Anatomy of the muscles of the lower jaw

The ability to move is given specifically to the lower jaw, and not the upper, for the sake of several benefits. Firstly, it is better to set in motion what is lighter; further, it is more appropriate and safe to set in motion an organ that is further removed from contact with important organs that the movement could injure; further, if the upper jaw were easy to move, then its articulation with the head would not be so carefully secured. Then, there is no need for the movements of the lower jaw to be more than three in number: this is the movement of opening the mouth and pharynx, the movement of closing and the movement of chewing and grinding.

The opening movement lowers the jaw down, the closing movement lifts the jaw, and the grinding movement rotates the jaw and moves it in both directions. It is clear that the closing movement must be carried out by the muscle located above, which contracts in an upward direction; the spreading movement is the opposite, and the rubbing movement should be carried out with muscles running obliquely. Therefore, two muscles are created to close the mouth, known as the temporalis muscles and also called the “torsion muscles.” In humans, their size is small, since the organ that they move in humans is small, spongy, light in weight, and its movements emanating from these two muscles are lighter. As for other animals, their lower jaw is larger and heavier than that of humans, and the movements imparted by these two muscles are when gnawing, cutting, biting and tearing out, cutting.

The two muscles mentioned are soft because they are located close to the beginning of the nerves, that is, to the brain, which is an extremely soft body. There is only one bone between them and the brain. For this reason, and also due to the danger that damage from them will spread to the brain if it happens, or pain that would lead the patient to sarsam, and similar ailments, the Creator, praise be to him! - buried these muscles at the place of their origin and exit in the bones of the “yoke” ^And led them under cover, like a vault formed by the bones of the “yoke,” and along the convolutions of a passage with enclosing edges, along which they followed for a considerable distance until they passed the “yoke.” This is done so that their substance gradually hardens, and they gradually move away from the place of their original exit.

Each of these muscles has a tendon that covers the edge of the lower jaw; when it contracts, it raises the jaw.

These two muscles are given assistance by two other muscles, passing inside the mouth and descending into the recesses in the lower jaw, for lifting something heavy requires measures that provide excess strength.

The tendon extending from these two muscles grows, for the sake of strength, not at their end, but in the middle. As for the muscle that opens the mouth and lowers the jaw, its fibers grow from needle-like processes located behind the ear. They merge into one muscle, but then, for the sake of greater strength, they separate into fibers, after which they are separated again, begin with meat, and, in order not to be damaged due to their considerable length, turn into muscle again. It's called the "double muscle." This muscle fits where the jaw curves toward the chin. By contracting, it pulls the jaw back, and the jaw inevitably drops down. Due to the fact that natural heaviness contributes to the lowering of the jaw, two muscles are sufficient and there is no need for an assistant.

As for the chewing muscles, there are two of them - one triangular muscle on each side. If we consider the corner located on the cheek as the apex, then two legs stretch from it, one of which goes down to the lower jaw, and the other rises towards the “yoke”. These muscles connect the straight base, and each corner is adjacent to the part of the base adjacent to it, so that the triangular muscle can contract in different ways. As a result, its movement is not direct, and it has the ability to deviate in different directions and make movements that grind and chew food.