SHOULDER-GIRDLE AND FORE LIMB
STERNUM
The sternum is always a distinguishing part of the bony structure of the
breast. In Crocodiles it is a cartilage to which the sternal ribs unite;
and upon its front portion a flat knife-like bone called the
interclavicle is placed. In lizards like the Chameleon, it is a
lozenge-shaped structure of thin bony texture, also bearing a long
interclavicle, which supports the clavicular bones, named collar bones
in man, which extend outward to the shoulder blades. Among mammals the
sternum is usually narrow and flat, and often consists of many
successive pieces in the middle line, on the under side of the body.
Among Bats the anterior part is somewhat widened from side to side, to
give attachment to the collar bones, but the sternum still remains a
narrow bone, much narrower than in Dolphins, and not differing in
character from many other Mammals, notwithstanding the Bat's power of
flight. The bone develops a median keel for the attachment of the
muscles of the breast, but something similar is seen in burrowing
Insectivorous mammals like the Moles. So that, as Von Meyer remarked,
the presence of a keel on the sternum is not in itself sufficient
evidence to prove flight.
Among birds the sternum is greatly developed. Broad and short in the
Ostrich tribe, it is devoid of a keel; and therefore the keel, if
present in a bird, is suggestive of flight. The keel is differently
developed according to the mode of attachment of the several pectoral
muscles which cover a bird's breast. In several water birds the keel is
strongly developed in front, and dies away towards the hinder part of
the sternum, as in the Cormorant and its allies. The sternum in German
Pterodactyles is most nearly comparable to these birds.
[Illustration: FIG. 36. COMPARISON OF THE STERNUM]
In the Solenhofen Slate the sternum is fairly well preserved in many
Ornithosaurs. It is relatively shorter than in birds, and is broader
than long; but not very like the sternum of reptile or mammal in form.
The keel is limited to the anterior part of the shield of the sternum,
as in Merganser and the Cormorant, and is prolonged forward for some
distance in advance of it. Von Meyer noticed the resemblance of this
anterior process to the interclavicle of the Crocodile in position; but
it is more like the keel of a bird's sternum, and is not a separate bone
as in Reptiles. In Pterodactyles from the Cretaceous rocks, the side
bones, called coracoids, are articulated to saddle-shaped surfaces at
the hinder part of the base of this keel, which are parallel in
Ornithocheirus, as in most birds, but overlap in Ornithodesmus, as in
Herons and wading birds.
[Illustration: FIG. 37. STERNUM IN ORNITHOCHEIRUS FROM THE CAMBRIDGE
GREENSAND
Showing the strong keel and the facets for the coracoid bones on its
hinder border above the lateral constrictions]
The keel was pneumatic, and when broken is seen to be hollow, and
appears to have been exceptionally high in Rhamphorhynchus, a genus in
which the wing bones are greatly elongated. Von Meyer found in
Rhamphorhynchus on each side of the sternum a separate lateral plate
with six pairs of sternal ribs, which unite the sternum with the dorsal
ribs, as in the young of some birds. The hinder surface of the sternum
is imperfectly preserved in the toothless Pterodactyles of Kansas.
Professor Williston states that the bone is extremely thin and
pentagonal in outline, projecting in front of the coracoids, in a stout,
blunt, keel-like process, similar to that seen in the Pterodactyles of
the Cambridge Greensand. American specimens have not the same notch
behind the articulation for the coracoid to separate it from the
transverse lateral expansion of the sternal shield. The lateral margin
in the Cambridge Greensand specimens figured by Professor Owen and
myself is broken; but Professor Williston had the good fortune to find
on the margin of the sternum the articular surfaces which gave
attachment to the sternal ribs. The margin of the sternal bone thickens
at these facets, four of which are preserved. The sternum in
Ornithostoma was about four and a half inches long by less than five and
a half inches wide. The median keel extends forward for rather less than
two inches, while in the smaller Cambridge species of Ornithocheirus it
extends forward for less than an inch and a half.
A sternum of this kind is unlike that of any other animal, but has most
in common with a bird; and may be regarded as indicating considerable
power of flight. The bone cannot be entirely attributed to the effect of
flight, since there is no such expanded sternal shield in Bats. The
small number of sternal ribs is even more characteristic of birds than
mammals or reptiles.
THE SHOULDER-GIRDLE
The bones which support the fore limb are one of the distinctive regions
of the skeleton defining the animal's place in nature. Among most of the
lower vertebrata, such as Amphibians and Reptiles, the girdle is a
double arch--the arch of the collar bone or clavicles in front, and the
arch of the shoulder-blade or scapula behind. The clavicular arch, when
it exists, is formed of three or five parts--a medium bar named the
interclavicle, external to which is a pair of bones called clavicles,
reaching to the front of the scapulae when they are present; and
occasionally there is a second pair of bones called supraclavicles,
extending from the clavicles up the front margins of the scapulae. Thus
the clavicular arch is placed in front of the scapular arch. The
supraclavicles are absent from all living Reptiles, and the clavicles
are absent from Crocodiles. The interclavicle is absent from all mammals
except Echidna and Ornithorhynchus. Clavicles also may be absent in some
orders of mammals. Hence the clavicular arch may be lost, though the
collar bones are retained in man.
The scapular arch also is more complicated and more important in the
lower than in the higher vertebrata. It may include three bones on each
side named coracoid, precoracoid, and scapula. But in most vertebrates
the coracoid and precoracoid appear never to have been segmented so as
to be separated from each other; and it is only among extinct types of
reptiles, which appear to approximate to the Monotreme mammals, that
separate precoracoid bones are found; though among most mammals,
probably, there are stages of early development in which precoracoids
are represented by small cartilages, though few mammals except Edentata
like the Sloths and Ant-eaters, retain even the coracoids as distinct
bones. Therefore, excepting the Edentata and the Monotremes, the
distinctive feature of the mammalian shoulder-girdle appears to be that
the limbs are supported by the shoulder-blades, termed the scapulae.
Among reptiles there are several distinct types of shoulder-girdle.
Chelonians possess a pair of bones termed coracoids which have no
connexion with a sternum; and their scapulae are formed of two widely
divergent bars, divided by a deeper notch than is found in any fossil
reptiles. Among Lizards both scapula and coracoid are widely expanded,
and the coracoid is always attached to the sternum. Chameleons have the
blade of the scapula long and slender, but the coracoid is always as
broad as it is long. Crocodiles have the bone more elongated, so that it
has somewhat the aspect of a very strong first sternal rib when seen on
the ventral face of the animal. The bone is perforated by a foramen,
which would probably lie in the line of separation from the precoracoid
if any such separation had ever taken place. The scapula, or
shoulder-blade, of Crocodiles is a similar flat bone, very much shorter
than the scapula of a Chameleon, and more like that of the New Zealand
Hatteria. Thus there is very little in common between the several
reptilian types of shoulder-girdle.
[Illustration: FIG. 38. COMPARISON OF SCAPULA AND CORACOID IN THREE
PTERODACTYLES AND A BIRD]
In birds the apparatus for the support of the wings has a far-off
resemblance to the Crocodilian type. The coracoid bones, instead of
being directed laterally outward and upward from the sternum, as among
Crocodiles, are directed forward, so as to prolong the line of the
breast bone, named the sternum. The bird's coracoid is sometimes
flattened towards the breast bone among Swans and other birds; yet as a
rule the coracoid is a slender bar, which combines with the still more
slender and delicate blade of the scapula, which rests on the ribs, to
make the articulation for the upper arm bone. Among reptiles the scapula
and coracoid are more or less in the same straight line, as in the
Ostrich, but in birds of flight they meet at an angle which is less than
a right angle, and where they come in contact the external surface is
thickened and excavated to make the articulation for the head of the
humerus. There is nothing like this shoulder-girdle outside the class of
birds, until it is compared with the corresponding structure in these
extinct animals called Pterodactyles. The resemblance between the two is
surprising. It is not merely the identity of form in the coracoid bone
and the scapula, but the similar angle at which they meet and the
similar position of the articulation for the humerus. Everything in the
Pterodactyle's shoulder-girdle is bird-like, except the absence of the
representative of the clavicles, that forked #V#-shaped bone of the bird
which in scientific language is known as the furculum, and is popularly
termed the "merry-thought." This kind of shoulder-girdle is found in the
genera from the Lias and the Oolitic rocks, both of this country and
Germany.
In the Cretaceous rocks the scapula presents, in most cases, a different
appearance. The coracoid is an elongated, somewhat triangular bone,
compressed on the outer margin as in birds, but differing alike from
birds and other Pterodactyles in not being prolonged forward beyond the
articulation for the humerus. In these Cretaceous genera, toothed and
toothless alike, the articulation for the upper arm bone truncates the
extremity of the coracoid, so that the bone is less like that of a bird
in this feature. Perhaps it shows a modification towards the crocodilian
direction. The scapula, which unites with the coracoid at about a right
angle, is similarly truncated by the articular surface for the humerus;
but the bone is somewhat expanded immediately beyond the articulation,
and compressed; and instead of being directed backward, it is directed
inward over the ribs to articulate with the neural arches of the early
dorsal vertebrae in the genera found in strata associated with the Chalk.
As the bone approaches this articulation, it thickens and widens a
little, becoming suddenly truncated by an ovate facet, which exactly
corresponds to the transversely ovate impression, concave from front to
back, which is seen in the neural arches of the dorsal vertebrae on which
it fits. This condition is not present in all Cretaceous Pterodactyles.
It does not occur in the Kansas fossil, named by Professor Marsh,
Nyctodactylus. And it appears to be absent from the Pterodactyles of the
English Weald, named Ornithodesmus.
[Illustration: FIG. 39. THE NOTARIUM
An ossification which gives attachment to the scapulae seen in the
early dorsal vertebra of Ornithocheirus
(From the Cambridge Greensand)]
[Illustration: FIG. 40. RESTORATION OF THE SHOULDER-GIRDLE IN THE
CRETACEOUS ORNITHOCHEIRUS
Showing how the scapulae articulate with a vertebra and the
articulation of the coracoids with the sternum. The humeral
articulation with the coracoid is unlike the condition shown
in other Ornithosaurs]
There is no approach to this transverse position of the scapulae among
birds. And while the form of the bones in the older genera of
Ornithosaurs is singularly bird-like, the angular arrangement in this
Cretaceous genus is obtained by closely approximating the articulations
on the sternum, so that the coracoids extend outward as in reptiles,
instead of forward as in birds; and the extremities of the scapulae
similarly approximate towards each other. This rather recalls the
relative positions of scapula and coracoid among crocodiles. If
crocodile and bird had been primitive types of animals instead of
surviving types, it might almost seem as though there had been a cunning
and harmonious blending of one with the other in evolving this form of
shoulder-girdle.
THE FORE LIMB
The bones of the fore limb, generally, correspond in length with the
similar parts of the hind limb. The upper arm bone corresponds with the
upper leg bone, and the fore-arm bone is as long as the fore-leg bone;
then differences begin. The bones which correspond to the back of the
hand in man, termed the metacarpus, are variable in length in
Pterodactyles--sometimes very long and sometimes short. The wing
metacarpal bone is always stout, and the others are slender. The
extremity of the metacarpus was applied to the ground. Three small
digits of the hand are developed from the three small metacarpal bones,
and terminate in large claws.
The great wing finger was bent backward, and only touched the ground
where it fitted upon the wing metacarpal bone. It appears sometimes to
have been as long as the entire vertebral column.
Owing to the circumstance that the joint in the arm in Pterodactyles was
not at the wrist as among birds, but between the metacarpus and the
phalanges, it follows that the fore limb was longer than the hind limb
when the metacarpus was long; but the difference would not interfere
with the movements of the animal, either upon four feet or on two feet,
for in bats and birds the disproportion in length is greater.
HUMERUS OR UPPER ARM BONE
The first bone in the fore-arm, the humerus, is remarkable chiefly for
the compressed crescent form of its upper articular end, which is never
rounded like the head of the upper arm bone in man, and secondly for the
great development of the external process of bone near that end, termed
the radial crest. Sir Richard Owen compared the bone to the humerus of
both birds and crocodiles, but in its upper articular end the crocodile
bone may be said to be more like a bird than it is like the
Pterodactyle. In flying reptiles the articular surface next to the
shoulder-girdle is somewhat saddle-shaped, being concave from side to
side above and convex vertically, while most animals with which it can
be compared have the articular head of the bone convex in both
directions. A remarkable exception to this general rule is found in some
fossil animals from South Africa, which, from resemblance to mammals in
their teeth, have been termed Theriodonts. They sometimes have the head
of the bone concave from side to side and convex in the vertical
direction. To this condition Ornithorhynchus makes a slight
approximation. The singular expansion of the structure called the radial
crest finds no close parallel in reptiles, though Crocodiles have a
moderate crest on the humerus in the same position; and in Theriodonts
the radial crest extends much further down the shaft of the humerus. No
bird has a radial crest of a similar kind, though it is prolonged some
way down the shaft in Archaeopteryx. In Pterodactyles it sometimes
terminates outward in a smooth, rounded surface, which might have been
articular if any structure could have articulated with it. There is also
a moderate expansion of the bone on the ulnar side in some
Pterodactyles, so that the proximal end often incloses nearly
three-fourths of an ovate outline. The termination of the radial crest
is at the opposite end of this oval to the wider articular part of the
head of the bone, in some specimens from the Cambridge Greensand. The
radial crest is more extended in Rhamphorhynchus. All specimens of the
humerus show a twist in the length of the bone, so that the end towards
the fore-arm, which is wider than the shaft, makes a right angle with
the radial crest on the proximal end, which is not seen in birds. The
shaft of the humerus is always stouter than that of the femur, though
different genera differ in this respect.
The humerus in genera from rocks associated with the Chalk presents two
modifications, chiefly seen in the characters of the distal end of the
bone. One of these is a stout bone with a curiously truncated end where
it joins the two bones of the fore-arm; and the other is more or less
remarkable for the rounded form of the distal condyles. Both types show
distinct articular surfaces. The inner one is somewhat oblique and
concave, the outer one rounded; the two being separated by a concave
channel, so that the ulna makes an oblique articulation with the bone as
in birds, and the radius articulates by a more or less truncated or
concave surface.
[Illustration: FIG. 41. COMPARISON OF THE HUMERUS IN PTERODACTYLE AND
BIRD]
ULNA AND RADIUS
[Illustration: FIG. 42. COMPARISON OF THE BONES OF THE FORE-ARM IN
BIRD AND ORNITHOSAUR]
The bones of the fore-arm are similar to each other in size, and if
there be any difference between them the ulna is slightly the larger.
There is some evidence that in Rhamphorhynchus the upper end of the ulna
was placed behind the radius, probably in consequence of the mode of
attachment of those bones to the humerus. The ulna abutted towards the
inner and lower border, while the radius was towards the upper border,
consequent upon the twist in the humerus. This condition corresponds
substantially with the arrangement in birds, but differs from birds in
the relatively more important part taken by the radius in making the
articulation. The bones are compared in Dimorphodon with the Golden
Eagle drawn of the same size (Fig. 42). In birds the ulna supports the
great feathers of the wing, and this may account for the size of the
bone. The ulna is best seen at its proximal end in the specimens from
the Cambridge Greensand, where there is a terminal olecranon
ossification forming an oblique articulation, which frequently comes
away and is lost. It is sometimes well preserved, and indicated by a
suture. The examples of ulna from the Lias show a slight expansion of
the bone at both ends, and at the distal end toward the wrist the
articulation is well defined, where the bone joins the carpus. The
larger specimens of the bone are broken. The distal articular surface is
only connected with the proximal end of the bone in small specimens: it
always shows on the one margin a concavity, followed by a prominent
boss, and an oblique articulation beyond the boss. On the side towards
the radius, on the lower end of the shaft there is an angular ridge,
which marks the line along which the ulna overlaps the radius. The
lower end of the radius has a simple, slightly convex articulation,
somewhat bean-shaped. No rotation of these bones on each other was
possible as in man. There is a third bone in the fore-arm. This bone,
named the pteroid, is commonly seen in skeletons from Solenhofen. It was
regarded by Von Meyer as having supported the wing membrane in flight.
Some writers have interpreted it as an essential part of the
Pterodactyle skeleton, and Von Meyer thought that it might possibly
indicate a fifth digit in the hand. The only existing structure at all
like it is seen in the South African insectivorous mammal named
_Chrysochloris capensis_, the golden mole, which also has three bones in
the fore-arm, the third bone extending half-way up towards the humerus.
In that animal the third bone appears to be behind the others and
adjacent to the ulna. In the German fossils the pteroid articulated with
a separate carpal or metacarpal bone, placed on the side of the arm
adjacent to the radius, and the radius is always more inward than the
ulna. If the view suggested by Von Meyer is adopted, this bone would be
a first digit extending outward and backward towards the humerus. That
view was adopted by Professor Marsh. It involves the interpretation of
what has been termed the lateral carpal as the first metacarpal bone,
which would be as short as that of a bird, but turned in the opposite
direction backward. The first digit would then only carry one phalange,
and would not terminate in a claw, but lie in the line of the tendon
which supports the anterior wing membrane of a bird.
The third bone in the fore-arm of Chrysochloris does not appear to
correspond to a digit. The bone is on the opposite side of the arm to
the similar bone of a Pterodactyle, and therefore cannot be the same
structure in the Golden Mole. The interpretation which makes the pteroid
bone the first digit has the merit of accounting for the fifth digit of
the hand. All the structures of the hand are consistent with this view.
The circumstance that the bone is rarely found in contact with the
radius, but diverging from it, shows that it plays the same part in
stretching the membrane in advance of the arm, that the fifth digit
holds in supporting the larger wing membrane behind the arm.
According to Professor Williston, the American toothless Pterodactyle
Ornithostoma has but a single phalange on the corresponding first toe of
the hind foot, and that bone he describes as long, cylindrical, gently
curved, and bluntly pointed. There is some support for this
interpretation; but I have not seen any English or German Pterodactyles
with only one phalange in the first toe.
The wing in Pterodactyles would thus be stretched between two fingers
which are bent backward, the three intermediate digits terminating in
claws.
THE CARPUS
The wrist bones in the reptilia usually consist of two rows. In
Crocodiles, in the upper row there is a large inner and a small outer
bone, behind which is a lunate bone, the remainder of the carpus being
cartilaginous. Only one carpal is converted into bone in the lower row.
It is placed immediately under the smaller upper carpal. In Chelonians,
the turtle and tortoise group, the characters of the carpus vary with
the family. In the upper row there are usually two short carpals, which
may be blended, under the ulna; while the two under the radius are
commonly united. The lower row is made up of several small bones.
Lizards, too, usually have three bones in the proximal row and five
smaller bones in the distal row.
The correspondence of the distal carpals with the several metacarpal
bones of the middle hand is a well-known feature of the structure of the
wrist.
Von Meyer remarks that the carpus is made up of two rows of small bones
in the Solenhofen Pterodactyles; while in birds there is one row
consisting of two bones. The structure of the carpus is not distinct in
all German specimens; but in the short-tailed Solenhofen genera the
bones in the two rows retain their individuality.
In all the Cretaceous genera the carpal bones of each row are blended
into a single bone, so that two bones are superimposed, which may be
termed the proximal and distal carpals. One specimen shows by an
indication of sutures the original division of the distal carpal into
three bones; and the separated constituent bones are very rarely met
with. Two bones of the three confluent elements contribute to the
support of the wing metacarpal, and the third gives an articular
attachment to the bone which extends laterally at the inner side of the
carpus, which I now think may be the first metacarpal bone turned
backward towards the humerus. The three component bones meet in the
circular pneumatic foramen in the middle of the under side of the distal
carpal. There is no indication of division of the proximal carpal in
these genera into constituent bones.
[Illustration: FIG. 43. CARPUS FROM ORNITHOCHEIRUS
(Cambridge Greensand)]
This condition is somewhat different from birds. In 1873 Dr. Rosenberg,
of Dorpat, showed that there is in the bird a proximal carpal formed of
two elements, and a distal carpal also formed of two elements. Therefore
the two constituents of the distal carpal in the bird which blends in
the mature animal with the metacarpus, forming the rounded pulley joint,
may correspond with two of the three bones in the Cretaceous
Pterodactyle _Ornithocheirus._
The width of a proximal carpal rarely exceeds two inches, and that of a
distal carpal is about an inch and three-quarters. Two such bones when
in contact would not measure more than one inch in depth. The lower
surface shows that the wing had some rotary movement upon the carpus
outward and backward.
METACARPUS
[Illustration: FIG. 44. METACARPUS IN TWO ORNITHOSAURS]
The metacarpus consists of bones which correspond to the back of the
hand. The first digit of the hand in clawed animals has the metacarpal
bone short, or shorter than the others. Among mammals metacarpal bones
are sometimes greatly elongated; and a similar condition is found in
Pterodactyles, in which the metacarpal bone may be much longer than the
phalange which is attached to it. Two metacarpal bones appear to be
singularly stouter than the others. The first bone of the first digit,
if rightly determined, is much shorter than the others, and is, in fact,
no longer than the carpus (Fig. 43). It is a flat oblong bone, attached
to the inner side of the lower carpal, and instead of being prolonged
distally in the same direction as the other metacarpal bones, is turned
round and directed upward, so that its upper edge is flush with the base
of the radius, and gives attachment to a bone which resembles a terminal
phalange of the wing finger. According to this interpretation it is the
first and only phalange in the first digit. The bone is often about half
as long as the fore-arm, terminates upward in a point, is sometimes
curved, and frequently diverges outward from the bones of the fore-arm,
as preserved in the associated skeleton, being stretched towards the
radial crest of the humerus. This mode of attachment of the supposed
first metacarpal, which is true for all Cretaceous pterodactyles, has
not been shown to be the same for all those from the Solenhofen Slate.
There is no greater anomaly in this metacarpal and phalange on the
inner side being bent backward, than there is in the wing finger being
bent backward on the outer side. The three slender intervening digits
extend forward between them, as though they were applied to the ground
for walking.
The bone which is usually known as the wing metacarpal is frequently
stouter at the proximal end towards the carpus than towards the
phalange. At the carpal end it is oblong and truncated, with a short
middle process, which may have extended into the pit in the base of the
carpal bone; while the distal terminal end is rounded exactly like a
pulley. There is great difference in the length of the metacarpus. In
the American genus Ornithostoma it is much longer than the fore-arm. In
Rhamphorhynchus it is remarkably short, though perhaps scarcely so short
as in Dimorphodon or in Scaphognathus. The largest Cretaceous examples
are about two inches wide where they join the carpus. The bone is
sometimes a little curved.
Between the first and fifth or wing metacarpal are the three slender
metacarpal bones which give attachment to the clawed digits. They bear
much the same relation to the wing metacarpal that the large metatarsal
of a Kangaroo has to the slender bones of the instep which are parallel
to it.
The facet for the wing metacarpal on the carpus is clearly recognised,
but as a rule there is no surface with which the small metacarpals can
be separately articulated. One or two exceptional specimens from the
Cambridge Greensand appear to have not only surfaces for the wing
metacarpal, but two much smaller articular surfaces, giving attachment
to smaller metacarpals; while in one case there appears to be only one
of these additional impressions. It is certain that all the animals from
the Lias and Oolites have three clawed digits, but at present I have
seen no evidence that there were three in the Cretaceous genera, though
Professor Williston's statements and restoration appear to show that the
toothless Pterodactyles have three. Another difference from the Oolitic
types, according to Professor Williston, is in the length of the slender
metacarpals of the clawed phalanges being about one-third that of the
wing metacarpal, but this is probably due to imperfect ossification at
the proximal end; for at the distal end the bones all terminated on the
same level, showing that the four outer digits were applied to the
ground to support the weight of the body. The corresponding bone in the
Horse and Oxen is carried erect, so as to be in a vertical line with the
bones of the fore-arm; and the same position prevails usually, though
not invariably, with the corresponding bone in the hind limb, while in
many clawed mammals the metacarpus and metatarsus are both applied upon
the ground. In Pterodactyles the metatarsal bones are preserved in the
rock in the same straight line with the smaller bones of the foot, or
make an angle with the shin bone, leading to the conviction that the
bones of the foot were applied to the ground as in Man, and sometimes as
in the Dog, and were thus modified for leaping. Just as the human
metacarpus is extended in the same line with the bones of the fore-arm,
and the movement of jointing occurs where the fingers join the
metacarpus, so Pterodactyles also had these bones differently modified
in the fore and hind limbs for the functions of life. The result is to
lengthen the fore limb as compared with the hind limb by introducing
into it an elevation above the ground which corresponds to the length of
the metacarpus, always supposing that the animal commonly assumed the
position of a quadruped when upon the earth's surface.
This position of the metacarpus is a remarkable difference from Birds,
because when the bird's wing is at rest it is folded into three
portions. The upper arm bone extends backward, the bones of the fore-arm
are bent upon it so as to extend forward, and then at the wrist the
third portion, which includes the metacarpus and finger bones, is bent
backward. So that the metacarpus in the Pterodactyle differs from birds
in being in the same line as the bones of the fore-arm, whereas in birds
it is in the same line with the digit bones of the hand. It is worthy of
remark that in Bats, which are so suggestive of Pterodactyles in some
features of the hand, the metacarpals and phalanges are in the same
straight line; so that in this respect the bat is more like the bird.
But Pterodactyles in the relation of these bones to flight are quite
unlike any other animal, and have nothing in common with the existing
animals named Reptiles.
THE HAND
From what has just been said it follows that the construction of the
hand is unique. It may be contrasted with the foot of a bird. The bone
which is called, in the language of anatomists, the tarso-metatarsus,
and is usually free from feathers and covered with skin, is commonly
carried erect in birds, so that the whole body is supported upon it; and
from it the toes diverge outward. It is formed in birds of three
separate bones blended together. In the fore limb of the Pterodactyle
the metacarpus has the same relation to the bones of the fore-arm that
the metatarsus has to the corresponding bones of the leg in a bird. But
the three metacarpal bones in the Pterodactyle remain distinct from each
other, perhaps because the main work of that region of the skeleton has
devolved upon the digit called the wing finger, which is not recognised
in the bird. In the Pterodactyles from the Solenhofen Slate there is a
progressive number of phalanges in the three small digits of the hand,
which were applied to the ground. This number in the great majority of
species follows the formula of two bones in the first, three bones in
second, and four in the third; so that in the innermost of the clawed
digits only one bone intervenes between the metacarpal and the claw. The
fingers slightly increase in length with increase in number of bones
which form them.
[Illustration: FIG. 45. CLAW PHALANGE FROM THE HAND IN
ORNITHOCHEIRUS. (Half natural size)]
[Illustration: FIG. 46. METACARPUS AND DIGITS OF THE HAND IN BIRDS
WITH CLAWS]
The terminal claw bones are unlike the claws of Birds or Reptiles. They
are compressed from side to side, and extremely deep and strong, with
evidence of powerful attachment for ligaments, so that they rather
resemble in their form and large size the claws of some of the
carnivorous fossil reptiles, often grouped as Dinosauria, such as have
been termed Aristosuchus and Megalosaurus. In the hand of the Ostrich
the first and second digits terminate in claws, while the third is
without a claw. But these claws of the Ostrich and other birds are
slender, curved, and rather feeble organs. In the Archaeopteryx, a fossil
bird which agrees with the Pterodactyles in retaining the separate
condition of the metacarpal bones and in having the same number of
phalanges in two of the fingers of the fore limb, the terminal claws are
rather more compressed from side to side, and stronger than in the
Ostrich, but not nearly so strong as in the Pterodactyle. The
Archaeopteryx differs from the Pterodactyle in having no trace of a wing
finger. The first metacarpal bone is short, as in all birds; and the
first phalange scarcely lengthens that segment of the first digit of the
Bird's hand to the same length as the other metacarpal bones. It
therefore was not bent backward like the first digit in Pterodactyles.
The wing finger, from which the genius of Cuvier selected the scientific
name--Pterodactyle--for these fossils, yields their most distinctive
character. It is a feature which could only be partly paralleled in the
Bat, by making changes of structure which would remove every support to
the wing but the outermost digit of that animal's hand. In the Bat's
hand the membrane for flight is extended chiefly by four diverging
metacarpal bones. There are only two or three phalanges in each digit in
its four wing fingers. In Pterodactyles the metacarpal bones are, as we
have seen, arranged in close contact, and take no part in stretching the
wing.
THE WING FINGER
In Birds there is nothing whatever to represent the wing finger of the
Pterodactyle, for it is an organ external to the finger bones of the
bird, and contains four phalanges. The first phalange is quite different
from the others. Its length is astonishing when compared with the small
phalanges of the clawed fingers. The articular surface, which joins on
to the wing metacarpal bone, is a concave articulation, which fits the
pulley in which that bone ends. The pulley articulation admits of an
extension movement in one direction only. Many specimens show the wing
finger to be folded up so as to extend backward. The whole finger is
preserved in other specimens straightened out so as to be in line with
the metacarpus. This condition is well seen in Professor Marsh's
specimen of Rhamphorhynchus, which has the wing membrane preserved, in
which all bones of the fore-arm metacarpus and wing finger are extended
in a continuous curve. The outer surface of the end of the first bone of
the wing finger overlaps the wing metacarpal, so that a maximum of
strength and resistance is provided in the bony structures by which the
wing is supported. There is, therefore, in flight only one angular bend
in the limb, and that is between the upper arm bone and the fore-arm.
An immense pneumatic foramen is situate in a groove on the under side of
the upper end of the first phalange in Ornithocheirus, but is absent in
specimens from the Kimeridge clay. This bone is long and stout. It
terminates at the lower end in an obliquely truncated articular surface.
Specimens occur in the Cambridge Greensand which are 2 inches broad at
the upper end and nearly 1-1/2 inch wide at the lower end. An imperfect
bone from the Chalk is 14-1/2 inches long. The bones are all flattened.
Specimens from the Chalk of Kansas at Munich are 28 inches long. The
second phalange is concave at the upper articular end and convex in the
longer direction at the lower end. The articular points of union between
the several phalanges form prominences on the under side of the finger
in consequence of the adjacent bones being a little widened at their
junction. It should be mentioned that there is a proximal epiphysis or
separate bone to the first phalange, adjacent to the pulley joint of the
metacarpal bone, which is like the separate olecranon process of the
ulna of the fore-arm. It sometimes comes away in specimens from the
Chalk and Cambridge Greensand, leaving a large circular pit with a
depressed narrow border. On the outer side of this process is a rounded
boss, which may possibly have supported the bone, if it were applied to
the ground with the wing folded up, like the wing of a Bat directed
upward and backward at the animal's side.
The four bones of the wing finger usually decrease progressively in
length, so that in Rhamphorhynchus, in which the length of the animal's
head only slightly exceeds 3-1/2 inches, the first phalange is nearly as
long, the second phalange is about 3-1/4 inches, the third 2-3/4 inches,
and the fourth a little over 2 inches. Thus the entire length of the
four phalanges slightly exceeds 11 inches, or rather more than three
times the length of the head. But the fore-arm and metacarpus in this
type only measure 3 inches. Therefore the entire spread of wings could
not have been more than 2 feet 9 inches.
The largest Ornithosaur in which accurate measurements have been made is
the toothless Pterodactyle Ornithostoma, also named Pteranodon, from
North America. In that type the head appears to have been about 3 or 4
feet long, and the wing finger exceeded 5 feet; while the length of the
fore-arm and metacarpus exceeded 3 feet. The width of the body would not
have been more than 1 foot. The length of the short humerus, which was
about 11 inches, did not add greatly to the stretch of the wing; so that
the spread of the wings as stretched in flight may be given as probably
not exceeding 17 or 18 feet. A fine example of the wing bones of this
animal quite as large has been obtained by the (British Museum Natural
History). Many years ago, on very fragmentary materials, I estimated the
wings in the English Cretaceous Ornithocheirus as probably having a
stretch of 20 feet in the largest specimens, basing the calculation
partly upon the extent of the longest wings in existing birds relatively
to their bones, and partly upon the size of the largest associated bones
which were then known.