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 classes 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 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 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
massive. 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.
[Illustration: 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.
[Illustration: 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 interspace 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 crassirostris_ 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 canal 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 canal, 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
associated with birds, it is possibly also represented by the great bony
plates which cover the ribs in Chelonians, and combine to form the
shield which covers the turtle's back. The structure is as
characteristic of reptiles as of birds, but is not necessarily
associated with either.
[Illustration: FIG. 26
The upper figures show the side and back of a dorsal vertebra of
Ornithocheirus compared with corresponding views of the side and
back of a dorsal vertebra of a Crocodile]
There are two remarkable modifications of the early dorsal vertebrae in
some of the Cretaceous Pterodactyles. First, in the genus Ornithodesmus
from the Weald the early dorsal vertebrae are blended together into a
continuous mass, like that which is found in the corresponding region of
the living Frigate-bird, only more consolidated, and similar to that
consolidated structure found behind the dorsal vertebrae, known as the
sacrum, made by the blending of the vertebrae into a solid mass which
supports the hip bones. Secondly, in some of the Cretaceous genera of
Pterodactyles of Europe and America the vertebrae in the front part of
the back are similarly blended, but their union is less complete; and in
genera Ornithocheirus and Ornithostoma--the former chiefly English, the
latter chiefly American--the sides of the neural spines are flattened to
form an oval articular surface on each side, which gives attachment to
the flattened ends of their shoulder-blade bones named the scapulae. This
condition is found in no other animals. Three vertebrae appear to have
their neural arches thus united together. The structure so formed may be
named the notarium to distinguish it from the sacrum.
SACRUM
For some mysterious reason the part of the backbone which lies between
the bones of the hips and supports them is termed the sacrum. Among
living reptiles the number of vertebrae in this region is usually two, as
in lizards and crocodiles. There are other groups of fossil reptiles in
which the number of sacral vertebrae is in some cases less and in other
cases more. There is, perhaps, no group in which the sacrum makes a
nearer approach to that of birds than is found among these
Pterodactyles, although there are more sacral vertebrae in some
Dinosaurs. In birds the sacral vertebrae number from five to twenty-two.
In bats the number is from five to six. In some Solenhofen species, such
as _Pterodactylus dubius_ and _P. Kochi_ and _P. grandipelvis_, the
number is usually five or six. The vertebrae are completely blended. The
pneumatic foramina in the sacrum, so far as they have been observed, are
on the under sides of the transverse processes; while in the
corresponding notarial structure in the shoulder girdle the foramina are
in front of the transverse processes. Almost any placental mammal in
which the vertebrae of the sacral region are anchylosed together has a
similar sacrum, which differs from that of birds in the more complete
individuality of the constituent bones remaining evident. The transverse
processes in front of the sacrum are wider than in its hinder part; so
that the pelvic bones which are attached to it converge as they extend
backward, as among mammals. The bodies of the vertebrae forming the
sacrum are similar in length to those of the back. Each transverse
process is given off opposite the body of its own vertebra, but from a
lower lateral position than in the region of the back, in which the
vertebrae are free.
[Illustration: FIG. 27. SACRUM OF RHAMPHORHYNCHUS
Showing the complete blending of the vertebrae and ribs as in a bird,
with the well-defined Iliac bones, produced chiefly in front of the
acetabulum for the head of the femur.]
The hip bones are closely united with the sacrum by bony union, and
rarely appear to come away from the sacral vertebrae, as among mammals
and reptiles, though this happens with the Lias Pterodactyles. In the
Stonesfield Slate and Solenhofen Slate the slender transverse processes
from the vertebrae blend with the ilium of the hip girdle, and form a
series of transverse foramina on each side of the bodies of the
vertebrae. In the Cambridge Greensand genera the part of the ilium above
the acetabulum for the articular head of the femur appears to be always
broken away, so that the relation of the sacrum to the pelvis has not
been observed. This character is no mark of affinity, but only shows
that ossification obliterated sutures among these animals in the same
way as among birds.
The great difference between the sacrum of a Pterodactyle and that of a
bird has been rendered intelligible by the excellent discussion of the
sacral region in birds made by Professor Huxley. He showed that it is
only the middle part of the sacrum of a chicken which corresponds to the
true sacrum of a reptile, and comprises the five shortest of the
vertebrae; while the four in front correspond to those of the lower part
of the back, which either bear no ribs or very short ribs, and are known
as the lumbar region in mammals, so that the lower part of the back
becomes blended with the sacrum, and thus reduces the number of dorsal
vertebrae. Similarly the five vertebrae which follow the true sacral
vertebrae are originally part of the tail, and have been blended with the
other vertebrae in front, in consequence of the extension along them of
the bird's hip bones. This interpretation helps to account for the great
length of the sacrum in many birds, and also explains in part the
singular shortness of the tail in existing birds. The Ornithosaur sacrum
has neither the lumbar nor the caudal portions of the sacrum of a bird.
THE TAIL
The tail is perhaps the least important part of the skeleton, since it
varies in character and length in different genera. The short tails seen
in typical pterodactyles include as few as ten vertebrae in
_Pterodactylus grandipelvis_ and _P. Kochi_, and as many as fifteen
vertebrae in _Pterodactylus longirostris_. The tails are more like those
of mammals than existing birds, in which there are usually from six to
ten vertebrae terminating in the ploughshare bone. But just as some
fossil birds, like the Archaeopteryx, have about twenty long and slender
vertebrae in the tail, so in the pterodactyle Rhamphorhynchus this region
becomes greatly extended, and includes from thirty-eight to forty
vertebrae. In Dimorphodon the tail vertebrae are slightly fewer. The
earliest are very short, and then they become elongated to two or three
times the length of the early tail vertebrae, and finally shorten again
towards the extremity of the tail, where the bones are very slender. In
all long-tailed Ornithosaurians the vertebrae are supported and bordered
by slender ossified ligaments, which extend like threads down the tail,
just as they do in Rats and many other mammals and in some lizards.
Professor Marsh was able to show that the extremity of the tail in
Rhamphorhynchus sometimes expands into a strong terminal caudal membrane
of four-sided somewhat rhomboidal shape. He regards this membrane as
having been placed vertically. It is supported by delicate processes
which represent the neural spines of the vertebrae prolonged upward. They
are about fifteen in number. A corresponding series of spines on the
lower border, termed chevron bones, equally long, were given off from
the junctions of the vertebrae on their under sides, and produced
downward. This vertical appendage is of some interest because its
expansion is like the tail of a fish. It suggests the possibility of
having been used in a similar way to the caudal fin as an organ for
locomotion in water, though it is possible that it may have also formed
an organ used in flight for steering in the air.
[Illustration: FIG. 28. EXTREMITY OF THE TAIL OF _RHAMPHORHYNCHUS
PHYLLURUS_ (MARSH)
Showing the processes on the upper and under sides of the vertebrae
which make the terminal leaf-like expansion]
The tail vertebrae from the Cambridge Greensand are mostly found isolated
or with not more than four joints in association. They are very like the
slender type of neck vertebrae seen in long-necked pterodactyles, but are
depressed, and though somewhat wider are not unlike the tail vertebrae of
the Rhamphorhynchus. The pneumatic foramen in them is a mere puncture.
They have no transverse processes or neural spines, nor indications of
ribs, or chevron bones.
The hindermost specimens of tail vertebrae observed have the neural arch
preserved to the end, as among reptiles; whereas in mammals this arch
becomes lost towards the end of the tail. The processes by which the
vertebrae are yoked together are small. There is nothing to suggest that
the tail was long, except the circumstance that the slender caudal
vertebrae are almost as long as the stout cervical vertebrae in the same
animal. No small caudal vertebrae have ever been found in the Cambridge
Greensand. The tail is very short, according to Professor Williston, in
the toothless Ornithostoma in the Chalk of Kansas.