İKİNCİ FEN · ÜÇÜNCÜ BÂB · BİRİNCİ FASIL · Dördüncü Madde
Fourth Article
Concerning the movement, benefits, and muscles of the lower jaw.
O esteemed one! Know that anatomists have said: There are many benefits to the upper jaw being immobile and the lower jaw mobile, and among these is that it is more suitable and easier to move what is light. Another is that moving the jaw which does not carry important organs susceptible to harm is safer and more beautiful. Another is that because the upper jaw is immobile, the joint of the upper jaw is firmly and durably connected to the head joint.
The mobile lower jaw has three movements: one is the opening of the mouth, another is the closing of the mouth, and a third is the chewing and grinding movement. The action of opening the mouth lowers the jaw. The action of closing raises the jaw. The chewing and grinding movement draws and rotates the jaw from side to side. Thus, two muscles (the temporal muscles) have been created to close the jaw, which descend from above and draw the jaw upwards. Because the human jaw [122/b] is light and does not require hard and tearing tasks like an animal’s jaw, these two muscles were created in the smallest form and originate from the very delicate body, the brain. Being near it, they are also found to be soft and delicate, for only a thin bone separates them from the brain. And as they emerge from the brain, two pairs of bones cover them, passing them through a veil, so that as these bones move away from their origin, their substance gains some hardness. Each of these two muscles has a large nerve which encircles the edge of the lower jaw and, when they converge, raise the jaw and adhere it to the upper jaw. There are also two other muscles assisting these, which come from within the mouth and descend into the hollow of the lower jaw. They have been formed from tendons emerging from their middle point for the strength of the nerves arising from the muscles of the mouth.
The muscles that open the mouth and lower the jaw descend from the protrusions of the styloid bone behind the ear, converging to form a single muscle. To gain additional power from it, there is a special short nerve which reaches and adheres to the point where the two jaw bones join below, drawing the palate downwards towards its underside. For this natural weight of the jaw aids in its descent. Therefore, two muscles are sufficient for it and it needs no other helper. Two muscles have been created for chewing, with a triangular muscle on each side. The apex of the triangle reaches the cheekbones, while its two stems extend, one descending to the lower jaw and the other rising towards the zygomatic region. Its base joins in a straight line between them, and each corner goes to its place. Thus, various movements arise from the convergence of the triangular muscle as described, resulting in chewing and grinding.
“We praise Allah who created and shaped all of this.”