INTERPRETATION OF PTERODACTYLES BY THEIR SOFT PARTS
THE ORGANS WHICH FIX AN ANIMAL'S PLACE IN NATURE
We shall endeavour to ascertain what marks of its grade of organisation
the Pterodactyle has to show. The organs which are capable of modifying
the bones are probably limited to the kidneys, the brain, and the organs
of respiration. It may be sufficient to examine the latter two.
PNEUMATIC FORAMINA IN PTERODACTYLES
[Illustration: FIG. 15. HEAD OF THE HUMERUS OF THE PTERODACTYLE
ORNITHOCHEIRUS
Showing position of the pneumatic foramen on the ulnar side of the
bone as in a bird]
Hermann von Meyer, the historian of the Ornithosaurs of the Lithographic
Slate, as early as 1837 described some Pterodactyle bones from the Lias
of Franconia, which showed that air was admitted into the interior of
the bones by apertures near their extremities, which, from this
circumstance, are known as pneumatic foramina. He drew the inference,
naturally enough, that such a structure is absolute proof that the
Pterodactyle was a flying animal. It was not quite the right form in
which the conclusion should have been stated, because the Ostrich and
other birds which do not fly have the principal bones pneumatic.
Afterwards, in 1859, the larger bones which Professor Sedgwick, of
Cambridge, transmitted to Sir Richard Owen established this condition as
characteristic of the Flying Reptiles of the Cambridge Greensand. It was
thus found as a distinctive structure of the bones both at the beginning
and the close of the geological history of these animals. Von Meyer
remarks that the supposition readily follows that in the respiratory
process there was some similarity between Pterodactyles and Birds. This
cautious statement may perhaps be due to the circumstance that in many
animals air cavities are developed in the skull without being connected
with organs of respiration. It is well known that the bulk of the
Elephant's head is due to the brain cavity being protected with an
envelope formed of large air cells. Small air cells are seen in the
skulls of oxen, pigs, and many other mammals, as well as in the human
forehead. The head of a bird like the Owl owes something of its imposing
appearance to the way in which its mass is enlarged by the dense
covering of air cells in the bones above the brain, like that seen in
some Cretaceous Pterodactyles. Nor are the skulls of Crocodiles or
Tortoises exceptions to the general rule that an animal's head bones may
be pneumatic without implying a pneumatic prolongation of air from the
lungs. The mere presence of air cells without specification of the
region of the skeleton in which they occur is not remarkable. The holes
by which air enters the bones are usually much larger in Pterodactyles
than in Birds, but the entrance to the air cell prolonged into the bones
is the same in form and position in both groups. So far as can be judged
by this character, there is no difference between them. The importance
of the comparison can only be appreciated by examining the bones side by
side. In the upper arm bone of a bird, on what is known as the ulnar
border, near to the shoulder joint, and on the side nearest to it, is
the entrance to the air cell in the humerus. In the Pterodactyle the
corresponding foramen has the same position, form, and size, and is not
one large hole, but a reticulation of small perforations, one beyond
another, exactly such as are seen in the entrance to the air cell in the
bone of a bird, in which the pneumatic character is found. For it is not
every bird of flight which has this pneumatic condition of the bones;
and Dr. Crisp stated that quite a number of birds--the Swallow, Martin,
Snipe, Canary, Wood-wren and Willow-wren, Whinchat, Glossy-starling,
Spotted-fly-catcher, and Black-headed Bunting--have no air in their
bones. And it is well known that in many birds, especially water birds,
it is only the upper bones of the limbs which are pneumatic, while the
smaller bones retain the marrow.
LUNGS AND AIR CELLS
[Illustration: FIG. 16. LUNGS OF THE BIRD APTERYX PARTLY OPENED ON
THE RIGHT-HAND SIDE
The circles are openings of the bronchial tubes on the surface of the
lung. The notches on the inner edges of the lungs are impressions of
the ribs (After R. Owen)]
[Illustration: FIG. 17. THE BODY OF AN OSTRICH LAID OPEN TO SHOW THE
AIR CELLS WHICH EXTEND THROUGH ITS LENGTH (After Georges Roche)]
It may be well to remember that the lungs of a bird are differently
conditioned from those of any other animal. Instead of hanging freely
suspended in the cone-shaped chamber of the thorax formed by the ribs
and sternum, they are firmly fixed on each side, so that the ribs deeply
indent them and hold them in place. The lungs have the usual internal
structure, being made up of branching cells. The chief peculiarity
consists in the way in which the air passes not only into them, but
through them. The air tube of the throat of a bird, unlike that of a
man, has the organ of voice, not at the upper end in the form of a
larynx, but at the lower end, forming what is termed a syrinx. There is
no evidence of this in a fossil state, although in a few birds the rings
of the trachaea become ossified, and are preserved. But below the syrinx
the trachaea divides into two bronchi, tubes which carry the ringed
character into the lungs for some distance, and these give off branches
termed bronchial tubes, the finer subdivisions from which, in their
clustered minute branching sacs, make up the substance of the lung.
There is nothing exceptional in that. But towards the outer or middle
part of the ventral or under surface of the lungs, four or five rounded
openings are seen on each side. Each of these openings resembles the
entrance of the air cell into a bone, since it displays several smaller
openings which lead to it. Each opening from the lung leads to an air
cell. Those cells may be regarded as the blowing out of the membrane
which covers the lungs into a film which holds air like a mass of soap
bubbles, until the whole cavity of the body of a bird from neck to tail
is occupied by sacculated air cells, commonly ten in number, five on
each side, though two frequently blend at the base of the neck in the
region of the #V#-shaped bone named the clavicle or furculum, popularly
known as the merry-thought. Most people have seen some at least of
these semi-transparent bladder-like air cells beneath the skin in the
abdominal region of a fowl. The cells have names from their positions,
and on each side one is abdominal, two are thoracic, one clavicular, and
one cervical, which last is at the base of the neck. The clavicular and
abdominal air cells are perhaps the most interesting. The air cell
termed clavicular sends a process outward towards the arm, along with
the blood vessels which supply the arm. Thus this air cell, entering the
region of the axilla or arm-pit, enters the upper arm bone usually on
its under side, close to the articular head of the humerus, and in the
same way the air may pass from bone to bone through every bone in the
fore limb. The hind limbs similarly receive air from the abdominal air
cell, which supplies the femur and other bones of the leg, the sacrum,
and the tail. But the joints of the backbone in front of the sacrum
receive their air from the cervical air sac. The air cells are not
limited to the bones, but ramify through the body, and in some cases
extend among the muscles. A bird may be said to breathe not only with
its lungs, but with its whole body. And it is even affirmed that
respiration has been carried on through a broken arm bone when the
throat was closed, and the bird under water.
Birds differ greatly in the extent to which the aircell system prolonged
from the lungs is developed, some having the air absent from every bone,
while others, like the Swift, are reputed to have air in every bone of
the body.
Comparison shows that in so far as the bones are the same in Bird and
Ornithosaur, the evidence of the air cells entering them extends to
resemblance, if not coincidence, in every detail. No living group of
animals except birds has pneumatic limb bones, in relation to the lungs;
so that it is reasonable to conclude that the identical structures in
the bones were due to the same cause in both the living and extinct
groups of animals. It is impossible to say that the lungs were identical
in Birds and Pterodactyles, but so far as evidence goes, there is no
ground for supposing them to have been different.
THE LUNGS OF REPTILES
[Illustration: FIG. 18. THE SIDE OF THE BODY OF A CHAMELEON
Ribs removed to show the sacculate branched form of the lung]
There is nothing comparable to birds, either in the lungs of living
reptiles or in their relation to the bones. The Chameleon is remarkable
in that the lung is not a simple bladder prolonged through the whole
length of the body cavity, as in a serpent, but it develops a number of
large lateral branches visible when the body is laid open. Except near
the trachaea, where the tissue has the usual density of a lizard lung,
the air cell is scarcely more complicated than the air bladder of a
fish, and does not enter into any bone of the skeleton. And although
many fishes like the Loach have the swim bladder surrounded by bone
connected with the head, it offers no analogy to the pneumatic condition
of the bones in the Pterodactyle.
THE FORM OF THE BRAIN CAVITY
[Illustration: FIG. 19. THE FORM OF THE BRAIN]
But the identity of the pneumatic foramina in Birds and Flying Reptiles
is not a character which stands by itself as evidence of organisation,
for a mould of the form of the brain case contributes evidence of
another structural condition which throws some light on the nature of
Ornithosaurs. Among many of the lower animals, such as turtles, the
brain does not fill the chamber in the dry skull, in which the same
bones are found as are moulded upon the brain in higher animals. For the
brain case in such reptiles is commonly an envelope of cartilage, as
among certain fishes; and except among serpents, the Ophidia, the bones
do not completely close the reptilian brain case in front. The brain
fills the brain case completely among birds. A mould from its interior
is almost as definite in displaying the several parts of which it is
formed as the actual brain would be. And the chief regions of the brain
in a bird--cerebrum, optic lobes, cerebellum--show singularly little
variation in proportion or position. The essential fact in a bird's
brain, which separates it absolutely from all other animals, is that the
pair of nerve masses known as the optic lobes are thrust out at the
sides, so that the large cerebral hemispheres extend partly over them as
they extend between them to abut against the cerebellum. This remarkable
condition has no parallel among other vertebrate animals. In Fishes,
Amphibians, Reptiles, and Mammals the linear succession of the several
parts of the brain is never departed from; and any appearance of
variation from it among mammals is more apparent than real, for the
linear succession may be seen in the young calf till the cerebral
hemispheres grow upward and lop backward, so as to hide the relatively
small brain masses which correspond to the optic lobes of reptiles,
extending over these corpora-quadrigemina, as they are named, so as to
cover more or less of the mass of the cerebellum. From these conditions
of the brain and skull, it would not be possible to mistake a mould
from the brain case of a bird for that of a reptile, though in some
conditions of preservation it is conceivable that the mould of the brain
of a bird might be distinguished with difficulty from that of the brain
in the lowest mammals. Taken by itself, the avian form of brain in an
animal would be as good evidence that its grade of organisation was that
of a bird as could be offered.
THE BRAIN IN SOLENHOFEN PTERODACTYLES
It happens that moulds of the brain of Pterodactyles, more or less
complete, are met with of all geological ages--Liassic, Oolitic, and
Cretaceous. The Solenhofen Slate is the only deposit in Europe in which
Pterodactyle skulls can be said to be fairly numerous. They commonly
have the bones so thin as to show the form of the upper surface of the
mould of the brain, or the bones have scaled off the mould, or remain in
the counterpart slab of stone, so as to lay bare the shape of the brain
mass.
In the Museum at Heidelberg a skull of this kind is seen in the
long-tailed genus of Pterodactyles named Rhamphorhynchus. It shows the
large rounded cerebral hemispheres, which extend in front of cerebral
masses of smaller size a little below them in position, which perhaps
are as like the brain of a monotreme mammal as a bird.
The short-tailed Pterodactylus described by Cuvier has the cerebral
hemispheres very similar to those of a bird, but the relations of the
hinder parts of the brain to each other are less clear.
The first specimen to show the back of the brain was found by Mr. John
Francis Walker, M.A., in the Cambridge Greensand. I was able to remove
the thick covering of cellular bone which originally extended above it,
and thus expose evidence that in the mutual relations of the fore and
hind parts of the brain bird and ornithosaur were practically identical.
Another Cambridge Greensand skull showed that in the genus
Ornithocheirus the optic lobes of the brain are developed laterally, as
in birds. That skull was isolated and imperfect. But about the same time
the late Rev. W. Fox, of Brixton, in the Isle of Wight, obtained from
Wealden beds another skull, with jaws, teeth, and the principal bones of
the skeleton, which showed that the Wealden Pterodactyle Ornithodesmus
had a similar and bird-like brain. In 1888 Mr. E. T. Newton, F.R.S.,
obtained a skull from the Upper Lias, uncrushed and free from
distortion. This made known the natural mould of the brain, which shows
the cerebral hemispheres, optic lobes, and cerebellum more distinctly
than in the specimens previously known. In some respects it recalls the
Heidelberg brain of Rhamphorhynchus in the apparently transverse
subdivision of the optic lobes, but it is unmistakably bird-like, and
quite unlike any reptile.
IMPORTANCE OF THE BRAIN AND BREATHING ORGANS
So far as the evidence goes, it appears that these fossil flying animals
show no substantial differences from birds, either in the mould of the
brain or the impress of the breathing organs upon the bones. These
approximations to birds of the nervous and respiratory systems, which
are beyond question two of the most important of the vital organs of an
animal, and distinctive beyond all others of birds, place the
naturalist in a singular dilemma. He must elect whether he will trust
his interpretation to the soft organs, which among existing animals
never vary their type in the great classes of vertebrate animals, and on
which the animal is defined as something distinct from its envelope the
skeleton and its appendages the limbs, or whether he will ignore them.
The answer must choose substantially between belief that the existing
order of Nature gives warrant for believing that these vital
characteristics which have been discussed might equally coexist with the
skeleton of a mammal or a reptile, as with that of a bird, for which
there is no particle of evidence in existing life. Or, as an
alternative, the fact must be accepted that birds only have such vital
organs as are here found, and therefore the skeleton, that may be
associated with them, cannot affect the reference of the type to the
same division of the animal kingdom as birds. The decision need not be
made without further consideration. But brain and breathing organs of
the avian type are structures of a different order of stability in most
animals from the bones, which vary to a remarkable extent in almost
every ordinal group of animals.
TEMPERATURE OF THE BLOOD
The organs of circulation and digestion are necessarily unknown. There
are reasons why the blood may have been hot, such as the evidences from
the wings of exceptional activity; though the temperature depends more
upon the amount of blood in the body than upon the apparatus by which it
is distributed. We speak of a Crocodile as cold-blooded, yet it is an
animal with a four-chambered heart not incomparable with that of a
bird. On the other hand, the Tunny, a sort of giant Mackerel, is a fish
with a three-chambered heart, only breathing the air dissolved in water,
which has blood as warm as a mammal, its temperature being compared to
that of a pig. Several fishes have blood as warm as that of Manis, the
scaly ant-eater; and many birds have hotter blood than mammals. The term
"hot-blooded," as distinct from "cold-blooded," applied to animals, is
relative to the arbitrary human standard of experience, and expresses no
more than the circumstance that mammals and birds are warmer animals
than reptiles and fishes.
The exceptional temperature of the Flying Fish has led to a vague
impression that physical activity and its effect upon the amount of
blood which vigour of movement circulates, are more important in raising
an animal's temperature than possession of the circulatory organs
commonly associated with hot blood, which drive the blood in distinct
courses through the body and breathing organs. Yet the kind of heart
which is always associated with vital structures such as Pterodactyles
are inferred to have possessed from the brain mould and the pneumatic
foramina in the bones, is the four-chambered heart of the bird and the
mammal. Considering these organs alone--of which the fossil bones yield
evidence--we might anticipate, by the law of known association of
structures, that nothing distinctly reptilian existed in the other soft
part of the vital organisation, because there is no evidence in favour
of or against such a possibility.