Month: May 2009

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OK, where were we? By looking at the medial and lateral pterygoid muscles we found ourselves in the infratemporal fossa.
This space is bounded superiorly by the sphenoid bone (and the temporal bone), laterally by the ramus of the mandible, medially by the lateral plate of the pterygoid process and anteriorly by the maxilla. You can reach this place on a skull by sticking your finger down through the zygomatic arch and deep to the mandible, or from posterior to the ramus of the mandible. By doing this you also see the routes by which structures may enter or leave the infratemporal fossa.
What’s so special about this region? It has some important structures running through it, and by knowing where the infratemporal fossa is you will also know where these structures run. As you find out more about the sphenoid bone and the nerves and vessels that pass through it you will become aware that they all pass from or to the infratemporal fossa. It’ll give you a chance to link parts of the head together in your own mind.
The common carotid artery ascends in the neck and splits into internal and external carotid arteries. The internal carotid artery disappears inside the skull and the external carotid artery continues to ascend posterior to the mandible. It terminally bifurcates as the maxillary artery and the superficial temporal artery. The maxillary artery dives through the infratemporal fossa, its branches extending to parts of the face (e.g. the mandible, teeth, the cheek, the scalp) and supplying blood to those muscles of mastication that we also looked at. Importantly, it also gives off a branch called the middle meningeal artery that passes superiorly through a small foramen (spinosum) in the sphenoid bone. This artery is important in supplying blood to the dura mater (connective tissue surrounding the central nervous system), and very important when considering blows to the side of the head, such as encountered in boxing.
So now you can locate the maxillary artery as it runs from the external carotid artery through the infratemporal fossa, and if you pick up a skull and take a look at this region hopefully you will be able to imagine its course in three dimensions.
In this same region the blood is drained through a plexus of veins known as the pterygoid venous plexus (or simply, pterygoid plexus). We’re starting to see that the term pterygoid is associated with the infratemporal region, because of those pterygoid processes (Arnold’s glossary says, “pterygoid: adjective, Greek pteryx = wing, and eidos = shape; hence, wing-shaped”). This plexus of veins surrounds the structures here, draining blood from the cavernous sinus inside the cranium, and linking to the facial vein, a more superficial structure. There’s an important anatomical link here between superficial and intracranial venous structures, and this has an interesting clinical relevance. There are no valves in the veins here, so infection may pass from superficial or oral structures back to the cavernous sinus, causing serious problems.
So there are two important blood vessels in this region. What about nerves? If we talk about cranial nerves in general, we have 3 cranial nerves involved in the infratemporal fossa. Hopefully you’ll remember that the facial nerve (CN VII) passes through the parotid salivary gland (but doesn’t innervate it) and branches to supply the muscles of facial expression. The parotid gland lies over the region that we’ve been looking at, so the facial nerve must pass very near to the infratemporal fossa. In fact, the facial nerve drops out of the skull from the stylomastoid foramen, a little posterior to the infratemporal fossa.
However, parasympathetic fibres from the facial nerve making their way down to the sublingual and submandibular glands do pass through the infratemporal fossa. The chorda tympani, a nerve carrying those fibres, passes through the middle ear and leaves the skull via the petrotympanic fissure, a fissure in the temporal bone a little anterior to the stylomastoid foramen. The chorda tympani passes through the infratemporal fossa to join with the lingual nerve, and passes inferiorly to the lower salivary glands.
Aha – the lingual nerve. Do you remember what that’s a branch of? Our next cranial nerve is the trigeminal nerve (CN V). The lingual nerve is a sensory branch from the mandibular nerve (also known as V3), the third branch of the trigeminal nerve. The mandibular nerve leaves the cranium through the foramen ovale (see here, and here), passing into the infratemporal fossa. All its branches pass through the infratemporal fossa, including those motor branches passing to the muscles of mastication that we looked at earlier (making the infratemporal fossa a great landmark for finding and remembering where all these nerves are).
Ok, the final cranial nerve. If the facial nerve passes through the parotid gland but doesn’t supply secretomotor fibres to tell it to squeeze juice, what parasympathetic fibres do? The glossopharyngeal nerve (CN IX) sends fibres on a fascinating route through the skull (described here, but best looked at with a skull or a model, and multiple sources to work it all out) that eventually makes it to the infratemporal fossa. Here, right next to the mandibular nerve shortly after it appears from the oval foramen, these glossopharyngeal fibres synapse at the otic ganglion. There are a few ganglia in the head that you should know about, and this is one of them. Postganglionic parasympathetic fibres then pass from the otic ganglion as part of the auriculotemporal nerve to the parotid gland, and it’s these fibres that tell the parotid gland to secrete saliva.
So just by finding one region in the skull you can now locate 2 major vascular structures, important parts of 3 cranial nerves, and you’ve started to learn about the bones and foramina of the skull itself. It’s a good region to spend time looking at.

In my anatomy session this week we started looking at the anatomy of the head (as I may have mentioned several times, probably my favourite area, anatomically-speaking). In this station, we looked at the movements of the jaw, the muscles that make those movements, some of the bones involved (bones of the skull – get looking at skulls now because this is crucial for this term) and found that we were looking at the infratemporal fossa. We described what this was and noted a number of important structures passing through it.
So what are the movements of the mandible? The temporomandibular joint (TMJ), between the temporal bone and the condylar process of the mandible, allows for some interesting movements, and the left and right TMJs work together to allow us to bite, chew and express ourselves. It isn’t a particularly standard synovial joint structurally, histologically, or in the way that it articulates. Opening the mouth involves both the hinging of these joints and the forward translocation of the mandible out from the joint cavity.
Back to movements. We can elevate the jaw, depress it, move it from side to side, protrude it, and retract it. Combining these movements and making slightly different movements on either side allows us to get to grips with our food and chew it. There is a great deal of power exerted through our teeth when we bite. So what muscles give what movements? We can feel the masseter muscle getting thicker as it contracts if we palpate the region just anterior to our ears when we clench our jaws (yes, do it now). The masseter muscle is a superficial muscle attached to the zygomatic arch and to the ramus and angle of the mandible.
We can also palpate the temporalis muscle (touch your “temples”, or temporal region) when we clench our jaws. This is another superficial muscle but has a broad, curving attachment to the bones that it covers (frontal, parietal and temporal bones). Its fibres all come together to pass deep to the zygomatic arch and insert into the coronoid process of the mandible, giving it not only great power from its size but also a mechanical advantage relative to the pivot point of the TMJ. This is a really powerful biting (or mandible elevating) muscle. It’s really, really powerful and much larger in animals that need to rip other animals apart with their teeth.
So if the temporalis muscle is a major elevator of the jaw, the masseter aids this movement, but is also involved in the side to side movement. There are two more muscles of mastication, and both are deep to the mandible. The medial and lateral pterygoid muscles both attach to the lateral plate of the pterygoid process of the sphenoid bone. You can see what this is here on Wikipedia, but you really need to go to the lab and pick up a skull to get the best feel for where it is.
The lateral pterygoid muscle is rather different to the other muscles of mastication as its fibres run horizontally. All the other muscles run in almost vertical directions, but the horizontal fibres of the lateral pterygoid muscle are able to protract the jaw. The sphenoid bone doesn’t move, the fibres of the lateral pterygoid muscle contract, and they pull the head of the mandible anteriorly in the TMJ.
The medial pterygoid muscle runs inferiorly from the pterygoid process down to the deep surface of the mandible, on the other side of the mandible from the masseter muscle. This muscle also helps elevate the mandible.
What causes depression? I mean, what muscles depress the mandible? Gravity has a role here, but the strap muscles in the anterior neck (digastric, geniohyoid, mylohyoid) are able to pull the mandible down. The masseter and temporalis muscles are able to retract a protruded jaw, by the way.
By looking at the pterygoid plates and the muscles there, we’ve put ourselves into the infratemporal fossa. What’s that then? Well, I’m a bit worn from writing all that so I’ll write up the infratemporal fossa stuff a bit later.
Where to poke?

Palpating the masseter muscle.

Palpating the temporalis muscle.