In the trachea there are tracheal rings, also known as tracheal cartilages. There are generally sixteen to twenty individual cartilages in the tracheal, which varies from person to person. These cartilages reside above one another, lying horizontally. Each one is about one millimeter thick, with a depth of around four millimeters. The tracheal cartilages have an order, starting with the peculiar tracheal cartilages. These are the first and last rings in the trachea. The first cartilage is broad and divided at the end. It sometimes blends in with the following cartilage, depending on the structure of the trachea. The last cartilage is broad in the middle and thick, with a triangular shape for a lower border. This piece curves downward, extending into the two bronchi until forming an imperfect circle that encloses the bronchus. When a person ages, the tracheal cartilages may begin to calcify. Their normal state is one of elasticity. Two or more cartilages often merge in the trachea, either partially or completely, and can be bifurcated.