I. Review of Osteology
Before beginning your consideration of this region, it is vitally important that you review the bones of the skull and mandible. This will orient you to the region and help make understanding this very compact space more clear. To complete the review which follows, you should read with skull in hand and have pipe cleaners, dental floss threaders, cellophane tape, and string available.
View the skull from the side; the so-called Norma lateralis. Identify the bones which make up the lateral projection of the braincase. These include the frontal, parietal, temporal, zygomatic, spheroid, and occipital bones (fig. 1). On the lateral sides of the frontal, parietal, and temporal bones are two curved lines, the temporal lined. These mark the attachments of the components of the temporalis muscle to the skull. Anteriorly note that the lines end at a vertical projection of the zygomatic bone, the frontal process. Posteriorly the lower temporal line becomes continuous with the zygomatic arch or cheer: bone . The space bounded by the temporal lines, the zygomatic arch, and the frontal process of the zygomatic bone is the temporal fossa. The main occupant of the temporal fossa is the temporalis muscle. In addition to its attachment to the skull, the temporaTis muscle attaches to the coronoid process of the mandible. Identify the coronoid and condylar processes of the mandible. Try to imagine how the large, semicircular temporalis muscle converges onto its tendon of insertion on the coronoid process. Palpate your own temporalis muscle, both at the posterior border of the frontal process of the zygomaiic bone and just in front of your ear. By clenching your teeth you can observe the jaw-closing functions of the different parts of this muscle.
Once all of these bony landmarks have been found, the boundaries of
the infratemporal fossa can be easily delineated. Replace the mandible
and follow the description of the boundaries. The lateral wall is formed
by the ramus of the mandible. Near the center of this ramus note the mandibular
foremen. At the anterior end of the fossa the maxilla forms the boundary.
At the top of this anterior wall is the inferior orbital fissure. At the
medial extreme of the anterior wall is the pterygomaxillary fissure. The
medial wall of the infratemporal fossa is formed by the lateral pterygoid
plate. The roof of the fossa is mainly the temporal fossa, but note that
the infratemporal fossa is somewhat deeper than the temporal fossa so that
a small bony roof, formed mainly by the spheroid, can be identified. Two
important foramina lie m this roof: the foremen ovate and the foremen spinosum.
II. Contents of the Infratemporal Fossa
By now you should have a pretty good idea of the bony landmarks surrounding and forming the infratemporal fossa and of the foramina opening into it. Before proceeding to a dissection of the region, it will be useful to review the contents of the infratemporal fossa, so that you can develop an idea of what you should expect to find. The infratemporal fossa contains the pterygoid muscles, the maxillary artery and its branches, the pterygoid venous plexus, and branches of the mandibular division of the trigeminal nerve. In addition, the maxillary division of the trigeminal nerve courses in the roof of the fossa.
The pterygoid muscles are muscles of mastication. They arise from the two sides of the lateral pterygoid plate and insert onto the mandible (fig. 5). The deeper and larger medial pterygoid muscle courses from the medial side of the plate to the angle of the mandible. It acts to pull the mandible up and forward during chewing. The lateral pterygoid courses from the lateral side of the plate to the condylar process of the mandible. It has two heads. The larger oblique head acts much as the medial pterygoid muscle. The smaller horizontal head pulls the mandibular condyle forward during jaw opening.
The main arterial supply to the infratemporal fossa is the maxillary artery (fig. 6). This is the largest terminal branch of the external carotid artery (the other terminal branch is the superficial temporal artery, which supplies the contents of the temporal fossa). The maxillary artery arises just posterior to the neck of the mandible in the substance of the parotid gland and courses somewhat obliquely through the fossa to end in the pterygomaxillary fissure. Through its course It usually lies lateral (superficially to the lateral pterygoid muscle, but it can sometimes lie on the deep side of the muscle. It has an impressive number of branches, but the important ones can be learned with careful attention to the course of the artery. Two early branches are the middle meningeal and the inferior alveolar. The middle meningeal artery courses superiorly from the maxillary and leaves the infratemporal fossa through the foremen spinosum, thereby entering the middle cranial fossa, where it supplies the meninges. The inferior alveolar artery courses inferiorly from the maxillary, enters the mandibular foremen, and courses in the body of the mandible, supplying the mandibular teeth. Numerous muscular branches leave the maxillary artery during its course. Especially important among these are the deep temporal artery which courses on the deep surface of the temporalis muscle, and the buccal artery which supplies the cheek region. The terminal branch of the maxillary artery is the sphenopalatine artery. It enters the pterygomaxillary fissure and will penetrate the sphenopalatine foremen to supply the soft palate and nasal regions. However, before leaving the infratemporal fossa, it gives off branches to the upper molar teeth (posterior superior alveolar), palate (descending palatine) and orbit (infraorbital).
The veins corresponding to the branches of the maxillary artery form a plexus on the lateral and medial surfaces of the lateral pterygoid muscle called the pterygoid plexus (fig. 7). The pterygoid plexus communicates directly with the internal jugular venous system, via the retromandibular vein, but also with the facial vein, via a deep facial vein, with the cavernous sinus of the skull via a small tributary coursing through the foremen ovate, and with a plexus of veins on the pharynx via the sphenopalatine foremen. You should keep in mind that all of these veins are valveless, so that blood can flow in either direction in them, depending on the prevailing pressure gradients.
Any time spent mastering the distribution of the maxillary artery will be time well spent, since the distribution of the branches of the mandibular division of the trigeminal nerve will be similar.
The mandibular division of the trigeminal nerve enters the infratemporal fossa through the foremen ovate (fig. 8). Unlike the other divisions of the trigeminal nerve, the mandibular division contains motor axons which supply skeletal muscle. Branches of this division innervate the muscles of mastication: the medial and lateral pterygoid muscles, the temporalis, and the masseter, all via branches in the infratemporal fossa. In addition the mandibular nerve innervates the mylohyoid muscle and the anterior belly of the digastric muscle outside of the fossa.
Upon entering the infratemporal fossa, the mandibular nerve usually forms anterior and posterior branches. The anterior branches include muscle branches and the buccal nerve, an important source of SENSORY innervation to the skin of the cheek. (Remember that the buccinator muscle receives its motor innervation from buccal branches of the facial nerve.)
1. The auriculotemporal nerve arises as two roots that encircle the middle meningeal artery. It courses posteriorly! in the infratemporal fossa, deep to the lateral pterygoid muscle, and ascends just in front of the ear, in the company of the superficial temporal artery. It provides mainly sensory innervation to the scalp, although it also receives autonomic axons from the otic ganglion destined for the parotid gland.
The otic ganglion lies at the top of the infratemporal fossa, just below the foremen ovate, behind the medial pterygoid muscle, and just anterior to the middle meningeal artery (fig. 9). Parasympathetic preganglionic axons course to the otic ganglion from their source in the glossopharyngeal nerve via the lesser petrosal nerve. The lesser petrosal nerve passes through a canal in the temporal bone and emerges in the infratemporal fossa in or near the foremen ovate. These axons synapse in the ganglion and postganglionic axons join the auriculotemporal nerve to supply the parotid grand. In addition, sympathetic postganglionic axons derived from the cells of the superior cervical ganglion enter the otic ganglion from a plexus on the middle meningeal artery. They course through the ganglion without synapsing, join the auriculotemporal nerve' and supply blood vessels in the parotid gland.
2. The lingual nerve descends on the deep surface of the lateral pterygoid muscle where it receives a small branch, the chorda tympani nerve (fig. 8). The chorda tympani nerve arises as a branch of the facial nerve in the petrous portion of the temporal bone. Instead of coursing through the facial canal with the main part of the nerve, it courses through a small canal in the temporal bone, through the middle ear cavity (hence its name), and emerges from the skull at the posterior extreme of the infratemporal fossa. It carries special sensory fibers associated with taste and parasympathetic preganglionic axons destined for the submandibular ganglion. After receiving this branch, the lingual nerve courses deep to the mucosa of the floor of the mouth, passes beneath the duct of the submandibular gland and supplies mainly sensory innervation to the anterior V3 of the tongue. These include general sensation and special sensation associated with taste buds. In addition, axons from the lingual nerve supply the submandibular ganglion. Some of the parasympathetic postganglionic axons from submandibular ganglion cells join the lingual nerve to supply glands in the mucous membrane of the floor of the mouth.
3. The inferior alveolar nerve courses with the inferior alveolar artery, enters the mandibular foremen and courses forward in the mandibular canal, ending as it emerges anteriorly from the mental foremen as the mental nerve. It supplies sensory innervation to the mandibular (lower) teeth and gums and to the skin of the chin region. Just before entering the mandibular foremen, the inferior alveolar nerve gives off a mylohyoid branch. This branch courses inferiorly, along the inner-side of the mandible and supplies the mylohyoid muscle and the anterior belly of the digastric muscle.
The maxillary division of the trigeminal nerve makes only a cameo appearance in the infratemporal fossa. It courses through the roof of the fossa, leaving the skull via the foremen rotundum and then entering the orbit through the inferior orbital fissure. It terminates anteriorly as the infraorbital nerve. Thus the maxillary nerve courses over the pterygopalatine fossa, and communicates with it. The maxillary nerve has numerous sensory branches to the face (infraorbital and zygomatic branches) and maxillary (upper) teeth (anterior and posterior superior alveolar branches). It also contains axons of sensory and autonomic neurons the lacrimal gland and the mucosa of the palate and pharynx. These latter are not originally part of the trigeminal nerve, but simply "hitch a ride" with it so that they deserve special consideration.
Parasympathetic preganglionic axons derived from the facial nerve leave the main trunk of the nerve and course in a special canal in the temporal bone as the greater petrosal nerve (fig. 10). Sympathetic postganClionic axons derived from cell bodies in the superior cervical ganglion course on the surface of the internal carotid artery and enter the skull with the artery at the carotid canal. Just before the internal carotid artery enters the cavernous sinus. some of these axons leave to form the deep petrosal nerve, course a short distance in the temporal bone, and join with the greater petrosal nerve to form the nerve of the ptervgoid canal. (Recall the proximity of the carotid canal and the opening of the pterygoid canal from your examination of the base of the skull.) This mixed nerve courses through the pterygoid canal to end in the sphenopalatine foremen. The sympathetic axons course through the adjacent pterygopalatine ganglion without synapsing and are distributed with the nerve and arterial branches emanating from it, to the mucosa of the nasal and pharyngeal regions. The parasympathetic fibers entering the pterygopalatine ganglion synapse there and distribute to glands in the same mucous membranes. In addition, some of the postganglionic axons pass to the lacrimal gland by way of first the maxillary nerve, then one of its branches, the zygomatic nerve, and finally by way of one of the branches of the ophthalmic division of the trigeminal nerve, the lacrimal nerve (fig. 10). Sensory axons of the maxillary division of the trigeminal nerve from the nasal, palatine, orbital, and lacrimal regions take courses similar to these autonomic neurons (but in the opposite direction) and pass through the pterygopalatine ganglion without synapsing. Their cell bodies (equivalent to those found In the dorsal root ganglia) lie in the trigeminal ganglion.
Plan for Dissection
By now you should be familiar with the boundaries and contents of the infratemporal fossa, so that a detailed dissection plan will seem somewhat silly. Your individual dissection plan should, as always, keep in mind the structures that you wish to see as well as those structures you want to preserve for your demonstration of the region. The instructions which follow are only designed to be very general ones.
First, you will want to orient yourself to the region on your cadaver. Find the palpable bony boundaries of the fossa: the zygomatic arch the frontal process of the zygomatic bone, the angle of the mandible, and the external acoustic meatus (alveolar), palate (descending palatine) and orbit (infraorbital).
Identify the masseter and temporalis muscles and the coronoid and condylar
processes of the mandible. In order for you to expose the infratemporal
fossa, you will have to remove its lateral wall, the ramus of the mandible,
and the zygomatic arch. However, you will want to be especially careful
in doing so, as you will not want to destroy the contents of the fossa
in the process. The best way to insure success is to recall the superficial
structures of the fossa and to move them out of the way before you remove
the bone. The procedure described in your dissector is a good starting
point. Removal of the zygomatic arch, temporalis muscle, and the coronoid
process of the mandible should be done first. In removing the rest of the
mandibular ramus, don't spend too much time looking for the "lingula".
Your objective will be to make your saw cut above the level of the mandibular
foremen, so that the important inferior alveolar vessels and nerves will
be preserved. To do this, run the tip of a probe down the inside of the
mandible until you feel it catch in the mandibular foremen. Note the length
of the probe to the tip from the mandibular notch. Mark this distance on
the outside of the mandible and make the third saw cut above this line.
The third saw cut should be done especially carefully to preserve the lateral
pterygoid muscle and maxillary artery, both of which course close to the
suggested area. Once the infratemporal fossa is opened, identify the two
heads of the lateral pterygoid muscle and then cut its posterior connections
and reflect it forward. If this is not possible, then remove the lateral
pterygoid muscle in pieces, taking care not to damage arteries branching
off of the maxillary artery. Follow a similar procedure for removing or
reflecting the medial pterygoid muscle, so that the mandibular nerve can
be seen entering the infratemporal fossa through the foremen ovate. You
should now have a good idea of how and where to find most of the important
structures you want. If you are confused by what you see, review the contents
of the fossa with your dried skull, and try to relate what you see to what
you have learned on the skull.