The backbone is indeed a marvel.

The fitness of the construction consists, like that of the American truss-bridge, in attaining the maximum of strength with the minimum of weight.

It is brought about by dispensing with every cubic millimeter of bone which can be spared without weakening the vertebrae for the various stresses and strains to which they were subjected, and these must have been tremendous in an animal from sixty to seventy feet in length.

The bodies of the vertebrae are of hour-glass shape, with great lateral and interior cavities; the arches are constructed on the T-iron principle of the modern bridge-builder, the back spines are tubular, the interior is spongy, these devices being employed in great variety, and constituting a mechanical triumph of size, lightness, and strength combined.

Comparing a great chambered dinosaurian (_Camarasaurus_) vertebra (see above) with the weight per cubic inch of an ostrich vertebra, we reach the astonishing conclusion that it weighed only twenty-one pounds, or half the weight of a whale vertebra of the same bulk.