Part 8 (1/2)
ULNA AND RADIUS
[Ill.u.s.tration: 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 ab.u.t.ted 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 circ.u.mstance 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 const.i.tuent 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 const.i.tuent bones.
[Ill.u.s.tration: 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 const.i.tuents 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
[Ill.u.s.tration: 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 a.s.sociated 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 s.h.i.+n 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 a.s.sumed the position of a quadruped when upon the earth's surface.