Part 5 (2/2)
THE BACKBONE, OR VERTEBRAL COLUMN
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 cla.s.ses of vertebrate animals than either the head or limbs. It is divided into neck bones, the cervical vertebrae; back bones, the dorsal vertebrae; loin bones, the lumbar vertebrae; the sacrum, or sacral vertebrae, 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 vertebrae. 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 vertebrae, 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 s.h.i.+eld. 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 vertebrae 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 vertebrae. 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 vertebrae are longer, but the length visibly shortens in the sixth and seventh.
Sometimes the vertebrae 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 ma.s.sive. The vertebrae 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 vertebrae 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 vertebrae. 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 vertebrae 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 vertebrae have no cervical ribs.
[Ill.u.s.tration: FIG. 24 UNITED ATLAS AND AXIS OF ORNITHOCHEIRUS (Cambridge Greensand)]
The articular surfaces between the bodies of the vertebrae, in the neck, are transversely oval. The middle part of this articular joint is made by the body of the vertebra; its outer parts are in the neural arch. In front this surface is a hollow channel, often more depressed than in any other animals. The corresponding surface behind is convex, with a process on each side at its lower outer angles (Fig. 25). It is a modification of the cup-and-ball form of vertebral articulation, which at the present day is eminently reptilian. Serpents and Crocodiles have the articulations similarly vertical, but in both the form of the articulation is a circle. In Lizards the articular cup is usually rather wider than deep, when the cup and ball are developed in the vertebrae; it differs from the vertical condition in pterodactyles in being oblique and much narrower from side to side. Only among Crocodiles and Hatteria is there a double articulation for the cervical rib, though in neither order have rib or vertebra in the neck the bird-like proportions which are usual in these animals. Pterodactyles show no resemblance to birds in this vertebral articulation. A Bird has the corresponding surface concave from side to side in front, but it is also convex from above downward, producing what is known as the saddle-shaped form which is peculiarly avian, being found in existing birds except in part of the back in Penguins. It is faintly approximated to in one or two neck vertebrae in man. Professor Williston remarks that in the toothless Pterodactyles of Kansas the hinder ball of the vertebral articulation is continued downward and outward as a concave articulation upon the processes at its outer corners. There are no mammals with a cup-and-ball articulation between the vertebrae, so that for what it is worth the character now described in Ornithosaurs is reptilian, when judged by comparison with existing animals.
[Ill.u.s.tration: FIG. 25. CERVICAL VERTEBRA OF ORNITHOCHEIRUS From the Cambridge Greensand]
Low down on each side of the vertebra, at the junction of its body with the neural arch, is a large ovate foramen, transversely elongated, and often a little impressed at the border, which is the entrance of the air cell into the bone. These foramina are often one-third of the length of the neck vertebrae in specimens from the Cambridge Greensand, where the neck bones vary from three-quarters of an inch to about two and a half inches in length, and in extreme forms are as wide as long. The width of the inters.p.a.ce between the foramina is one-half the width of the vertebrae, though this character varies with different genera and species. Several species from the Solenhofen Slate have the neck long and slender, on the type of the Flamingo. In others the neck is thick and short--in the _Scaphognathus cra.s.sirostris_ and _Pterodactylus spectabilis_. Some genera with slender necks have the bones preserved with a curved contour, such as might suggest a neck carried like that of a Llama or a Camel. The neck is occasionally preserved in a curve like a capital #S#, as though about to be darted forward like that of a bird in the act of striking its prey. The genera of Pterodactyles with short necks may have had as great mobility of neck as is found among birds named Ducks and Divers; but those Pterodactyles with stout necks, such as Dimorphodon and Ornithocheirus, in which the vertebrae are large, appear to have been built more for strength than activity, and the neck bones have been chiefly concerned in the muscular effort to use the fighting power of the jaws in the best way.
THE BACK
The region of the back in a Pterodactyle is short as compared with the neck, and relatively is never longer than the corresponding region in a bird. The shortness results partly from the short length of the vertebrae, each of which is about as long as wide. There is also a moderate number of bones in the back. In most skeletons from Solenhofen these vertebrae between the neck and girdle of hip bones number from twelve to sixteen. They have a general resemblance in form to the dorsal vertebrae in birds. The greatest number of such vertebrae in birds is eleven. The number is small because some of the later vertebrae in birds are overlapped by the bones of the hip girdle, which extend forward and cover them at the sides, so that they become blended with the sacrum.
This region of the skeleton in the Dimorphodon from the Lias is remarkable for the length of the median process, named the neural spine, which is prolonged upward like the spines of the early dorsal vertebrae of Horses, Deer, and other mammals. In this character they differ from living reptiles, and parallel some Dinosaurs from the Weald. The bones of the back in Ornithocheirus from the Cambridge Greensand show the under side to be well rounded, so that the articular surfaces between the vertebrae, though still rather wider than deep, are much less depressed than in the region of the neck. The neural ca.n.a.l for the spinal cord has become larger and higher, and the sides of the bone are somewhat compressed. Strong transverse processes for the support of the ribs are elevated above the level of the neural ca.n.a.l, at the sides of vertebrae compressed on the under sides, and directed outward. Between these lateral horizontal platforms is the compressed median neural spine, which varies in vertical height. The articulation of the ribs is not seen clearly. Isolated ribs from the Stonesfield Slate have double-headed dorsal ribs, like those of birds. In some specimens from the Solenhofen Slate like the Scaphognathus, in the University Museum at Bonn, dorsal ribs appear to be attached by a notch in the transverse process of the dorsal vertebra, which resembles the condition in Crocodiles. Variations in the mode of attachment of ribs among mammals may show that character to be of subordinate importance. Von Meyer has described the first pair of ribs as frequently larger than the others, and there appear in Rhamphorhynchus to be examples preserved of the sternal ribs, which connect the dorsal ribs with the sternum. Six pairs have been counted. A more interesting feature in the ribs consists in the presence behind the sternum, which is shorter than the corresponding bone in most birds, of median sternal ribs. They are slender #V#-shaped bones in the middle line of the abdomen, which overlapped the ends of the dorsal ribs like the similar sternal bones of reptiles. Such structures are unknown among Birds and Mammals. There is no trace in the dorsal ribs of the claw-like process, which extends laterally from rib to rib as a marked feature in many birds. Its presence or absence may not be important, because it is represented by fibro-cartilage in the ribs of crocodiles, and may be a small cartilage near the head of the rib in serpents, and is only ossified in some ribs of the New Zealand reptile Hatteria. So that it might have been present in a fossil animal without being ossified and preserved. Although the structure is a.s.sociated with birds, it is possibly also represented by the great bony plates which cover the ribs in Chelonians, and combine to form the s.h.i.+eld which covers the turtle's back. The structure is as characteristic of reptiles as of birds, but is not necessarily a.s.sociated with either.
[Ill.u.s.tration: FIG. 26
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