The skeleton of the elephant has been greatly influenced by the tremendous mass it must support. Incredibly, it weighs about 16.5 percent of an elephant´s total weight. As a comparison, cattle have a skeletal structure that only weights about 10 percent of the total weight. The backbone is the mechanism by which soft tissues are 'hung', consisting of sturdy vertebrae with high, strong neural spines in the chest (thoracic) area. Similar to Man, the elephant has an almost vertical pelvis that is greatly expanded. This influenced the prime reasons why it is difficult for an untrained eye in the past to know the sex of a particular elephant; the genital organs in the male are inside the body, and the female´s genitals are outside the body but are difficult to spot from the rear. The ribs extend along most of the backbone and form an enormous barrel-shaped cage. The limbs are composed of segments in direct line with one another resulting in a rigid pillar of support for the huge mass of the elephant: it is as though an elephant is walking on four thick and upright pillars, which are long in the upper segment and short in the lower. Also, the majority of the marrow cavities in the leg bones have been replaced with a spongy bone aiding in the legs great strength and relatively light weight. As in all mammals, elephants have seven neck vertebrae. Unlike other herbivores, the elephants vertebrae evolved to have fused and relatively flat discs, which are able to handle the weight of the elephant´s tusks and head.