Variation in the thoracic index
The magnitude of this index is in the order of quadruped > fetus > child > adult.
In large hoofed animals with degenerated clavicles, the anterior portion of the thorax is extremely narrow and longitudinal in order to efficiently support the body weight (anterior half of the body) by the serratus abdominis (serratus anterior in human) muscle. In aquatic animals, the cross-section of the thorax is almost circular because it is freed from weight bearing, and the body is torpedo-like. In adult humans, the thorax is flattened anteriorly and posteriorly. This and the kyphosis of the thoracic portion of the spinal column seem to help bring the center of gravity of the thorax closer to the spinal column.