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 Tanystrophœus. 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.
German naturalists affirm on clear evidence that the Solenhofen Pterodactyles have seven cervical vertebræ. In many specimens there can be no doubt about the number, because the neck bones are easily distinguished from those of the back by their size; but the number is not always easy to count.
As in Birds, the first vertebra, or atlas, in Pterodactyles is extremely short, and is generally—if not always—blended with the much longer second vertebra, named the axis. The front of the atlas forms a small rounded cup to articulate with the rounded ball of the basioccipital bone at the back of the skull. The third and fourth vertebræ are longer, but the length visibly shortens in the sixth and seventh.
Sometimes the vertebræ are slender and devoid of strong spinous processes. This is the condition in the little Pterodactylus longirostris and in the comparatively large Cycnorhamphus Fraasii, in which there is a slight median ridge along the upper surface of the arch of the vertebra. This condition is paralleled in birds with long necks, especially wading birds such as the Heron. Other Ornithosaurs, such as Ornithocheirus from the Cretaceous rocks, have the neck much more massive. The vertebræ are flattened on the under side. The arch above the nervous matter of the spinal cord has a more or less considerable transverse expansion, and may even be as wide as long. These vertebræ have proportions and form such as may be seen in Vultures or in the Swan. In either case the form of the neck bones is more or less bird-like, and the neural spine may be elevated, especially in Pterodactyles with long tails.
One of the most distinctive features of the neck bones of a bird is the way in which the cervical ribs are blended with the vertebræ. They are small, and each is often prolonged in a needle-like rod at the side of the neck bone.
In Ornithocheirus the cervical rib similarly blends with the vertebra by two articulations, as in mammals, so that it might escape notice but for the channel of a blood vessel which is thus inclosed. In several of the older Pterodactyles from Solenhofen the ribs of the neck vertebræ remain separated, as in a Crocodile, though still bird-like in their form, anterior position, and mode of attachment. In Terrapins and Tortoises the long neck vertebræ have no cervical ribs.