 This is Dr. Sanjay Sanyal, Professor of Department Chair. So this is going to be a dissection demonstration of the muscles of mastication and the temporomandibular joint. This is the supine cannibar. We are showing the right side of the face. As you know, the muscles of mastication are distributed over three different regions. We have the temporalis muscle in the temporal region. We have the meseter muscle in the parotid region. And we have the two other, the tericoid muscles in the infratemporal fossa. So therefore we go layer by layer. So let's start off. This is the superficial temporal fascia that we are retracting right now. This was attached to the zygomatic arch and we have cut it and we have reflected it. The next fascia that we see underneath is this one here. This is the deep temporal fascia. The deep temporal fascia was also attached to the zygomatic arch. It is a very thick at the lower portion and it has got two layers. The outer layer is attached to the outer margin of the zygomatic arch. The inner layer is attached to the inner surface of the zygomatic arch. We have cut that and we are reflecting that also. The first muscle that comes to a view is this one here. This is the temporalis muscle. The temporalis muscle, it takes origin from the inferior temporal line in contrast to the temporal fascia, deep temporal fascia, which takes attachment from the superior temporal line. This temporalis muscle fibers, the vertical fibers, anti-fibers go down and the posterior fibers are almost horizontal while the in-between fibers are oblique. All these fibers, they converge under the zygomatic arch and they will go and get inserted onto the coronoid process of the mandible, which I shall show you just now, and to the temporal crest of the mandible. So what is the action of this temporalis muscle? The temporalis muscle, the anti-fibers, they are responsible for closure of the jaw and the posterior fibers, they are meant for protrusion of the mandible and they are also responsible for small side-to-side chewing movements of the epsilon side. The next muscle that comes into a view is this one here. This is the mesetior muscle. The mesetior muscle takes origin from the zygomatic arch and the fibers, they are very strong and they are covered by this aponeurotic sheet. They get inserted onto the mesetor tuberosity on the lateral surface of the angle of the mandible. It's a quadragonal shaped muscle. What does this muscle do? This was also a powerful closure of the mandible, therefore it elevates the jaw. The superficial fibers, they are responsible for protrusion of the mandible. So these are the two main actions of the mesetior. Now what we shall do is we will go slowly into the temporal fossa. Now this temporal fossa entry is not an easy task. We are going to go to the infrared temporal fossa. How do we enter the infrared temporal fossa? So first we have to cut the zygomatic arch here and we are removing this piece of zygomatic arch. Next we have to cut the attachment origin of the mesetior muscle. Now we are removing it. Then comes into a view is the ramus of the mandible. And here you can see this is the head of the mandible. This is the neck of the mandible and this is the coronoid process of the mandible and the small elevation in the front is the infrared temporal crest of the mandible. And we can see now clearly how the temporalis muscle is getting attached to the coronoid process and to the temporal crest of the mandible. We can clearly see that here. The next thing we had to do was we had to disarticulate the temporal mandible joint. And that is what we have done here. And in the middle we can see an articular disc that I will show you just now when I remove the mandible. Then we had to saw off the ramus of the mandible and then we had to turn it over. And that is what we did. Once we turn this, now what is coming to our view is the infrared temporal fossa. So the boundaries of the infrared temporal fossa are laterally it is bounded by the ramus of the mandible, medially it is bounded by the lateral pterigoid plate, anteriorly it is bounded by the maxilla, posteriorly it is bounded by the styloid process of the temporal bone, the mastoid process of the temporal bone, superiorly is the infrared temporal surface of the greater wing of sphenoid and inferiorly is this region where the medial pterigoid muscle is attached to the pterigoid tuberosity on the inner surface of the angle of the mandible. Now let's take a look at those two muscles. So once we are in the infrared temporal fossa, what do we see? We see one triangular shaped muscle here. This is the first muscle. This is the lateral pterigoid muscle. Lateral pterigoid muscle has got two heads. We can see the upper head and this is the lower head. The upper head takes origin from the infrared temporal crest of the greater wing of sphenoid and the fibers they go posteriorly and you can see they are getting attached to this structure here. This is the articular disc of the temporal mandibular joint. This is the upper head of the lateral pterigoid. We have retained the attachment here. The lower head of the lateral pterigoid, it takes attachment from the lateral surface of the lateral pterigoid plate. That's why it's called lateral pterigoid. And the fibers they go back, it's a bigger head than the upper head and they get inserted onto this portion of the mandible. This region of the mandible, if you see very carefully there's a small depression under the head of the mandible, this is the neck of the mandible and on the medial side of that there's a depression here. This is called the pterigoid phobia. That is where the lateral head, the lower lateral head of the pterigoid is attached. So what is the action of the lateral pterigoid? The upper head of the lateral pterigoid, when it contracts it pulls the articular disc. It pulls the articular disc forward and flattens it slightly. And it helps the mandible, the lower head to open the jaw by pulling the pterigoid phobia forward. So therefore when the person opens his jaw, the mandible protrudes forward on the articular tubercle and it depresses down. So that is the unique action of the lateral pterigoid. Now let's take the next one. This is the next muscle. This is the medial pterigoid. You can see we have cut it here, we had to cut it because as I told you the medial pterigoid is attached to the inner surface of the angle of the mandible and this rough area which is called the pterigoid tuberosity. The medial pterigoid also has got two heads an upper head and a lower head. The upper head takes origin from the posterior surface of the maxilla. The lower head takes origin from the medial surface of the lateral pterigoid plate which I can feel but you cannot see it. That's why this is called the medial pterigoid muscle. The fibres are roughly quadrangular in shape and therefore this is considered as the mirror image muscle of the masseter. The masseter is on the outside, the medial pterigoid is on the inside and it gets inserted under the pterigoid tuberosity. What do these two heads do? They are responsible also for closure of the mandible. Coming to the nerve supply. They are all supplied by branches of the mandibular nerve. CN5V3 and we can see the CN5V3 here. The nerve which I have picked up here this is the CN5V3 in the infrared temporal fossa. It enters into the infrared temporal fossa through the foramen ovale and it breaks up into multiple branches. The branches to the pterigoid are called the pterigoid branches. The branches to the temporalis are called the deep temporal nerves and the branches to the masseter are called the massetry branches. So they are the ones which supply all the muscles of mastication. Let's take a quick look at how the temporal mandibular joint moves. This is the temporal mandibular joint. This is the head of the mandible. This depression is the mandibular fossa of the temporal bone. So when the mandible opens, it moves like this. Of course we have cut it here and that's what opens the jaw and when the mandible goes up, it closes the jaw. All the muscles of mastication, temporalis muscle, the masseter muscle and the medial pterigoid muscle, they are primarily responsible for closing the jaw. Only the lateral pterigoid is primarily responsible for opening the jaw. So this is the main action. There are accessory actions. Like for example, the posterior fibres of the temporalis are responsible for retribution of the mandible while all the other muscles, they are responsible for protrusion of the mandible. Additionally, the two pterigoids, they are responsible for side to side chewing movements of the contralateral side while the temporalis muscle is responsible for side to side chewing movements of the same side. Additionally, we have something called accessory muscles of mastication. The suprahyoid muscles, they help in opening the jaw. Once the hyoid bone is stabilized by the infrahyoid muscles and the platysma also helps to open the jaw. That brings me to a few points of clinical correlation. When the masseter muscle is located here, as I had mentioned earlier it gives a powerful downward pull on the zygomatic arch, which we have removed. This is the zygomatic arch. In rare instances, the zygomatic arch has even been fractured by the downward pull of the masseter. So therefore, this is given a strong counter traction by this thick, strong, deep temporal fascia. That's one point. The second point is, if there is an injury of the CN5V3, that is the mandibular branch of trigeminal nerve, then all the muscles of mastication will be paralyzed and what will be the manifestation? The manifestation will be when the person tries to open his jaw, that side lateral teregoid will not be able to open the jaw. So therefore, the jaw will deviate towards the paralyzed side. So that's an important manifestation of paralysis of the CN5V3, that is the mandibular division of trigeminal nerve. Incidentally, the mandibular division of trigeminal nerve also supplies the anterior digastric, the mylohyoid, tensor palatin E and tensor timpani. So therefore, the analysis of the tensor timpani will produce hypoacusis, but that's a different topic altogether. So these are the various muscles of mastication that we wanted to show you, temporalis muscle, the masseter muscle, and the two teregoids, the lateral teregoid and the medial teregoid, with their action on the temporal mandibular joint. Thank you very much for watching. Dr. Sanjay Sanyal, Sunny Out. Sherwin Weeks is the camera person. If you have any questions or comments, please put them in the comment section below. Have a nice day. Please like and subscribe.