Part 4

Like all other backboned animals, birds have a chain of small bones running along the back, and enclosing a canal in which runs the spinal marrow. In most vertebrates some of the individual vertebræ in the region of the hind limb, the sacral region, are somewhat intimately fused together, forming a more solid structure for the support of the pelvis. In birds the strong coupling of the vertebræ is more marked, and extends to the dorsal region. The mechanical value of this to a flying animal is clear; it is analogous to the tight coupling of an express train, and prevents the back from bending from side to side under the strain produced by the powerful movements of the muscles in flight. The tail vertebræ show some curious modifications in different birds. In the typical carinate bird, the last few vertebræ are fused into a piece which is called the ‘plough-share bone,’ or ‘pygostyle.’ The name of this bone sufficiently indicates its shape; the expanded end of the bone serves as a firm base, upon which rest the strong tail feathers. Now, in the ostrich tribe there are no rectrices comparable in size to those of the flying carinates. Here there is no pygostyle, but the individual vertebræ are small and disconnected. They are, however, few in number, whereas in the Archæopteryx they are numerous, though, oddly enough, not so numerous altogether as are the tail vertebræ of some flying birds. Each individual vertebra in the Archæopteryx supports a pair of rectrices, which are thus arranged in a series, and not in one row. A very distinctive peculiarity of the vertebræ of birds is the saddle-shaped centrum. The centrum of the vertebra is the solid piece which underlies the canal of the spinal cord, the walls of the latter being formed by the neural arches, which unite above to form a neural spine. In other vertebrates the centra are flat (mammals), or procœlous (the concavity being forward), or opisthocœlous (the concavity posterior), or amphicœlous (concave on both sides). This latter form of vertebra is frequently met with in archaic forms belonging to various groups. It occurs, for example, in many fishes. Such reptiles as Hyperodapedon and the Geckos have the same kind of vertebræ. Among birds there is no existing genus or species which is to be thus characterised; but the extinct Ichthyornis had clearly biconcave vertebræ.

Shoulder Girdle.

This series of bones serves as the intermediary between the fore limb and the vertebral column. It consists of three distinct elements. There is, first of all, a sword-blade-like bone with sharp edges, which lies along the vertebral column--the scapula. To the end of this is firmly attached a somewhat shorter bone, which approaches its fellow as it joins the sternum below; this bone is known as the coracoid (52, fig. 8). The angle between these two bones is, in flying birds, a considerable one, but in the ostrich tribe they are almost in the same straight line; this is really connected with the power of flight, for it has been shown by careful measurements that, in birds which still have wings that bear every appearance of being functional, and yet are not used for their legitimate purpose, the angle tends to approach the obtusity of the scapula and coracoid of the Ostrich. Birds have, besides these two bones, the merry-thought, or clavicle (58, fig. 8), which corresponds to our collar-bone. Its two halves are generally closely united to form one [**Symbol:]U-shaped or [**Symbol:]V-shaped bone; but sometimes they are separate, and then more or less rudimentary.

Wing.

We must enter into the matter of wing a little more closely--it is so important a feature of bird organisation. The wing, of course, although it performs so different a _rôle_, is the exact equivalent of the fore limb of mammals. We can easily recognise precisely the same bones, though they are diminished in number, and often of a different form. It will be noticed that in each case we can distinguish the three bones forming the arm, and which are known as the humerus, the radius, and ulna. The rest of the limb in the bird is not quite so obviously like the hand of the mammal; but a little attention will show that it is constructed upon a perfectly similar plan. The flexible wrist of the mammal is made up of many small bones; the hand itself is made up of a larger series still, of which those nearest to the wrist are technically termed the metacarpals, and those which follow, the phalanges. In many mammals there are five fingers; but there are many which have less, and the extreme is reached in the horse, which has to put up with a single finger and small rudiments of two others. Now the bird is better off in the way of fingers than the horse, as it has three fairly well-developed fingers, or rather two well developed and one less perfect. The shortest finger corresponds to the thumb of our hand. It is more freely movable than the others. The metacarpal bones of the second and third fingers are firmly welded together, and are long; each finger (as will be seen from a look at fig. 1, p. 4) has one or two phalanges, as the case may be. Now in mammals the end phalanx of each finger is tipped with a nail, or with a hoof. The powerful claws of the tiger, used for tearing, and the solid hoof of the ox or horse, upon which the creature walks, are one and the same thing. It might be supposed that the hand of the bird, which is not an organ of offence or meant to walk with, might be shorn of these appendages. But this is not the case: every bird has at least two nails (fig. 9), of a long and rather claw-like form when well developed, and sometimes three nails, that is, one to each of its fingers. It looks, therefore, very much as if the wing of the bird had been formed out of a limb that was once an organ for climbing or walking with. There is a curious bird, found in British Guiana, which is known as the Hoatzin (figs. 9, 11). In the very young nestlings of the hoatzin the claws of the fingers are so conspicuous that they are actually used by the callow chick to climb with, before the feathers of the wings have grown sufficiently to enable them to use their wings in the proper way in which a bird should; it has been said also, that other birds scramble about and use their claws when they are young. In the case of the hoatzin, it is stated that the thumb and the first finger can be brought together so as to lay hold definitely of an object. A very important thing to notice about the wing bones is that they are capable of but little movement upon each other. There are two hinges, one at the elbow, and the other at the wrist; but the radius and ulna cannot move round each other, as they can in our arms, and the fingers are fixed and rigid. This would be most unfortunate if the wing had to be used as a walking or climbing limb; but it is most useful in relation to the function which the wing has to perform--that of flight. The strength of the downward stroke would be enfeebled if the bones were in a limp condition and moved upon each other. They offer, too, a firm foothold for the thick quills of the big feathers of the wing. It has been mentioned that all the evidence at our disposal points to the view that the wing has become gradually moulded into an organ of flight, from a condition in which it played a different part. The earliest bird of which we have any record had wings which were much less perfect as flying organs than those of modern birds. It seems pretty plain that the bones in that antique bird were much less rigidly fixed together, and it is equally clear that the fingers were very much more loosely attached to one another. They were also more on an equality as regards size; the great disparity evident in fig. 12 is not to be seen in the Archæopteryx. All this, of course, shows that the Archæopteryx could not have possessed the ample pinion of its more vigorous descendants of to-day. The fossil Archæopteryx looks a little like a crow would look after receiving at close quarters a charge of duck shot; but a closer examination will show that in reality all the bones are there, on one side at least. Out of the disjecta membra of the fossil numerous ‘restorations’ have been put together, which are as diverse as the minds which imagined them. We cannot really say with certainty what were the precise relations of the hand to the feathers. It seems most probable that the hand of this ‘mediæval’ bird still retained the ordinary functions of a hand; that it served its possessor to lay hold of convenient branches, from which it fluttered feebly to others. One bold speculator has insisted upon the probability that the Archæopteryx had the requisite five fingers of the presumed ancestral type; but there are no traces of them, except in so far as the lie of the feathers enables a hint to be gathered. Boring operations, or at least prospecting in the interior of the stony slab on which the fossil lies, might reveal some additional fingers; but the operation would be fraught with too obvious perils to a nearly unique object. There are a good many birds which do not, and some which cannot, fly. To the first category belong such birds as the domestic ducks and fowls, and some of the rails. These birds, when put to it--when chased by a dog, for example--can often fly; but as a rule they do not, or at most only flutter along. The Ostrich tribe and a few other birds have totally lost the power of flight. But though this is the case, the bony structure of the hand remains the same in the Ostrich and in the American Rhea; in the Cassowary, however, and the Apteryx of New Zealand, the fingers are reduced to one. The last stage in the atrophy of the organ of flight is seen in the giant and extinct birds of New Zealand, the Moa or Dinornis, in which no trace of a wing has been so far discovered. But in some of these birds in which the wing is reduced in size, or so simplified in structure that it can no longer serve its legitimate purpose, it is made use of for other purposes. When the Ostrich skims along the surface of the sandy deserts where it is often found, it holds out both wings, which are compared to sails; they possibly serve rather as the pole of the tight-rope walker, to preserve the balance of the bird when hurrying along at full speed. In the Secretary Vulture of Africa the wings can be used for flying, but they are also used as weapons wherewith to combat the poisonous snakes upon which the bird so usefully feeds. It strikes down the venomous serpent when the latter is attempting to strike the bird. The Chauna of South America has strong spurs upon its wings, which are used for fighting as well as for flying. But the most curious use to which wings are put is afforded by the Penguin. If the reader has never seen the ‘diving birds’ fed at the Zoological Gardens, let him go there on the first opportunity, and see how rapidly and gracefully the Penguin ‘flies’ under water by the flapping of its wings. They are shorter than those of most birds, and the feathers have become flattened and almost scale-like, so as to offer no resistance to the water; at the same time the bones of the wing are flattened, so that a broad surface is provided, which of course acts like an oar. With this oar-like wing the Penguin can outswim a small fish.

Sternum and Ribs.

The breast-bone or sternum (fig. 8, p. 13) of birds shows the same relation to the power of flight that is shown by so many, if not by all, parts of the skeleton. It is relatively a very large bone, and is in all perfectly flying birds furnished in the middle line, below, with a strongly marked keel, the presence of which has given its name to the great group of birds called carinates. The ostrich tribe, from whose sterna the keel is absent, are termed ‘ratite,’ or ‘raftlike.’ The reason for the keel is the attachment of the great pectoral muscle, which is the most important muscle of flight. The sternum often offers useful characters to the systematist. The surface of the bone is sometimes in various degrees fenestrate, or more or less deeply incised, the one condition being an exaggeration of the other, and both the conditions being due to defective ossification. The sternum is attached to the vertebral column by the ribs, which are well developed in all birds, but vary very much in number. A highly characteristic feature of the ribs of birds is a small bony projection of the hinder margin of a certain number of them, called the uncinate processes. These are present in all birds, with the single and remarkable exception of the South American Screamers (_Chauna_, _Palamedea_), a group of birds occupying a rather isolated position, and showing resemblances to a great many different groups.

Pelvis.

The hind limbs are attached to the vertebral column by means of a considerable bony structure known as the pelvic girdle (fig. 13). This mass of bone is in reality composed of three pairs of elements, though they are in the adult strongly compacted together. The main bone, which is firmly attached to the vertebral column, is the ilium; with this is almost completely fused the ischium; the very slender pubis is to a large extent free from these bones. The pelvis is in its form one of the most characteristic of the bones of the bird’s skeleton. In other animals the three bones are present, but they are directed away from each other; in the bird, as already described, the pubis is directed backwards, parallel to the ischium; in correspondence, perhaps, with its position it has become a feeble bone, and has but few muscles attached to it. The interest of the matter, however, is mainly in the fact that among the extinct Dinosaurs, a race of mesozoic reptiles, there were some in which the pelvis had a very bird-like structure, with the same feeble and recurrent pubis. This has been urged as a mark of affinity between the Dinosaurs and birds. The several bones of the pelvis are free from each other at the extremity, or almost so, in all the Ratites, and in the Tinamous, which are supposed to bear some relationship to the Ratites. The fact is interesting as being an example of the retention of a character by one group of birds which is only transitional and embryonic in another, for in all young birds the bones of the pelvis are separate; it is not until some time before hatching that they become fused together as we see them in the adult.

Hind Limb.

At first sight there appears to be a considerable difference between the fore limb and the hind limb. In both there is a long proximal bone, called humerus in the one case and femur in the other, followed by a pair of bones--the tibia and fibula--corresponding to the radius and ulna of the fore limb. But in the hind limb (fig. 13), the foot proper, consisting of metatarsals and phalanges, appears to come immediately after the tibia and fibula. In a sufficiently young bird, what is the apparent lower end of the tibia, and what is equally apparently the upper end of the metatarsus, are detachable; these two halves which are thus detachable are the tarsus, which is the equivalent of the carpus of the wing. The lower bone of the leg is on this account usually spoken of as the tarso-metatarsus. The lower part of this bone is made up of three fused elements, the separation of which from each other is clearly apparent at the lower end of the bone, where the phalanges are attached. In the Penguins the three bones are separated by grooves of a very marked character throughout. In some birds there is a fourth toe, the hallux; in these cases there is a small separate metatarsal loosely fixed to the lower end of the large conjoint metatarsals.

Gizzard and Alimentary Canal.

The gizzard (fig. 14) of the fowl is simply a part of the stomach which has especially hard and muscular walls, the other half remaining soft in texture; this latter is termed the proventriculus, and into it open the mouths of glands which secrete the digestive juice of the stomach. But the muscular part of the stomach--the gizzard--has to grind down the frequently hard food of the bird, so it has not merely a strong wall made of muscle, but also a very tough lining; the whole organ, therefore, forms a highly efficient mechanism for crushing and grinding the seeds and other hard vegetable food which is swallowed. It is rendered more useful still for this purpose by the pebbles which every bird takes care to swallow. The true and singular stories about the varied contents of an Ostrich’s stomach are founded upon the fact that, like other birds, it picks up stones, and with them occasionally other objects. But all birds do not possess a hard gizzard; in Hawks and fish-eating birds the walls are thinner, and the organ is flaccid instead of being rigid. By a very curious and unique exception certain Tanagers, a race of large, often bright-coloured, American, finch-like birds, have nothing at all that can be compared to the gizzard of other birds; this part of the alimentary canal is totally wanting. Now the difference between the gizzard of the grain-eating fowl and the flesh-eating hawk is chiefly a matter of diet. The celebrated anatomist, John Hunter, who lived in the last century, and wrote so much about the anatomy of all kinds of animals, including birds, found that he could feed a soft-stomached bird into one with a hard gizzard, and _vice versâ_.

We can pass briefly over the rest of the alimentary system, which does not vary a great deal in different birds. The intestines are always rather short, and are diversely coiled, the method of coiling being often characteristic of a particular group. A good way down the intestine are a pair of cæca, which may be entirely absent, as in the Hornbills, for example; and if present may be extremely short, as in the Sparrow, or very long, as in the Ostrich; various intermediate degrees exist. As in all vertebrated animals, two glands pour their secretion into the intestine; these are the pancreas and the liver. The secretion of the liver is the bile; this fluid is accumulated as it is formed in a largish bag--the gall-bladder, in those birds which possess one. Shakespeare used the epithet ‘pigeon-livered,’ which meant literally the absence of a gall-bladder; but, oddly enough, there are some kinds of pigeons which have a gall-bladder, while others, like the common pigeon, have not. The intestine ends in the cloaca, which is the common chamber into which the urinary and generative organs also open.

Tongue and Teeth.

In the inside of a bird’s mouth we find only one of the two things that we might expect to find: there is a tongue, but no teeth. We shall come back to the teeth immediately. The tongue is not so useful among the majority of birds as it is in most mammals. But some do make use of it to a great extent. If you watch a parrot eating its food, you will observe that its thick and fleshy tongue is of the greatest assistance in helping it to manipulate the pieces of food--to extract, for instance, the kernel from a seed or nut. It plays exactly the same part as it does with us. In one kind of parrot, called the ‘Brush-tongued Parakeet,’ the tongue is frayed out at the free end into a brush-like extremity. And there are some small birds, which peck at flowers and live upon honey, in which the tongue is thin and delicate, and frayed out in the same way; this allows them to suck up the juices of the flower. In the Hummingbird the tongue is rolled up so as to form two tubes running side by side, and the same power of sucking up juices is acquired by this means, which, curiously enough, is exactly paralleled by the proboscis of the butterfly. In other birds the tongue is sometimes merely a thin, flat, horny projection, and in others, again, it is just not absent altogether.

A little reflection about the habits of birds will show that they really do not want teeth; and we know that Nature is a most rigid economist: nothing superfluous is allowed in the body. Even rapacious birds like Owls and Hawks have no teeth, because they have a powerful beak and claws, with which the food may be as effectually torn to pieces. Birds such as the Pigeon, which feed upon grain, possess a gizzard--which we have had something to say about already--that performs effectually the function of a mill, grinding into a powder the hard grains of wheat and other seeds which the bird swallows. Nevertheless birds once did possess teeth. In earlier times of the history of this earth there were some birds whose jaws had as formidable a range of teeth as the mouth of many reptiles. They were fish-eaters, and have been named _Hesperornis_ and _Ichthyornis_. The first was something like a Diver in shape, the latter more like a Gull. A still more ancient bird, the oldest form of bird known to us, the _Archæopteryx_, had also toothed jaws. In fact, in the old days it was the rule for birds to have teeth, whereas now it is the rule, without a single exception, for birds to be toothless. Perhaps these ancient and extinct forms had some corresponding disadvantage when compared with their modern representatives; their teeth and claws, for example, may have been less effective. But although there is no bird now living which has real teeth, traces of these organs have been discovered in the young embryos of certain birds, which seems to be an absolute proof that they, at any rate, had for their first parents toothed birds. But although modern birds have no teeth, with enamel, dentine, and so forth, all complete, the horny beak has occasionally ridges which to some extent play the part of teeth. The inside of the Duck’s mouth is rough with such ridges, which occur also in some other birds. The large Flamingo was for some time regarded as a long-legged and awkward Duck that had partially adopted the habits of a Stork, partly on account of the fact that the inner edges of the beak were ridged in a fashion exactly like that of the Duck. But it happens that there is a Stork, a true Stork, in India, whose scientific name is _Anastomus_, which has similar ridges. Ducks feed to some extent upon shellfish, which the roughened edges of the beak are well suited to crush. The replacement in the course of ages of true teeth by horny teeth is seen--a curiously parallel case--in the Duck-billed Platypus of Australia, which has when adult horny plates instead of teeth, but when young has real teeth.

Heart.