Pre-lab Exercise: Spinal Cord and Ventral Body Wall
Primary Lab Objectives and Goals:
- Retract epaxial back muscles laterally; review the relationships between the superficial hypaxial muscles and the 3 layers of the deeper epaxial muscles.
- Perform laminectomies between vertebral levels T8-L4. This process will expose the spinal cord and also help you understand the relationship between the spinal cord, dural sacs, and specific parts of the vertebrae.
- Examine the spinal meninges, spinal cord, and spinal nerves. Understand how the spinal cord is anchored and how the spinal nerves exit the vertebral canal and dural sac.
- Examine the surface anatomy of the body wall and the intercostal spaces. This will be a chance to understand a common pathway for the ventral rami of the spinal nerves as well as blood flow in the body wall.
Note: the intercostal nerve and vessels lie between the internal intercostal and the innermost intercostal muscles. The main branches are tucked into a groove on the anterior inferior surface of each rib. Note the relationship between the vein, artery and nerve in the intercostal space. Proceed carefully here and use a blunt probe so that you do not accidentally sever these structures.
- Examine female pectoral anatomy with a particular emphasis of anatomy and lymphatics of the breast.
- Identify and reflect the superficial thoracic muscles. These are upper limb muscles (outgrowths of the body wall). They connect the upper limbs to the thorax.
NOTE: Steps 5 and 6 should be completed if at all possible. But they can wait until the beginning of the next lab as well.
1. PREPARE VERTEBRAL COLUMN.
- Mobilize and reflect erector spinae laterally as far as possible.
- Remove deep epaxial musculature (e.g. the multifidi and semispinalis thoracis) and expose the vertebral arches and transverse processes from about T8-L4.
- Use a chisel to cut through the laminae on each side of the exposed vertebrae from about T8 to L4.
Note - It is very common to inadvertently take off the transverse processes of the vertebrae when you are doing the laminectomy. Be sure to clear away as much of the muscle as you can. Then identify the transverse processes, which are projecting laterally or posterolaterally from the sides of the vertebra. You should place your chisel at the junction of the transverse process and the lamina. Angle the chisel slightly medially and give it a substantial (but not too substantial) whack with the hammer. This is a challenging process and we have provided a video here to give you some pointers.
- Once you have broken through the arches on each side, cut through the ligaments that connect the arches above T8 and below L4.
- Remove the freed segment(s) of vertebrae, exposing the dural sac of the spinal cord.
3. DURAL SAC
- Examine the dural sac in place, following it out into the intervertebral foramen.
- Using sharp-pointed scissors carefully cut a longitudinal slit through the dura and arachnoid mater.
You have entered the subarachnoid space where cerebrospinal fluid circulates.
- Identify the denticulate ligaments (see figure below) running through the subarachnoid space
The denticulate ligaments are lateral tooth-shaped extensions of pia mater that penetrate the arachnoid and fuse to the dura mater to help anchor the spinal cord within the vertebral canal.
- Study the way in which the dorsal and ventral roots of each spinal nerve are formed by the fusion of multiple rootlets.
- Note that their dural and arachnoid covering extends outward into the intervertebral foramen as far as the point where the two roots themselves fuse to from a mixed spinal nerve.
- Try to locate the dorsal root ganglion in the intervertebral foramen, just medial to where the two roots join to form the mixed spinal nerve.
Note: You have now exposed most of the constituents of a typical spinal nerve, from the cutaneous twigs of the dorsal ramus back to the component rootlets that coalesce to from the dorsal and ventral roots. The course of the ventral ramus is complicated by the formation of plexuses and by autonomic connections, both of which you will encounter in later dissections.
- Study the way in which the cauda equina is formed from a series of rootlets.
- Determine where the spinal cord ends, and locate the filum terminale, a central cord running from the conus medullaris (the end) of the spinal cord and into the sacrum.
- Transect a portion of the spinal cord in the T8 to T11 region and remove it so you can see the cross-section.
- Identify the white and grey matter and the ventral and dorsal horns.
4. BODY WALL AND INTERCOSTAL SPACE
- Reflect the scapula and attached serratus anterior laterally, exposing the posterior (dorsal) ends of the intercostal spaces.
- If necessary, dissect away iliocostalis and longissimus on one side to bare the ribs.
At this end of the intercostal space, the Internal Intercostal muscle becomes a ligamentous internal intercostal membrane. The ventral ramus of the spinal nerve therefore can be seen here by removing a piece of the External Intercostal muscle.
Note: the intercostal nerve and vessels lie between the internal intercostal and the innermost intercostal muscles. The main branches are tucked into a groove on the anterior inferior surface of each rib. Proceed carefully here and use a blunt probe so that you do not accidentally sever these structures.
- Expose the ventral ramus and trace it laterally, identifying its accompanying posterior intercostal vessels.
The ventral ramus cannot easily be traced medially toward the spinal cord, because it emerges deep to the ligaments which join the rib to the vertebral transverse process.
Surface Anatomy Exercise
At this point, you may choose to do this optional surface anatomy exercise regarding the bony landmarks of the thorax and the the anatomy of heart auscultation.
5. FEMALE PECTORAL ANATOMY: On female cadavers only (students with male cadavers should work with another group to see this dissection).
- Working as a team, role the cadaver over so that the ventral or anterior surface is facing up.
- The mammary glands are large modified sweat glands whose main substance lies in the superficial fascia of the pectoral region.
- Make an incision through the nipple and observe dense bands of connective tissue separating lobules filled with fat.
In the aged (non-cycling) female breast, duct and glandular tissue is not apparent.
6. GENERAL PECTORAL ANATOMY
- If you did not already do this in step 5 above, working as a team, role the cadaver over so that the ventral or anterior surface is facing up.
- Make a midline incision through the skin from the top of the sternum to the xiphoid process.
- Reflect the skin and superficial fascia to both sides.
- Identify the pectoralis major muscle and its attachements. Both the pectoralis major and the underlying pectoralis minor move the upper limb.
- Note – Pectoralis major is innervated by the medial and lateral pectoral nerves, named for where they branch off the cords of the brachial plexus. Pectoralis minor is usually innervated by only the medial pectoral nerve.
- Transect (cut) the pectoralis major along the attachments to the sternum and clavicle. Reflect it laterally.
- Identify the pectoralis minor and its attachements. Transect this muscle along the attachments to the ribs and reflect laterally.
- Transect the pectoralis major and pectoralis minor at their origins on the chest wall.
- Attempt to locate the medial and lateral pectoral nerves entering the deep surface of these muscles as you reflect them laterally. After reflecting pectoralis major and minor, the external intercostal muscles are now exposed in the intercostal spaces.
- Near the midline the external intercostal muscle becomes membranous, exposing the internal intercostal fibers. Examine the muscle fibers of the internal intercostals running nearly at right angles to those of the external intercostals.
- Along the lateral thorax, cut through the external and internal intercostal muscles to expose the 3rd layer of intercostal muscles, the innermost intercostal muscle. Attempt to locate the intercostal nerve and vessels between the 2nd and 3rd layers of muscles, just below the rib, in the intercostal groove.
- To perform the laminectomy you will cut between which two bony processes on the vertebral arch?
- What is the first layer of the meninges that you will encounter once you enter the vertebral canal?
- What forms the cauda equina?
1. Herniated Disc
The jelly-like nucleus pulposus (remnant of the notochord) may protrude through a ruptured annulus fibrosis of the intervertebral disk. This usually occurs posteriolaterally at the point where the ligaments binding together the vertebral bodies are deficient. Pressure on a spinal nerve may cause pain radiating along its cutaneous distribution. The most common place for a herniated disc to occur in humans is between L5 and S1. Why might this be, and where might you expect the pain to radiate?
2. Spinal Tap
A spinal tap usually is performed between vertebrae L.3 and L.4. This region of the spinal column lies below the end of the spinal cord (conus medullaris). The clinician avoids poking the needle into the substance of the cord which would cause permanent nerve damage.
3. Lymphatic Drainage of the Breast
Most of the lymphatic drainage of the breast enters the axillary lymph nodes. However, the drainage from the medial half may enter thoracic nodes, which are difficult to remove surgically. Tumors in the medial half of the breast have a worse prognosis than those of the lateral breast.