Primary Lab Objectives:
- Examine the retropharyngeal space.
- Separate the head from the vertebral column.
- Examine the prevertebral fascial space and associated structures, while considering the pathways from the neck to the thorax.
- Open the pharynx from the posterior aspect. Examine the interior of the pharynx. Locate the superior, middle and inferior constrictor muscles and examine their inner surface.
- Examine the larynx and upper trachea, and the muscles that move the vocal cords. Examine the vocal cords and folds, and the epiglottis.
In the last lab you examined many of the structures on the anterior surface of the neck, and looked briefly at the retropharyngeal space and its contents while moving them aside, but you have not yet dissected those structures in detail. Today you will explore the prevertebral area and the back of the pharynx (retropharyngeal space) in much more detail. In order to do this the head must be detached from the vertebral column to allow a posterior approach to the cervical viscera. This is a challenging and labor-intensive dissection.
The joints between the skull and the first cervical vertebra (the craniovertebral joints) are the logical sites for separation of the head from the vertebral column. You should consult with your instructor before starting. Some instructors prefer to remove a wedge of the occiput, while others prefer to detach the skull base intact. Regardless, this is a vigorous dissection involving chisels and hammers. Please remember to clean these tools and return them to the tool room after use!
NOTE: Please review the PRE-LAB exercise regarding the anatomy of the atlanto-occipital joint and the and larynx prior to carrying out this dissection.
1. RETROPHARYNGEAL SPACE
- Review the structures that pass through the foramen magnum:
- Brainstem/spinal cord
- vertebral arteries (left and right)
- cervical roots of the accessory nerves (left and right).
- Review the hypoglossal nerve (CNXII) where it enters the hypoglossal canal.
- Review the structures that exit the jugular foramen:
- glossopharyngeal nerve (CNIX),
- vagus nerve (CNX),
- accessory nerve (CNXI)
- internal jugular vein (draining the sigmoid sinus from the cranium)
Make sure you have reflected the sternocleidomastoid muscle on both sides, having separated each muscle from deeper structures all the way to the mastoid process.
- Place the fingers of both hands posterior to the carotid sheaths.
- Push your fingers medially until they meet posterior to the cervical viscera. Your fingers are now in the retropharyngeal space.
- Work your fingers superiorly as far as the basilar part of the occipital bone, superior limit of the retropharyngeal space.
- Work your fingers inferiorly toward the superior thoracic aperture.
- Note that the retropharyngeal space extends into the superior mediastinum.
2. SEPARATION OF THE ATLANTO-OCCIPITAL JOINT
- Turn the cadaver to the prone position
- If not already done when you went through Lab 1 (epaxial muscles), now use a scalpel to detach the semispinalis capitis muscle, splenius capitis muscle, obliquus capitis superior muscle, and the rectus capitis posterior major and minor muscles from the occipital bone.
NOTE: we encourage you to review your procedure in Lab 1 and go over the suboccipital muscles one more time as they are fair game for the upcoming exam because of their relevance to understanding the functional and clinical anatomy of the neck.
- Looking from the inside of the skull along the clivus of the occipital bone, identify the tectorial membrane, which is continuous with the posterior longitudinal ligament.
- Superior to the anterior border of the foramen magnum, cut the tectorial membrane and reflect it inferiorly as far as possible.
- Anterior to the tectorial membrane, identify the cruciate ligament of the atlas.
- Identify its three parts: superior longitudinal band, transverse ligament of the atlas, inferior longitudinal band.
- Use a scalpel to cut the superior longitudinal band.
- Anterior to the cruciate ligament of the atlas, identify the left and right alar ligaments. The alar ligaments extend from the dens to the lateral margins of the foramen magnum, and they control lateral rotation and side-to-side movements of the head.
- Use a scalpel to cut the alar ligaments close to the dens.
Note that the rotation of the head is now very easy and extensive.
Note if you do not choose to remove an occipital wedge, you can skip this section and proceeed to the next section where you separate the atlantooccipital joints. This can be decided on a case-by-case basis with your instructor.
Beginning of optional wedge removal procedure:
Following the steps that are outlined below, you will use a chisel to loosen the atlas and cut or chisel through the posterior arch of the atlas. Access to this very tight space may be facilitated by "wedging" the occipital bone, which may be done as follows (not all groups will use this approach, please discuss this with your instructor before proceeding):
- Identify the posterior atlanto-occipital membrane, which spans the interval between the atlas (C1) and the occipital bone.
- Use scissors to incise the posterior atlanto-occipital membrane transversely from the left vertebral artery to the right vertebral artery (these may not actually still be intact depending on the brain removal method used. If they are still there, preserve them).
NOTE: The posterior atlanto-occipital membrane is deep to all musculature. This membrane needs to be completely transected in order to remove the head. Note the vertebral arteries entering into lower lateral corners of the membrane. This is how the vertebral arteries make the transition from the transverse foramina to the foramen magnum.
- Use a scalpel or chisel to scrape the occipital bone clean of remaining muscle fibers and pericranium. Identify a line between the midline of the lambdoid suture and the lateral margin of the foramen magnum on both the right and left sides of the skull. This will define an “occipital wedge.”
- Use a saw to cut along the lines from mid-lambdoid suture to the foramen magnum. Loosen the wedge of bone with a chisel and mallet and remove it.
Disarticulating the atlanto-occipital joint
NOTE: in this step you are ONLY separating the base of the skull from the vertebral column and the capitis muscles that attach to the base of the skull. ALL of the remaining structures (pharynx, carotid arteries, jugular vein, cranial nerves) should be kept intact!
- Use bone cutters (if an occipital wedge was cut as shown below) or a small chisel to remove the posterior arch of the atlas.
- Use a scalpel to open the capsule of the atlanto-occipital joint on both sides.
- Force a chisel into each atlanto-occipital joint and disarticulate it.
Note: If the occipital bone was NOT wedged as described above this may have already been accomplished when you loosened the atlas to get at the posterior arch.
- Turn the cadaver to the supine position.
- Retract the cervical viscera and contents of the carotid sheath anteriorly.
- Re-identify the sympathetic trunk and the superior cervical sympathetic ganglion on the anterior surface of the cervical vertebral column.
- On the left side, sever the internal carotid nerve just superior to the superior cervical ganglion and leave the sympathetic trunk attached to the vertebral column.
- On the right side, reflect the sympathetic trunk and the superior cervical ganglion with the head and cervical viscera. Make every effort to protect cranial nerves IX, X, XI, and XII where they emerge from the base of the skull near the internal jugular vein.
- Insert a scalpel blade between the transverse process of the atlas and the occipital bone and sever the rectus capitis lateralis muscle on each side.
- More medially, sever the rectus capitis anterior and longus capitis muscles.
IMPORTANT NOTE: the carotid artery, internal jugular vein, and CN IX, X, and XII are IMMEDIATELY ANTERIOR to the rectus capitis lateralis and IMMEDIATELY LATERAL to rectus capitus anterior. You want to be absolutely certain you are only cutting the muscles, but preserving the other structures.
- Push the head anteriorly to detach it from the vertebral column.
HINT: This may be more effectively done with the trunk vertical. When the head moves forward there will be some significant sound of tissue separating. If you maintain the retropharyngeal well, as described above, and keep it held forward (including the carotid sheath and other structures) with your hand or a probe, no damage should occur.
3. PREVERTEBRAL SPACE
You examined this in the last session, but now you will be able to examine it more closely.
- On the anterior surface of the cervical vertebral column, examine the prevertebral fascia. The prevertebral fascia covers the prevertebral muscles (the longus colli and longus capitis muscles) and the lateral vertebral muscles (the anterior, middle and posterior scalene muscles).
- Study the sympathetic trunk.
- Identify the superior, middle (when present), and inferior cervical sympathetic ganglia. Frequently, the inferior cervical ganglion is fused with the 1st thoracic ganglion to form the cervicothoracic (or stellate) ganglion.
4. INTERIOR OF THE PHARYNX
The pharyngeal muscles consist of three constrictors that have separate attachments anteriorly, but overlap posteriorly, and meet in the midline at a tendinous insertion called the pharyngeal raphe.
- Use a probe to clean the buccopharyngeal fascia from the posterior surface of the pharynx.
- Identify the inferior pharyngeal constrictor muscle. The anterior attachments of the inferior pharyngeal constrictor muscle are the oblique line of the thyroid cartilage and the lateral surface of the cricoid cartilage. The posterior attachment of the inferior pharyngeal constrictor muscle is the pharyngeal raphe.
- Beginning near the thyroid cartilage, use blunt dissection to clean the superior border of the inferior pharyngeal constrictor muscle.
- Identify the middle pharyngeal constrictor muscle. The anterior attachments of the middle pharyngeal constrictor muscle are the greater horn of the hyoid bone and the inferior portion of the stylohyoid ligament. The posterior attachment of the middle pharyngeal constrictor muscle is the pharyngeal raphe.
- Note that the inferior part of the middle pharyngeal constrictor muscle lies deep to the inferior pharyngeal constrictor muscle.
- Use blunt dissection to clean the superior border of the middle pharyngeal constrictor muscle.
- Indentify the superior pharyngeal constrictor muscle. The anterior attachment of the superior pharyngeal constrictor muscle is the pterygomandibular raphe and its posterior attachments are the pharyngeal raphe and pharyngeal tubercle of the occipital bone.
Note that the inferior part of the superior pharyngeal constrictor muscle lies deep to the middle pharyngeal constrictor muscle.
- Use blunt dissection to define the superior border of the superior pharyngeal constrictor muscle. The dense connective tissue membrane that attaches the superior edge of the superior constrictor to the base of the skull is the pharyngobasilar fascia.
- To find the stylopharyngeus muscle, palpate the greater horn of the hyoid bone. About one finger’s width above the greater horn of the hyoid bone, the stylopharyngeus muscle passes between the superior and the middle pharyngeal constrictor muscles.
- Use a probe to clean the posterior and lateral surfaces of the stylopharyngeus muscle.
- Identify the glossopharyngeal nerve (CNIX) which courses around the lateral surface of the stylopharyngeus muscle to enter the pharynx.
NOTE: The hypoglossal nerve takes a similar course as the glossopharyngeal, but it can be found the stylopharyngeous and stylohyoid muscles.
- Examine the inferior border of the inferior constrictor muscle.
- Note that the most inferior fibers of the inferior constrictor muscle are continuous with the circular fibers of the esophagus. At this location the recurrent laryngeal nerve enters the pharyngeal wall.
- Re-identify the contents of the carotid sheath but this time from the posterior view.
- Follow the internal carotid artery superiorly as far as possible.
- Note that the internal jugular vein is lateral to the internal carotid artery.
- Also within the carotid sheath you should find the vagus nerve.
- Note that the hypoglossal nerve (CNXII) passes lateral to the internal and external carotid arteries.
- Cut the raphe of the constrictor muscles posterior to the larynx. You will examine the interior of the pharynx in more detail in the next lab.
- Review the external features of the larynx that have been dissected previously:
- Infrahyoid muscles
- Sternohyoid muscles
- Omohyoid muscles
- Sternothyroid muscles
- Thyrohyoid muscles
- Internal branch of the superior laryngeal nerves
- Superior laryngeal arteries
- External branch of the superior laryngeal nerves
- On the external surface of the larynx, identify the cricothyroid muscle.
The proximal attachment of the cricothyroid muscle is the lateral surface of the cricoid cartilage and its distal attachment is the inferior margin of the thyroid cartilage. The cricothyroid muscle tilts the thyroid cartilage anteriorly, which lengthens the vocal fold. The cricothyroid muscle is innervated by the external branch of the superior laryngeal nerve.
- To expose the posterior surface of the larynx, push the head forward and allow the chin to rest on the thoracic wall.
- Palpate the lamina of the cricoid cartilage.
- Lateral to the lamina identify the piriform recess.
- Remove the mucosa from the piriform recess.
- Immediately deep to the mucosa identify the internal branches of the superior laryngeal nerve and the inferior laryngeal nerve.
- Use a probe to strip the mucosa from the lamina of the cricoid cartilage and expose the posterior cricoarytenoid muscle.
The proximal attachment of the posterior cricoarytenoid muscle is the posterior surface of the lamina of the cricoid cartilage and its distal attachment is the muscular process of the arytenoid cartilage. The posterior cricoarytenoid muscle causes the arytenoid cartilage to rotate, moving the vocal process laterally (abduction of the vocal folds).
- Superior to the posterior cricoarytenoid muscle, identify the arytenoid muscle.
The inferior attachment of the lateral cricoarytenoid muscle is the superior surface of the arch of the cricoid cartilage and its superior attachment is the muscular process of the arytenoid cartilage. The lateral cricoarytenoid muscle causes the arytenoid cartilage to rotate, moving the vocal process medially (adduction of the vocal fold).
- Use scissors to cut the arytenoid muscles, the laminae of the cricoid cartilage, and the trachea in the median plane.
- Open the posterior surface of the larynx and observe that the laryngeal cavity has three parts
- Vestibule – superior to the vestibular folds
- Ventricle – the depression between the vestibular fold and the vocal folds
- Infraglottic cavity – inferior to the vocal folds and continuous with the trachea
- Examine the epiglottis and note that it moves posteriorly during swallowing to close the laryngeal inlet.
- Inspect the mucosa that lines the interior of the larynx and identify the vestibular folds (false vocal folds) and the vocal folds (true vocal folds). The vocal ligament is located within the vocal folds.
- Examine the ventricle. The ventricle may extend into a recess called the saccule.
NOTE: it will be helpful to use the models of the larynx in the model room to study these structures. Some of these structures are very small and a model can help you to understand the relationship and how all of the parts of the larynx work together. Remember that models cannot be removed from the model room.
There are several conditions of the vocal folds which can cause the voice to sound quiet, weak or hoarse. Vocal fold paresis (paralysis) can be uni- or bi-lateral, and is caused by compression of or trauma to the recurrent laryngeal nerve. Due to the course of these nerves this can be a side effect of both heart and thyroid surgery. Though generally benign, polyps and nodules (abnormal growths or swellings on the vocal folds) can prevent the folds from being fully adducted, causing hoarseness and changes in vocal range.
A cricothyrotomy (aka cricothyroidotomy or, more commonly, "cric") is an emergency surgical procedure that allows for the placement of an endotracheal tube when airway control is not possible by the more conventional means of endotracheal (ET) intubation (the procedure of passing a tube through the oral cavity, phraynx, larynx, and finally into the trachea). For example, extensive trauma to the jaw, crush injuries of the larynx, or foreign body obstruction may make passage of an ET tube impossible, although the most common reason for cricothyrotomy is in instances where the patient may have a suspected cervical spine injury in which case the head manipulations required to place an ET tube (tilting the head back) CANNOT be performed. The procedure is performed by making a small incision in the cricothyroid ligament and membrane using a scalpel or a large-bore cannula and then placing an endotracheal tube in the resulting opening. The procedure is preferable to a tracheostomy (described below) mainly because there are generally no large midline structures or vessels overyling the cricothyroid ligament and membrane, although the thyroid gland may sometimes extend into this region in which case some moderate, but still manageable, bleeding can occur. As the opening that is created is very close to the vocal cords and often can only accomodate a relatively small-bore tube, a cricothyrotomy is generally used only for short-term airway management. For longer term airway control, a tracheostomy is preferred.
Whereas endotracheal intubation and/or cricothyroidectomy are good short-term solutions for establishing an airway, patients in need of long-term airway management (e.g. those who will be on a ventilator for a long period of time or those for who there is permament airway compromise) usually require tracheostomy. This is usually performed midway between the laryngeal prominence (the Adam’s apple in the male) and the jugular notch, at about the level of the second or third tracheal ring. This involves dividing the isthmus of the thyroid gland. Any higher and the vocal cords may be damaged or the trachea narrowed, any lower may result in bleeding from the inferior thyroid vein and/or arteries, or, if really low, the left brachiocephalic vein. In infants and young children the procedure may also damage the thymus. Due to the technical nature of the procedure, it is RARELY used in emergency situations but instead is usually performed by an otolaryngology surgeon under general anesthesia.