Lab 3A
disarticulation of head,
posterior view of pharynx

The goal is to separate the head from the neck, but to preserve the connections between the cervical viscera and the base of the skull.

Begin posteriorly by cutting through the posterior longitudinal ligament and looking for the cruciate and alar ligaments of the craniovertebral joint:

Anterior and posterior to the vertebral bodies are the anterior longitudinal ligament, and the posterior longitudinal ligament. These ligaments run along the anterior and posterior aspects of the vertebral column and are major players in the stability of the vertebral column.

The most superior part of the posterior longitudinal ligament is called the tectorial membrane, which runs from the posterior surface of the axis (the second cervical vertebra) to the inside of the occipital bone.

What you will see as you look into the base of the skull and the inside of the vertebral canal is the meningeal layer of the dura mater, which passes through the foramen magnum and continues within the vertebral canal. Often the meningeal dura is fused to the tectorial membrane.

Using a scalpel, cut horizontally through the dura/tectorial membrane just superior to the foramen magnum. Use forceps to peel the tough connective tissue away from the skull and expose the cruciate and alar ligaments. Feel for the dens as a bony landmark to let you know that you’re looking in the right region.

You’re looking for the cruciate ligament (or cruciform ligament), which is part of the atlanto-occipital joint, and holds the dens (odontoid process) of the axis tightly against the inner anterior surface of the atlas vertebra. Just lateral and superior to the cruciate ligament look for the alar ligaments (or check ligaments), which connect the dens to the occipital bone. After appreciating the cruciate and alar ligaments, cut through them - this will be necessary to complete the disarticulation of the skull.

Now separate the cervical viscera from the vertebral column and pre-vertebral muscles. From both side of the neck, work your fingers into the pre-vertebral space. The goal is to separate (by blunt dissection) the trachea, esophagus, carotid aa., internal jugular vv., and cranial nerves from their attachment to the cervical vertebrae and the pre-vertebral muscles. Once you’ve made that separation, continue it up to the base of the skull, and down toward the thoracic vertebrae.

Once you’ve separated the cervical viscera up to the base of the skull, use a scalpel to cut through soft tissue structures that are running along the anterior surface of the cervical spine. You’ll have to do this blindly to some degree. Be sure to safeguard the cervical viscera.

After you cut through those structures anterior to the cervical spine, go back to the posterior approach and use a chisel to disarticulate the joint between the axis and the occipital bone, or the atlas and the axis. You’ll probably have to go back and fourth between the anterior and posterior aspects of the vertebral column a few time to complete the disarticulation, just be sure not to cut through the cervical viscera. Ask a member of the teaching staff to give you a hand if you get stuck.

When you’ve disarticulated the skull from the vertebral column and pulled the skull and cervical viscera forward you’ll have a clear view of the anterior surface of the vertebral column.

Locate the scalene muscles again. Recall that the roots of the brachial plexus pass between the anterior and middle scalenes.

Medial to the scalene muscles look for the longus colli mm. The slips of longus colli mm. connect vetebrae to vertebrae, and function to flex and/or stabilize the cervical spine.

Superior to the longus colli mm. are the longus capitis mm. The longus capitis mm. attach to the transverse processes of cervical vertebrae and to the occipital bone. Since you cut through the soft tissue anterior to the cervical spine to remove the head, you will have cut through the longus capitis mm.

Now turn your attention to the posterior view of the cervical viscera. Spend some time cleaning off connective tissue and looking for the structures you can see from this posterior view.

Begin by identifying the broad, thin, flat pharyngeal constrictor mm. These muscles wrap around the back of the pharynx, and initiate swallowing. The three pharyngeal constrictors attach anteriorly to bony and cartilagenous elements of the pharynx, and the left and right halves of each muscle meet posteriorly at a midline raphe. You’ll probably need to pull connective tissue off the posterior surface of the muscles to get a clear view of them. The superior and inferior constrictor mm. are not too difficult to find. To differentiate the middle constrictor m. you’ll have to clean off connective tissue to get a clear view of the muscle fascicles, and then look for changes in the fascicle orientation to find the superior and inferior borders of the middle constrictor.

The stylopharyneus m. is a fairly thin, cylindrical muscle that attaches to the styloid process of the temporal bone, runs superficial to the superior constrictor, and then passes between the superior and middle constrictors before attaching to the thyroid cartilage. Running with the stylopharyngeus m., often on its deep surface, is a small branch of the glossopharyngeal n. (CN IX), which supplies the motor innervation to the stylopharyngeus.

The remaining structures you should look for are things you have seen before, but you will be able to get a more complete view of them now that the cervical viscera is more accessible:

Stylohyoid m. - follow the stylopharyngeus muscle superiorly to the styloid process. Then find the stylohyoid m. also attached to the styloid process, but going anteriorly to attach to the hyoid bone.

Posterior belly of digastric m. - the posterior belly of the digastric m. runs right next to the stylohyoid m., though the digastric m. attaches to the mastoid process, not the styloid process.

Sympathetic trunks and ganglia - you may find separate inferior, middle, and superior ganglia. You will definitely see the superior cervical ganglion, because it’s the largest. Recall that the superior cervical ganglion is the “last stop” for preganglionic sympathetic neurons to synapse with post-ganglionic sympathetic neurons that are going to the face of the inside of the skull.

The superior cervical ganglion and sympathetic trunk may have been reflected with the skull and cervical viscera as in Figure 1.6, or it may have stayed with the vertebral column as in the image below:

Vagus n. - recall that the vagus n. runs in the carotid sheath with the carotid a. and internal jugular v., so you will find it in close association with those structures. Also look for the inferior ganglion of the vagus n., a few centimeters inferior to the jugular foramen. The inferior ganglion of the vagus contains the neuron cell bodies of visceral afferents (visceral sensory) neurons, it’s analogous to the dorsal root ganglion of spinal nerves.

Recurrent laryngeal nn. - see if you can find these as they pass underneath the inferior pharyngeal constrictor m.

Superior laryngeal n. - a branch of the vagus n. that goes to the larynx. The superior laryngeal n. has 2 terminal branches, the internal branch = internal laryngeal n, and the external branch = external laryngeal n. The external laryngeal n. provides motor innervation to the cricothyroid m, which is the only muscle of phonation that is external to the larynx, and also the only phonation muscle that is not innervated by the recurrent laryngeal n. The internal laryngeal n. passes through the thyrohyoid membrane and provides sensory innervation from the mucosa of the larynx.

Hypoglossal n. - follow CN XII toward the base of the skull, see how close you can get to the hypoglossal canal.

Spinal accessory n. - search for the spinal accessory nerve on the deep surface of the SCM and trapezius mm.

External carotid a. - you should have better access to the branches of the external carotid a. now. See if you can find the following:

superior thyroid a. - typically the first (most inferior) branch of the external carotid. Descends toward the thyroid gland. It gives off a superior laryngeal a., which accompanies the internal laryngeal branch of the superior laryngeal n.

lingual a. - typically the second branch of the external carotid a., supplies the tongue. This artery is visible from inside the mouth during the oral cavity dissection.

facial a. - passes over the ramus of the mandible at approximately the mid-p0int of the ramus to supply most of the face. Often it is inadvertently cut as it passes over the mandible. The facial a. has a torturous course, meaning there are lots of bends and curves to it.

occipital a. - runs posteriorly and ascends to supply the posterior region of the skull.

maxillary a. - the maxillary a. is the blood supply to the infratemporal fossa. Within the fossa it gives off several branches, including the middle meningeal a.

superficial temporal a. - this is the continuation of the external carotid a. after the maxillary a. branches off.

transverse facial a. - a branch of the superficial temporal a., runs toward the nose and supplies blood to the face.