belly. The occipital artery (N171) enters the area from a lateral position and can be found in the plane between the splenius capitis and the skull. Reflection of the semispinalis capitis reveals the semispinalis cervicis muscle attaching to the spinous process of C2. If a blood vessel is seen in this plane it is the deep cervical a., a branch of the costocervical trunk. The muscles of the suboccipital triangle (the inferior oblique, superior oblique, rectus capitis major and minor) may be exposed by pushing aside the splenius and semispinalis capitis muscles far enough to expose the transverse process of the atlas. The vertebral artery can be identified as it rests upon the superior surface of the atlas. Severing the posterior atlanto-occipital membrane will expose the entire length of the horizontally oriented artery. Note that the posterior ramus of the first cervical nerve emerges between the posterior arch of the atlas and the vertebral artery and supplies the muscles of the suboccipital triangle. The greater occipital nerve (dorsal ramus of second cervical nerve) emerges from below the inferior oblique muscle.


  6. To demonstrate the meninges and spinal cord in the inferior aspect of the vertebral canal remove enough true back muscle from the mid thoracic region on down to expose the vertebral spines and laminae. To remove the spinous processes and laminae and open the vertebral canal use a hammer and chisel or an autopsy saw with a special blade designed for this purpose. Refer to a lumbar or thoracic vertebra in order to plan the proper angle for the cuts in the laminae. Cutting into the pedicles of the vertebrae will not gain one access to the vertebral canal. The spines and laminae from the mid thoracic region to mid sacrum can be removed in one piece if the cuts are properly made. Note the ligaments which connect the bony parts:
    1. Supraspinous (N151) - remove muscle to demonstrate.
    2. Interspinous (N151) - remove muscle to demonstrate.
    3. Ligamentum flavum (N151) - yellowish in color joining the laminae.


  7. The epi dural space in the canal will contain some loose connective tissue and veins of the internal vertebral venous plexus (N166). Since these valveless veins communicate with veins in the pelvic cavity and with the venous sinuses in the skull they represent a pathway for metastatic spread of malignancy.


  8. The dural sac (N162) can now be viewed extending to the level of S2 and an extension toward the coccyx in the form of the coccygeal ligament (N153). The ligament is formed by a coalescence of all meningeal layers.


  9. Open the dural sac with a scalpel and note the filmy arachnoid (N162) layer. The pia (N162), the vascular layer of the cord, is intimately attached to nervous tissue. The arachnoid is collapsed onto the pia because there no longer is cerebrospinal fluid in the subarachnoid space.


  10. Note the lumbar enlargement of the spinal cord and the dorsal and ventral rootlets emerging from it to form dorsal and ventral roots (N162) of lumbar and sacral spinal nerves. Because of the discrepancy in length of the vertebral canal and spinal cord, the dorsal and ventral roots of lumbar and sacral nerves are stretched out into the cauda equina (N153). Locate the tapering end of the spinal cord, the conus medullaris (N153), at the level of L1 - L2. Extending from it to the end of the dural sac is a filament of pia, the filum terminale (N153).


  11. Follow dorsal and ventral roots as they leave the vertebral canal in a meningeal sleeve into an intervertebral foramen. Additional bone should be carefully chiseled away to expose these structures. In or close to the foramen note a swelling on the dorsal root. This is the dorsal root ganglion, site for the cell bodies of sensory fibers. Just lateral to the dorsal root ganglion (N163) the dorsal and ventral roots join to form the spinal nerve (N162). Almost immediately, the nerve will be found to divide into a dorsal ramus (N162)for supply of true back muscles and skin of the back and the ventral ramus (N162) to muscles of the limbs and trunk.


  12. In order to demonstrate the posterior longitudinal ligament (N151)cut dorsal and ventral roots of contiguous spinal nerves on one side and retract the cord and meninges to the opposite side. Note the configuration of the ligament as it relates to the intervertebral disc.


  13. Locate denticulate ligaments (N162). These are lateral extensions of the pia from the cord. They attach to the dura in a tooth-like configuration, and provide lateral stability to the cord.