We have already pointed out that there are eighteen ribs on each side, distinguished numerically as the first, second, third, and so on. The first eight are attached to the sternum and designated true ribs. The. remaining ten, having no such connection, are called false ribs. Although they are thus distinguished, they all possess certain common characteristics. They are long, flat, more or less curved or arched outward from the chest, and are, besides, somewhat twisted on themselves. They are all connected with the vertebrae above by two free-moving joints, and below they are attached to rods of cartilage (costal cartilages), through which the first eight become united by synovial articulations to the upper part of the side of the sternum, as already explained. Each rib possesses a head, a neck, and a tubercle at the superior extremity. The head fits into a hollow formed between the bodies of two vertebrae, where it is united by ligaments to form a free-moving joint. The tubercle forms another synovial articulation with the transverse process of the vertebra behind. The length of the ribs varies with the position they occupy. From the first to the ninth they increase in length, and then progressively they diminish to the last. Variation is also noticeable in the width, which increases from the first to the sixth or seventh, and then diminishes to the eighteenth.
The outward curve they make increases from the first to the last, and gives rotundity to the body in proportion as it is great or otherwise.