The backbone is a more deep-seated part of the skeleton than the head. It is more protected by its position, and has less varied functions to perform. Therefore it varies less in distinctive character within the limits of each of the classes of vertebrate animals than either the head or limbs. It is divided into neck bones, the cervical vertebræ; back bones, the dorsal vertebræ; loin bones, the lumbar vertebræ; the sacrum, or sacral vertebræ, which support the hind limbs; and the tail. Of these parts the tail is the least important, though it reaches a length in existing reptiles which sometimes exceeds the whole of the remainder of the body, and includes hundreds of vertebræ. It attains its maximum among serpents and lizards. In frogs it is practically absent. In some of the higher mammals it is a rudiment, which does not extend beyond the soft parts of the body. THE NECK The neck is more liable to vary than the back, with the habit of life of the animal. And although mammals almost always preserve the same number of seven bones in the neck, the bones vary in length between the short condition of the porpoise, in which the neck is almost lost, and the long bones which form the neck of the Llama, though even these may be exceeded by some fossil reptiles like Tanystrophoeus. In many mammals the neck bones do not differ in length or size from those of the back. In others, like the Horse and Ox, they are much broader and larger. There is the same sort of variation in the bones of the neck among birds, some being slender like the Heron, others broad like the Swan. But there is also a singular variation in number of vertebral bones in a bird's neck. At fewest there are nine, which equals the exceptionally large number found among mammals in the neck of one of the Sloths. Usually birds have ten to fifteen cervical vertebræ, and in the Swan there are twenty-three. Most of the neck bones of birds are relatively long, and the length of the neck is often greater than the remainder of the vertebral column. Reptiles usually have short necks. The common Turtle has eight bones in the neck, ten in the back. The two regions are sharply defined by the dorsal shield. Their articular ends are sometimes cupped in front, in the neck, sometimes cupped behind, or convex at both ends, or even flattened, or the articulation may be made exceptionally by the neural arch alone. Nine is the largest number of neck bones in existing Lizards, and there are usually nine in Crocodiles; so that reptiles closely approach mammals in number of the neck bones. It is remarkable that the maximum number in a mammal and in living reptiles should coincide with the minimum number in birds. Therefore the number of cervical vertebræ as an attribute of Mammal, Bird, or Reptile, can only be important from its constancy.