Chapter 12

The question why man cannot fly may be answered in a very simple and yet satisfactory manner: He has not been organically constructed for that purpose. That may seem like cutting the Gordian knot, but, after all, it is the only explanation that can be given. You might as well ask why man cannot clutch a perch with his foot after the manner of a bird or a monkey, for the response would be the same--his foot was made for walking, and not for prehensile purposes. On the other hand, the bird cannot grasp an object with its wings, while a man's hand is well adapted for the performance of such a function. Nature's motto in her whole realm seems to be: "Every creature after its kind."

When we look at the structure of the flying birds, we see at once that they were formed for swift locomotion through the air, just as plainly as the lithe skiff was made to glide over the water or the carriage to spin over the land. In the first place, the body of the bird is comparatively light--that is, in proportion to the width, strength, and extent of its wings. By its thick, light, airy covering of feathers its body is made still more buoyant, besides presenting a larger surface to the supporting air with very little additional weight. The tail, too, with its long, closely woven quills spread out like a fan, not only serves the purpose of a rudder for guiding the aerial craft, but is still more useful in helping to sustain the bird's weight in the up-buoying element.

It is interesting to note that the feathers on the bodies of the flying birds are arranged in tracts, with intervals here and there of quite, or almost, bare skin, called "apteria." Now, when a bird is carefully skinned, it will be seen that the feathered spaces have their own special slips of muscles inserted into the roots of the feathers, and when these muscles are contracted, they serve to raise the feathers, and must, therefore, be of some subsidiary value in flying, by making the bird's body more buoyant. Suggestive, indeed, is the fact that the plumes of the non-flyers are not arranged in tracts, but are evenly distributed over the body.

Nor is that all that Nature has done to carry out her evident purpose of making the bird a natural "flying machine." The body of the bird contains numerous air sacs, all connected with the lungs, and these, when inflated, are a great help in flying by making the bird light. More than that, many of the bones, though strong, have thin walls and are hollow, the cavities being connected with the lungs and air sacs, from which they are also filled with air, contributing another element of lightness to the aerial navigator. That the bird's bones are capable of being permeated with air can be demonstrated by actual experiment, and is, therefore, a scientifically established fact. It is easy enough to prove it in this way: Take a dead bird that has been beheaded, pass a syringe into its windpipe, tie it carefully so that the air cannot escape at the sides, then blow the air down through the tube, and you will be able to follow the passage of the air into the skin and other parts of the body. Now, if you will cut off one of the bones, you can detect the air passing from the cut surface; and, more than that, as a scientific English writer says, "if the experiment be made by using colored fluid instead of air--which is pumped in by a syringe--the fluid can be seen to ooze from the ends of any bone or muscle that has been cut across." Thus it is seen that the whole body of the fowl is so constructed that it can be pervaded with air.

However, while all parts of the bird's organism combine to produce the end in view, the special instruments of flight are the wings. They are really the fore limbs of the fowl, but differ in many respects from the fore limbs of the mammals. They are under the control of muscles of great comparative strength, as every one knows who has ever been beaten by the wings of even an ordinary barnyard fowl, which has meagre powers of flight. What a powerful stroke a large hawk or an eagle must be able to deliver! If man's arm muscles were as strong in proportion, he might have some hope of one day navigating the air on artificial wings, but it is due principally to this muscular weakness that Darius Green has never been able to make a success of his flying machine, and perhaps never will. He would not have the strength to wield wings large enough to sustain so much avoirdupois on the yielding air.

The wings are highly specialized members of the avicular organism, and hence differ in many important respects from the fore or pectoral limbs of the mammals. Beginning at the point nearest the body, let us examine one of these wonderful instruments. The wing proper begins at the shoulder joint, which hinges freely upon the shoulder in a shallow socket, into which the globular head of the first bone fits closely, and in which it is firmly held by the powerful muscles that control the organs of flight. The first bone is called the humerus, and is the largest and strongest bone of the wing, extending from the shoulder to the elbow. At the elbow, which is the first angle of the wing, reaching backward when the wing is folded, the humerus articulates in a wisely designed way with two other bones, called the ulna and radius, which together constitute the forearm and extend to the wrist joint. It must be remembered that, when the wing is closed, the forearm is the segment that reaches obliquely forward. The wrist joint is the second angle of the wing. In the wrist there are two small bones (the radiale and ulnare) which serve an important purpose in joining the forearm with what is known as the hand, and make possible the specialized movement of the two parts upon each other. The hand is the terminal segment of the wing, composed of the metacarpal bones and the digits or fingers. Of the last-named organs there are ordinarily three, forming a graceful tapering point to the wing, and giving to it the symmetry and proportion that are required for effective use. When the wing is folded, the hand extends obliquely downward and backward.

Now, these bones and their attendant ligatures are wonderfully and wisely contrived. The humerus moves freely in its socket in the shoulder, so that it can be swung in every required direction, and yet, as should be the case, its principal movement is up and down in a vertical line--the precise movement required for the effective wingstrokes in flight. But note further. The elbow joint, unlike that of the shoulder, is a rigid hinge, permitting motion in only one plane, that of the wing itself, or nearly so. The same is true of the wrist joint, which holds the hand firmly, allowing no motion save that which opens and closes the wing. The wisdom of this arrangement will be seen at a glance.

In the human arm the hand can be moved in every direction with the greatest freedom, and, moreover, the wrist may be turned and the hand laid on its back, its palm, its edge, or at almost any conceivable angle. This is a very convenient contrivance for man, but it would be a great misfortune for our avian friends if their wings would rotate so readily; for in that case they would not have sufficient rigidity to answer the purposes of flight, but would be twisted into every position by the assaults of the air currents. Besides, even in ordinary flight it would require a constant muscular effort to keep the wings in the proper position. How wisely Nature has devised the bird's flying apparatus! When outstretched, it is held firmly by the power of its own mechanism, with its broad under surface lying horizontally, and no breezy current can bend or twist it from its normal position.

The set of muscles that open the wing are called the extensors, and those that close it, the flexors. The former lie upon the back of the upper arm and the front of the forearm and the hand, their tendons passing over the convexities of the elbow and wrist, while the flexors occupy the opposite sides, and their tendons run up into the concavities of the joints. There are several powerful pectoral muscles which run out from the shoulder and breast, and operate upon the upper end of the humerus, and with these the wing is lifted and the strokes are made during flight.

Another mechanical contrivance deserves attention. An extremely elastic cord reaches over from the shoulder to the wrist joint, supporting a fold of skin that occupies the deep angle of the elbow, and that is covered with short, fluffy feathers. When the bird is flying, this cord is stretched and forms the front edge of that section of the wing. But, now, suppose the wing is closed, will not this cord make a cumbersome fold, flapping loosely in the angle of the elbow? Such would, indeed, be the case, did not its extreme elasticity enable it to contract to the proper length, so as to keep the wing's border straight and smooth.

Without the feathers the wing would be useless as an instrument of flight. The shorter plumes that shield the bases of the long quill feathers are called the coverts, which are found on both the upper and under surfaces of the wing. They are divided into several sets, according to the position they occupy, and are called the "primary coverts" (because they overlie the bases of the primaries), the "greater coverts," the "middle coverts," and the "lesser coverts." Forming a vast expansion of the bony and fleshy framework are the quills, or flight-feathers, called collectively the "remiges." These plumes mainly determine the contour of the wing, and constitute a thin, elastic surface for striking the air--one that is sufficiently resilient to give the proper rebound and yet firm enough to support the bird's weight. The longest quills are those that grow on the hand or outer extremity of the wing and are known as the primaries. What are called the secondaries are attached to the ulna of the forearm, while the tertiaries occupy the humerus and are next to the body. All these feathers are so placed relatively that the stiff outer vane of each quill overlaps the more flexible inner vane of its successor, like the leaves of certain kinds of fans, thus presenting an unbroken surface to the air. As to the structure of these plumes, they combine firmness, lightness, and mobility, the barbs and barbules knitting the more flexible parts together, so that they do not separate, but only expand, when the wing is unfolded.

While the primary purpose of wings is flight, there is quite a number of notable exceptions. A concrete example is the ostrich, whose wings are too feeble to lift it from the ground, but evidently aid the great fowl in running, as it holds them outspread while it skims over the plain, perhaps using them mainly as outriggers or balancing poles in its swift passage on its stilt-like legs. The penguins convert their wings into fins while swimming through the water, the feathers closely resembling scales.

There are birds of many kinds, and therefore a great variety of wings and modes of flight. Birds with short, broad, rounded wings, with the under surface slightly concave and the upper surface correspondingly convex, usually have comparatively heavy bodies, and race through the air with rapid wing-beats and rather labored flight, and compass only short distances. Among the birds of this kind of aerial movement may be mentioned the American meadowlark, the bob-white, and the pheasant. Other species propel themselves in rapid, gliding, and continued flight by means of long, narrow, and pointed wings, like the swifts, swallows, and goatsuckers, while many others, notably herons, hawks, vultures, and eagles, are distinguished by a vast alar expansion in proportion to their weight, and hence are able to sustain themselves in the air by sailing, with only a slight stroke at rare intervals. Such birds as the stormy petrel and the frigate-bird have wings that are broad, convex, and of great length in contrast with the lightness and small bulk of their bodies, for which reason they are able to sustain themselves in the air for days without rest. It is even thought that some of these wonderful birds of the limitless ocean sleep on the wing, though how such an hypothesis could be proved it would be difficult to say.

Even in this day of scientific research and astuteness, it must not be supposed that everything about the mechanics of avicular flight is understood. We may readily comprehend how a bird, without fluttering its wings, can poise in the air; but how can it move forward or in a circle, and even mount upward, without a visible movement of a pinion? And this some birds are able to do without reference to the direction of the ethereal currents. That, I venture to say, is still a mystery. It almost seems as if some of the masters of aerial navigation in the bird world were gifted with the ability to propel themselves forward by a mere act of volition.

An interesting article on the subject of bird flight appeared not long ago in one of the foremost periodicals of the country, a part of which is here quoted to show what a puzzling problem we have before us:

Recent developments in aerial navigation have renewed interest in the comparative study of the mechanical principles involved in the flying of birds. There is one exceedingly puzzling law in regard to birds and all flying creatures, the solution of which may work far-reaching influences in the construction of flying craft.

"This law, which has thus far perplexed scientists, is that the heavier and bigger the bird or insect, the less relative wing area is required for its support. Thus the area of wing surface of a gnat is forty-nine units of area to every one of weight. In graphic contrast to that, a condor (_Sarcorhamphus gryphus_) which weighed 16.52 pounds had a wing surface of 9.80 square feet. In other words, though the gnat needs wing surface in a ratio of forty-nine square feet per pound of weight, a great condor manages to sail along majestically with .59 of a square foot to at least a pound of weight. The unexplained phenomenon persists consistently throughout the whole domain of entomology and ornithology. Going up the scale from the gnat, it is found that with the dragon fly this ratio is 30 to 1, with the _tipula_, or daddy-longlegs, 14.5 to 1, the cockchafer only 5.15 to 1, the rhinoceros beetle 3.14 to 1.

"Among birds the paradoxical law that the smaller the creature the bigger the relative supporting wings holds good. A screech owl (_Scops zorca_) weighing one-third of a pound had 2.35 square feet of wing surface per pound of weight. A fish hawk (_Pandion haliaetus_) weighing nearly three pounds had a wing area of 1.08 square feet to each pound. A turkey buzzard weighing 5.6 pounds had a little less than one square foot of wing surface to each pound. A griffon vulture (_Gyps fulvus_) weighing 16.52 pounds had a wing surface of only .68 square feet to the pound.

"Students of aerial navigation who are devoting much attention to observations of birds say that if the peculiar law governing extant flying creatures could be fathomed the problem of human flight might be solved."

A BIRD'S FOOT

You will agree with me, after you have studied a bird's foot, that it is one of Nature's most wonderful contrivances, so admirably adapted for the purposes to which it is devoted that one cannot help feeling that a Divine Mind must have planned it, just as a man would make a watch for the express purpose of keeping time.

But what is properly included in a bird's foot? Here we shall have to correct a popular mistake, if we wish to be accurate, in the scientific sense of the term. Most people think that the avian foot consists only of the toes and claws, or the part that comes in direct contact with the ground or the perch. That, however, is an error, for the foot really comprises, in addition to the toes and claws, the first long bone of the limb, reaching from the base of the digits to the first joint. You will see, therefore, that the bird walks on its toes, not on its foot as a whole.

The long bone referred to--called the tarsus--corresponds to the instep of the human foot, that is, the foot proper, while the joint which extends backward, forming an angle with the next large bone, is really the bird's heel. Thus you perceive that most birds walk with their heels high in the air. What most people call the bird's "leg" is in reality the bird's foot, and what they call its "foot" comprises only its toes and claws.

To obtain a correct idea of the bird's entire walking apparatus, we begin with the uppermost part of the leg. As we proceed, it would be well to keep in mind the different parts of the human leg and foot. The highest bone is called the thigh bone or femur, which is, for the most part, enclosed in the general integument of the body, and is not entirely separate from it as is the thigh bone of the human leg. Among carvers it is known as the "second joint." It reaches forward and slightly downward, and is hidden under the feathers of the body. The upper end of the femur enlarges into a globular head, which fits into the socket of the hip in the pelvis, while the lower end meets another long bone, which extends obliquely backward and downward and with which it forms the knee joint.

The knee of the bird extends forward, as the human knee does when it is bent. By means of various nodules and tendons the femur is articulated with and fastened to the next large bone at the knee joint. This second bone is the leg proper, called in scientific language the crus. When, with its thick, palatable flesh, it is cooked and placed on the table, it is known as the "drumstick"--a favorite part of the fowl with hungry boys, vying, in their minds, with the "white meat" of the breast.

This important segment of the limb is composed of two bones, the larger of which is called the tibia, the smaller the fibula. At its lower end the tibia forms what is known as the ankle joint by articulating with the next long bone, which is commonly called the tarsus, although the proper name would be really metatarsus. It is not often that this bone is covered with flesh, and therefore it seldom finds its way to the table. Properly speaking, it is the larger part of the bird's foot, reaching obliquely upward and backward from the roots of the toes to the heel. If you will lift yourself upon your toes, holding your heels in the air, you will be able to form a correct idea of what the bird is doing whenever it stands or walks or perches.

The toes are fastened by means of well adapted joints to the lower end of the tarsus, and form what is popularly regarded as the bird's foot. When spoken of separately, these toes are called digits, and when spoken of collectively, they are called the podium. They are composed of small bones called phalanges or internodes, which are jointed upon one another like the several parts of the human fingers. The digits can be spread out for walking purposes, or bent around so as to clasp an object. The outer bone of each digit almost always bears a nail or claw, which is sometimes very strong and hooked, as is the case with the birds of prey, while in other species it is only slightly curved and is not meant as a weapon of offense or defense, but chiefly to enable the bird to "scratch for a living."

How do the birds, in perching and roosting, retain their hold so long on a limb without becoming weary? They do not need to make a conscious effort to do this, but are held by the mechanical action of certain muscles and tendons in the leg and foot. Of course, the bird can also control these muscles by an act of its will, but a large part of their action is automatic. In some species there is a muscle called the ambiens, which has its rise in the pelvis, passes along the inner side of the thigh, whence its tendon runs over the apex of the angle of the knee joint, and down the leg till it joins the muscles that flex the toes. Now when the bird's leg is bent at the joints, as is the case in perching, the tendons of this muscle are stretched over the knee and ankle joints, thus pulling the digits together, and causing them of their own accord to grasp the perch more or less tightly. When a bird wishes to unloose its hold, it simply rises on its feet and relaxes the tendons.

All birds by no means possess this particular muscle, but all the perchers have some muscular arrangement in the legs and toes that practically answers the same purpose. If you will bend your wrist backward as far as you can, you will observe that your fingers will have a tendency to curve slightly forward. This is caused by the stretching of the tendons over the convex part of your bent wrist joints.

The typical bird has four digits, three in front and one reaching backward. The hind toe is called the hallux, and corresponds to the thumb of the human hand, so that in grasping an object it can be made to meet any of the other toes. But many birds are not provided with a quartet of digits. The ostrich has only two, the inner and hinder toes being wanting. However, this great fowl does not experience any lack, for its feet are almost solid like hoofs, and quite flat, and hence are especially adapted for traveling across the sandy desert.

No bird has ever been found with more than four toes; and four seem to be ample for all purposes. A fifth toe for a bird would be as useless as a fifth wheel on a wagon. Quite a number of species have only three toes, most of them among the walkers and waders, and none, I believe, among the true perchers. Take the plovers and sanderlings, for example, which spend most of their time, when not on the wing, in running about on the ground, especially along the seashore or the banks of streams and lakes, and seldom, if ever, sit on a perch--in their case a fourth toe would be worse than a superfluous appendage; it would be an encumbrance, dragging along in the mud and mire. In these species it is the hind toe that is lacking, their three digits all being in front, where they are of the greatest service. There is another class of birds that have hind toes, though very much reduced because their owners do not perch, but scuttle about on the beach. This class includes the little spotted sandpipers which you often see running or flying along the shores of a river or lake.

Curious to tell, several species of woodpeckers are tridactyl--that is, three-toed--and still more curious is the fact that in their case the true hind toe is lacking, while the outer front toe is bent backward, or "reversed," as it is called, and is thus made to do service for a hind toe. The other species of woodpeckers have four toes, two in front and two behind, the outer one of the latter pair being a reversed digit. Why some of the woodpeckers should have four toes and others only three is an unsolved enigma, and is especially puzzling in view of the fact that the four-toed kinds do not seem to possess any advantage over their cousins. The tridactyl species are as expert climbers as any members of the family, and are extremely hardy birds, too, some of them dwelling the year round in cold northern climates, where the food question must often be a serious one.