Human Dissection Techniques – Advanced Anatomy Lab

Locate bony landmarks for dissection.  Palpate on self, skeleton, and cadaver.

Clavicles

Sternal notch – lies between clavicles

Xiphoid Process (end cartilage)

Pelvis – 2 boney prominences – anterior superior iliac spine

1st incision will extend from the sternal notch, down the body of the sternum, to the xiphoid process.  Locate the margin of the ribs.  Extend the incision distally toward the umbilicus.  Make incision around umbilicus, down to a line that connects the two anterior superior iliac spines.  The initial incision runs down along the body of the manubrium of the sternum.  You can press deeply because the bone from will prohibit you from going any further.  Be careful beyond the xiphoid process.  You do not want to puncture the abdominal cavity.

You should have successfully made the incision from the sternal notch to the xiphoid, and cut down to the bone to expose the layers of skin.

1)      Epidermis

2)      Subcutaneous Fascia – consists mostly of fat and connective tissue that extends from the skin all the way down to a dense fascia layer which covers the muscle.

As you extend the incision below the xiphoid process, you need to make sure you are not cutting deeper than the subcutaneous fascia layer.  The thickness of this layer is variable and depends on the relative obesity of the cadaver.

Extend the incision down the midline on the anterior abdominal wall; make another mid-clavicular incision (distally – parallel to midline incision)  This allows us to pull back the flaps of skin to further investigate.

Small cutaneous nerves that are extending through the deeper fascia covering the muscle coming to the skin, and traversing the superficial fascial space.  These nerves extend through the deep fascia to get to the skin.  These nerves innervate both the superficial compartment and the skin.  On the lateral flaps you will notice branches of the same nerves.  (Lateral cutaneous nerves)

Inguinal region —(Look for superficial lymph nodes.)  Use blade of scalpel to make incisions lateral to the penis.  Stroke the inguinal region vigorously.  On the lateral aspect of the scrotum, locate the saphenous vein, which comes up from the leg.  Lymph nodes should be nearby.

Lymph nodes – Size of a pea or larger

Same color as fat

Feel quite firm

Many lymphatic channels  or strands attach to each lymph node.  Multiple channels come into the inferior aspect of the node.  Fewer channels come out of the top.

Lab #2

Muscles of Back and Veterbral Column

On posterior side of cadaver-

Make incision along midline, over spinous processes of vertebra, from the nape of the neck to the sacral region.  Make about six transverse cuts which will allow us to now strip away skinNote the posterior cutaneous nerves.  Some may be stubs.  Also look for vessels that accompany nerves.

OBJECTIVE:   Expose vertebral column and see joints that those muscles move.  Open the vertebral column to expose the spinal cord which is protected within.  Clean away any subcutaneous tissues.  The first muscles you will see are not true back muscles.  (They exit the back and move the upper limb.)

Trapezius – Upon reflecting, you will find additional limb muscles deeper, including the rhomboids and the levator scapulae.  All these muscles will be reflected to see the true back muscles, which move the vertebral column.  Lower in the back, there is a large muscle that moves the upper limb, called the latissimus dorsi.

Reflect all superficial muscles – make an incision along either side of the midline, which is the attachment of all of the primary extrinsic muscles.  By doing that, you can get your hand deep into the muscle; Within a plane of some loose connective tissue, you should be able to separate that muscle from the deeper ones and simply reflect back and out of your way.

Intrinsic back muscles are exposed.  They are responsible for removing the vertebral column.  Several groups of intrinsic back muscles exist with complicated attachments.

A huge mass of muscle exists vertically along either side of the column – erector spinae group, which is particularly well developed in the lower back.

Use a scalpel to cut down either side of the spinous processes of the vertebrae; then use a chisel to scrape that muscle away from its other attachments.  You will be able to reflect that entire group of muscles to expose the vertebral column itself.

Cervical Region- Has additional intrinsic back muscles.  The most superficial is the spleneous capitus, which forms a V on the back of the neck.  The orientation of the muscle fibers of the spleneous capitus, form a V on the back of the neck.  These fibers are angling from the midline, upward and laterally, as they approach and attach to the skull.

By cutting along the midline, through the spleneous group, you can find a deeper group of muscles.  These run more vertically along the column and are called the semi-spinalis capitus.  All these extrinsic back muscles are going to be responsible for extending the spine and also rotational movements of the spine.

Using an articulated vertebral column, we can appreciate some of the surface landmarks that we see on the cadaver.

Spinous processes – right on the midline—are the bony prominences that extend posteriorly, and are for muscle attachments.  Look closely and you will see transverse processes, which also serve as muscular attachment points.  There are some joints between the pairs of vertebrae at each level.  (There is a limited amount of movement at each intervertebral joint.  The vertebral canal protects and houses the spinal cord.  To expose the spinal cord and its branches, you must cut approximately .5 cm on either side of the spinous process.

The part of the bone that is attached to the body extends posteriorly and is called the pedicle.  The portion of the vertebra that forms the roof over the canal is called the lamina.  The lamina meet at the point where the spinous process forms.  The pedicle and the lamina meet at a place that forms the transverse process.  We can also see articular processes form joints between the sequential vertebrae.

The size of the vertebral canal is approximately 1 cm in diameter.

GOAL:             Remove spinous processes and laminae along the entire length of the vertebral column.

After you have cleaned away as much of the soft tissue as possible, the next step is to take a saw and score along the entire length of the vertebral column.  After you have cleaned as much of the soft tissue as possible, the next step is to take a saw and score along the entire length of the vertebral column, just lateral to the spinous processes, cutting to the lamina.  This will insure that you expose the spinal cord and the surrounding tissues, without going too wide.  After you have done that initial saw cut, which should be relatively shallow, you want to finish the removal of the laminae by using a chisel and a hammer.  Angle the chisel in medially and then use the hammer to continue going through the bone and removing the vertebral column incrementally along the entire length.  Expose the spinal cord and the meninges.  You may be able to remove the vertebral column in one piece or in smaller sections.  (You should have just removed the laminae and spinous processes of the vertebral column.

You should also see the dura mater, which is one of the layers of the meninges.  (Dura mater – Medieval Latin, meaning literally “hard mother”.)  Incise the dura mater to see the other two coverings of the spinal cord.  (Slit the dura mater with a probe and separate.)

Arachnoid mater – spider web-like appearance; thin transparent membrane which provides a cushioning affect for the CNS.  Arachnoid and pia mater are sometimes called leptomeninges.  In a cadaver, the arachnoid is collapsed because the fluid is gone.

Pia mater – gentle mother or tender mother – adheres directly to the spinal cord.  Cut and remove dura mater.  Once you see the spinal cord, you will see it is significantly shorter than the vertebral column itself.  The spinal cord ends at approximately at vertebral level L2.  The spinal cord becomes more narrow at its inferior end, forming a structure called the conus medularis, an inferior cone-shaped structure.

Coming directly off of the conus medularisis a strand of tissue called the filum terminale, which is a specialization of the pia mater, which anchors the spinal cord to the coccyx.  It runs inferiorly within the vertebral column to do so.  The cauda equina mingle with the filum terminale.  They are dorsal and ventral nerve rootlets that will form spinal nerves forming at sequential vertebral levels, and exiting the vertebral column.  This is how the entire body receives innervation.

Look at the cranial area:

Focus on each spinal nerve.

Strands branch off of the spinal cord.  (We are at the posterior, dorsal side.)  Dorsal rami – contain sensory nerves and information going back to the spinal cord.  Ventral rami branch from the ventral or anterior side of the spinal cord, carrying motor information.

Denticulate ligament – another specialization of the pia mater, helps to anchor the spinal cord and hold it in position.  The denticulate ligaments come up the lateral edge of the spinal cord and through this space and anchor the spinal cord to the dura mater.  This landmark is helpful because it separates the dorsal or sensory rootlets from the ventral or motor rootlets.  Let’s follow those dorsal and ventral rootlets away from the spinal cord and ultimately, they will come together to form a spinal nerve from this level.  Following them away from the spinal cord, as you get to the position of the intervertebral foramen, there is a swelling that represents the dorsal root ganglion.  This is where the sensory nerve cells are found; For those axons that are traveling within the dorsal root.  Going further away you will find the major branches of the spinal nerve.  The spinal nerve itself is very short where all of the sensory and motor fibers are together.  (About 2 mm long)  Immediately you will find that the spinal nerve branches into a dorsal ramus, which is quite small because it only innervates the deep back muscles or the true back muscles, and provides cutaneous innervation as the posterior cutaneous nerve.

The ventral ramus is much larger because it has a much greater territory to supply.  This nerve wraps completely around the body wall; gives off the lateral cutaneous nerve and also the anterior cutaneous nerve.  The communicating ramus, which will link the spinal nerve to the sympathetic trunk, is part of the autonomic nervous system.

Cut a slice of thoracic cord:  (Less than a dime in diameter)

Gray matter – central, and looks whiter in the embalmed specimen

White matter – on the perimeter – has a darker appearance to it.

LAB #3

Abdominal Wall

Muscles of the anterior body wall –

1st Step – Reflect the pectoralis major muscle

Find the inferior margin of the muscle and run fingers medially.  Cut the muscle from the underlying ribs and from its attachment to the sternum and the clavicle.  This will allow you to reflect the muscle.  Do not be concerned about destroying the nerves or arteries to this muscle because they will all be found on the underside.  You will expose the pectoralis minor, which is attached to the anterior rib cage.  You should cut along its margin of attachment to reflect.  This will reveal the bony thorax showing the ribs and the intercostals spaces, which are filled with the intercostals muscles.

Remove a block of the intercostals muscles; Break a section of three or four ribs (2-3 cm long) and remove the segment.  Find a blood vessel which courses from the subclavian down along the inner surface of the ribs ——internal thoracic artery and internal thoracic vein.  As this artery courses down the underside of the thorax, it is sending slender branches to each of the intercostals spaces; so that arising from the interthoracic artery is the anterior intercostal artery, which will supply each intercostal space.

The next step is to reflect the external and internal transverses abdominal muscle.  Start by finding the upper margin of the external oblique.  The fibers are oriented towards the midline.  We will cut these fibers from the superior aspect to the umbilicus.  Place a pair of scissors into the superior margin, under the surface, to use as a guide.  Reflect the external oblique.  Be careful.  Do not damage the inguinal area; We will study later.

We have separated the external oblique from the internal oblique by doing a blunt dissection.  There is some fine connective tissue between the two.  It is very easy to separate the two once you have found the plane.  Identify the internal oblique by the direction of its fibers.  These fibers are at right angles to each other.

Cut along the margin of the internal oblique.  It is a thin muscle, so do not cut very deeply.  Do a blunt dissection beneath the internal oblique, so that you can reflect it and expose the underlying transversus muscle.  In the fine connective tissue between the second and third layer, we find the intercostal nerves.  They extend down to the abdomen and innervate these muscles.  These will appear in a regular fashion.

Rectus abdominus –

Found within a compartment called the rectus sheath, this thin, CT is really a tendon.  It is contributed to by each of the muscles we just dissected.  As the external oblique, internal oblique, and transversus muscles come toward the midline, the fibers stop.  Their tendon is a very thin, flat tendon.  That tendon splits as the muscles join together at the lateral margin of the rectus muscle.  We will cut the anterior margin of the rectus sheath.  There is an anterior and posterior lamina (or portion of the sheath).  There is another rectus abdominus within a sheath on the other side.  The rectus muscle is composed of several muscles which have come together.  They are really separate muscles which give rise to the tendonous inscriptions that can be seen.  Lay people refer to a “6-pack” as the perfect set of rectus abdominus muscles.  The muscles hypertrophy to cause this effect.  Since the rectus is made up of several muscles, one would expect more than one innervation.  Earlier we saw the nerves traveling between layers 2 and 3, coming around the body.  We should expect the nerves to continue into the rectus on its lateral margin.  During surgery you would not enter the abdomen by making an incision from the inferior to the superior aspect of rectus abdominus, as the nerves would be severed causing paralyses to this muscle.  One technique for entering the abdomen is to cut the anterior portion of the rectus sheath and reflect; Push muscle towards its innervations; Cut the posterior lamina and enter the abdominal cavity.

Another feature of the rectus muscle is its blood supply.  Just as is the case with the pectoralis major, the blood and nerve supply are on the undersurface.  A large artery travels on the underside of r. abdominus.  The internal thoracic artery, giving off the anterior intercostals, continues on below the margin of the rib cage and continues on the undersurface of the rectus muscle.  It then becomes the superior epigastric artery.  (Same vessel – new name).  Likewise, there is another artery that arises from the external iliac, which is the end of the aorta.  It arises and comes up on the under surface of the inferior portion of the rectus muscle.  They meet somewhere in the middle.  The meeting of these muscles introduces the concept of anastomotic connections.  If a blood cell was traveling up in the epigastric artery, it would be possible for that cell to continue into the superior epigastric artery.  The vessels represent a site for collateral circulation.  The upper portion of the rectus can actually receive its blood from the lower stem, or inferior epigastric.

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