 This is going to be a demonstration of the muscles of mastication. So let's start off with the first muscle that we can see here. This is the masseter muscle. The masseter muscle takes origin from a very strong tendon, which we can see here from the zygomatic arch. And we can notice that the fibers as they descend down, they twist slightly. And the fibers then get inserted onto the masseter tuberosity on the lateral surface of the angle of the mandible, which I shall show you a little later in more detail. All the muscles of mastication are supplied by the branches from CN5B3. Coming to the action of the masseter. The most important action of the masseter is it's a very powerful elevator of the jaw. That means it closes the mouth. The superficial fibers of the masseter, they cause protrusion of the mandible. When the masseter is contracting very strongly, it exerts a very strong pull on the zygomatic arch. There have been rare occasions when the zygomatic arch has even fractured. And to counter the pull of the masseter, there is a very strong fascia which we have cut. And that is the deep temporal fascia. That is the one which is inserted onto the zygomatic arch from above. And that gives a strong counter traction to the downward pull of the masseter. The next muscle that we can see here is this one. This is the temporalis muscle. Temporalis muscle takes origin from the inferior temporal line. And my instrument is tracing the inferior temporal line. It was covered by this deep fascia of the temporal muscle, which we have reflected. And if you take a look at the fibers, we notice that the anterior fibers are vertical. The posterior fibers are transverse and more horizontal. So therefore, the fibers converge in a fan-shaped fashion. And they go under the zygomatic arch where my instrument has disappeared. And they get inserted onto the coronate process of the mandible, which I shall show you. And to a small elevation just below the coronate process known as the temporal crest. The temporalis muscle and the anterior fibers. Vertical fibers are also responsible for elevation of the jaw. The posterior fibers, they are responsible for retrosion of the mandible. The temporalis muscle is also responsible for small side-to-side chewing movements of the ethylateral side. Now let me remove the zygomatic arch. And I shall reflect the masseter to show the inferior temporal fossa to show the other two muscles of mastication. So we have cut the zygomatic arch here and here. And now I have removed the zygomatic arch. We have already incised the origin of the masseter. And I'm going to reflect the masseter. This is the parotid duct. And we are pushing out the masseter from under that. So now we can see this is the ramus of the mandible. And we have cut that also here. I'm going to reflect this mandible now. And I'm going to push it under the parotid duct again. Just to bring you up to speed, this is the temporal mandible joint. So therefore this is the head of the mandible. And under that is the neck of the mandible. And this depression is referred to as the pterigoid fovea, which I shall tell you too just a little later. And this structure that we can see here, this is the coronoid process. And under that, this elevation is the temporal crest of the mandible. And this is where we can see the fibers of the temporalis muscle are getting inserted. So we have removed that, and now I'm reflecting it up. Once we reflect it up, now we have completely exposed the infra temporal fossa. Now let's take a look at the muscles that we can see here. So this is the cut portion of the ramus of the mandible. We can see this muscle here. This is the lateral pterigoid. The lateral pterigoid has been cut here. And we can see the cut portion because it was attached to the pterigoid fovea, head of the mandible. And it was also attached to the articular disc of the temporal mandibular joint. So that is where it was cut. So let's come to the lateral pterigoid first. The lateral pterigoid is a triangular shaped muscle. And we can see that it is roughly triangular. It's got two heads, an upper head and a lower head. The upper head takes origin from the infra temporal crest of the greater ring of sphenoid. And the lower head takes origin from this bone, which I can feel here, but we cannot see it because it's covered. The lower head takes origin from the lateral surface of the lateral pterigoid plate. Upper head gets attached to this cartilage here. This is the articular disc of the temporal mandible joint. And the lower head gets attached to the head of the mandible and to the pterigoid fovea. So what is the action of the lateral pterigoid? Later pterigoid is the only muscle of mastication which is responsible for opening the jaw. The upper head, because it is inserted onto the articular disc, when the person is about to open his jaw, it slightly flattens the articular disc. And then the lower head is responsible for opening the jaw and at the same time, it also pulls the head of the mandible slightly forwards and downwards. And that is how the person opens his jaw. This is a reconstructed CT scan to show the movements of the temporal mandible joint in opening and closing the mouth. And if the person opens his jaw widely, we know very well the person can get dislocation of the mandible. So that is the action of the lateral pterigoid. Now let's take a look at the next muscle. This is the next muscle of mastication. This is the medial pterigoid. The medial pterigoid is quadrangular shaped and it is considered as the mirror image muscle of the massager. Just like the massager is inserted on the outside, medial pterigoid is inserted on the inside. The medial pterigoid is also got two heads, an upper head and a lower head. The upper head is attached to the medial surface of the lateral pterigoid plate and the lower head is attached to the posterior surface of the maxilla. The two heads then continue down and they get inserted where my finger is disappeared onto the pterigoid tuberosity on the medial surface of the angle of the mandible. What is the action of the medial pterigoid? The medial pterigoid is also responsible for closure of the jaw. So therefore, the temporalis, meseter, medial pterigoid are all responsible for elevation of the mandible. Additionally, the medial pterigoid is also responsible for small side to side chewing movements of the contralateral side. The lateral pterigoid is also got side to side chewing movements which is seen more prominently in cows but humans also do it. It is responsible for large side to side lateral movements of the contralateral side. This nerve that we see here, this is the buccal nerve which comes out between the two heads of the lateral pterigoid and these structures that we can see here, the lingual nerve and the inferior alveolar nerve, they come out between the lateral pterigoid and the medial pterigoid and we can see that here. As I mentioned, the medial pterigoid and the meseter are considered as mirror image muscles because both are quadrangular shape. One is inserted on the outer surface, one is inserted on the inner surface of the angle of the mandible. So therefore, in fractures of the mandible, near the angle, like shown in this institution here, it is self-splinted by these two muscles. If there is injury to the mandibular division of trigeminal nerve which supplies all these muscles. The mandibular division supplies meseteric branch, pterigoid branch and deep temporal branch. My instrument has picked up the deep temporal branch from the CN5V3 which supplies the temporalis muscle. If there is an injury of the mandibular nerve and there's a paralysis of the muscles of mastication, most significant effect will be noted due to paralysis of the lateral pterigoid. The jaw will deviate towards the parallel side on attempted opening of the mouth. This is an important manifestation of paralysis of the lateral pterigoid muscle caused by injury to the CN5V3. In this particular cadaver, we notice that there's a small variation that temporalis muscle has got some additional tendons and we have cut that tendon here. This tendon was attached here and it was partly merging with the fibers of lateral pterigoid muscle. So these are the structures which I wanted to show you regarding the muscles of mastication. Thank you very much for watching. Dr. Sanjay Sanyal, Sanyal, please like and subscribe. If you have any questions or comments, please put them in the comment section below. Have a nice day.