Saliva constantly flows from the salivary glands in your mouth. Sometimes, the chemicals in saliva can crystalise to form a tiny stone. Salivary gland stones ('salivary calculi') can become lodged in the duct (tube) leading from the salivary gland, blocking the flow of saliva into the mouth. This can cause swelling and pain in the gland.
Hub assemblies and wheel bearings. Located between the brake drums/discs and the drive axle, the hub assembly is mounted to the holding bracket of the chassis on the axle side. On the drum/disc side, the wheel is connected to the hub assembly via bolts. The wheel bearing itself is inside the hub unit.
There are two basic types of wheel/rim systems. Vehicles that have spoke wheels have the wheels permanently (they aren't removed when the tire is changed) attached to the vehicle. Tires are mounted on rims that have no center discs. These rims slip over the spokes and are clamped to the spokes of the wheels. These wheel/rim systems are called spoke wheel/demountable rim systems. The second major system type is the disc wheel system. Here, the rim is permanently welded to a wheel with a disc in the center containing a mounting face with a bolt hole circle. This wheel is bolted onto the vehicle with studs that are inserted through holes in the hub, brake drum and wheel and fastened with lug nuts.