By their voice, too, birds are distinguished from the rest of the animal creation. Though there may be legends of singing serpents and of talking monkeys, a harsh scream or a growl is the only manifestation of the emotions through the voice which exists until we arrive at man. Among birds, the possession of a melodious voice is limited to that group which we term the Passeres. Other birds can scream or utter a dull note, while many are mute. So flexible is the voice organ of these creatures that they are the only animals that can imitate human speech. Here, however, it is not only the Passeres which can imitate the essential attribute of man. The Parrots, of course, are always supposed to be the birds which can talk, but this is far from being the truth. The hoarse utterances of most Parrots are left far behind in clearness of sound and correctness of imitation by the little Indian Mynah, which may be usually seen at the Zoological Gardens, and heard to speak. But the Parrot cannot sing. These are the only two groups of birds which have so elaborate and flexible an organ of voice. From this it might be inferred that some peculiarities of mechanism would distinguish the organ in question of these birds, and that is what we actually find to be the case. But, oddly enough, it is not only those birds which have a beautiful voice whose voice organs are so elaborate in structure. The harsh croak of the Raven issues from a syrinx which is as delicately fashioned as that which allows of the exquisitely varied tones of the Nightingale. The word ‘syrinx’ has been mentioned; that is the technical term for the voice organ of the bird, which is formed from a part of the windpipe, as in man and the mammalia, but from a different part of that tube. In man and in mammals the voice organ is placed in the throat just a little way down, at the prominence often spoken of as ‘Adam’s apple.’ This is a wider part of the tube, with larger rings of cartilage, which contains a pair of tightly stretched membranes that can be made to vibrate and cause a sound. In the bird, the voice organ is situated farther down, at the very point where the trachea forks into the two bronchi, one for each lung. Here are figures which illustrate the voice organ of a singing-bird (figs. 15, 16, 17). At this forking of the trachea the rings of the tube, which are of gristle or cartilage, become somewhat different in form. In the middle is a piece, which is often converted into bone, like the ‘three-way’ piece used to fix together the stick and the hoop of cane of a butterfly-net. To the upper side of this, and therefore within the tube, and directed upwards, is a little crescent-shaped piece of membrane (h, fig. 17); this can be set vibrating by the stream of air passing up and down the windpipe. At the sides of the syrinx there are shown in the figure (fig. 16) three pairs of muscles; these when they contract shorten the syrinx, and of course produce alterations in the note, just as the shortening of the tube in a cornet alters the sound. In many passerine birds, and in most other birds, there is only one pair of these muscles; but the Parrots agree with the passerines in having several pairs of muscles, and therefore a more easily alterable syrinx. In a good many birds there are no muscles at all in this place; for example, in the Storks, which have not by any means a flexible voice. The syrinx, in fact, is one of those organs which show a great deal of difference in different kinds of birds. But it is never entirely absent, though rather rudimentary in the Ostrich. The Australian Emu has a curious way of producing its sounds which is not found in any other bird. The cock and hen Emus can only be recognised by their voice, which is duller in the hen and sharper in the cock. When the bird is uttering its note, it seems almost to come from somewhere else, and not from the throat of the bird; the bird is something of a ventriloquist. The sound, which is a low bellow, is produced by a bag of skin opening into the windpipe some way up the neck; a current of air passing down the tube is believed to set the air in this bag in vibration, just as the air in a key may be caused to vibrate by blowing over its edge. Generally speaking, the windpipes of birds are straight tubes running to the lungs by the shortest route; but in the Cranes, and in a few other birds, the pipe is coiled upon itself once or twice, and the coils are even hidden in an excavation of the breast-bone. The increased length of tube gives a louder and more resonant note, such as we know characterises the Crane.
Lungs and Air-sacs.
It is not only by virtue of their powerful muscles and stiffened fore limbs that birds can fly. The body is rendered lighter in proportion to its bulk by air-cavities, which permeate everywhere, even into the substance of the bones. So thorough is this aëration in the Screamer of South America, that when the skin of the recently dead bird is roughly pressed it crackles. Curiously enough, there seems to be no very definite relation between the degree of thoroughness to which the aëration of the body is carried out and the capacity for flight. The Screamer, that has just been mentioned, is fuller of air-cavities than the Frigate-bird, in which the art of flying is carried to the highest extreme—the ‘triumph of the wing,’ as Michelet says in ‘L’Oiseau.’ Anyone who has the opportunity of dissecting a Hornbill will be struck by the large and abundant air-spaces between the muscles. This applies even to the Ground Hornbill of Abyssinia; and yet the latter, as its name denotes, lives upon the ground, while the flight of other hornbills is heavy and most unsuggestive of lightness of body. These air-spaces are in direct communication with the windpipe. It is much easier to understand their arrangement by the actual dissection of a bird. We must first get a notion of the position and form of the lungs, which differ very much from the lungs of other animals. In a rabbit, for example, or any other mammal, the lungs lie freely on each side of the heart, and are capable of being pushed here and there after the body is opened, and of much expansion and diminution of volume during the movements of respiration. But the lungs of all birds are tightly fixed to the wall of the chest cavity, being, as it were, moulded on to the ribs and vertebræ; when they are carefully picked away from their place, they retain the impressions of the bones which they touch. There is no great possibility here of independent movements on the part of the lungs. Respiration is effected in a totally different manner; it is, in fact, bound up with the mechanical filling of the air-spaces. Each of the two lungs is contained within a large compartment, which is bounded externally by an obliquely disposed septum, often spoken of, on account of its direction, as the ‘oblique septum.’ Others call it the diaphragm, imagining that it is the equivalent of the diaphragm in the mammal, that partly fleshy, partly tendinous plate which shuts off the cavity of the chest, in which lie the heart and lungs, from the cavity of the abdomen, in which lie the intestines, stomach, and liver. Now, this oblique septum does not by any means closely invest the lungs; on the contrary, a deep space is thereby shut off, at the bottom of which are the lungs. This cavity is subdivided by two partitions into three separate compartments. It requires a very skilful manipulation to show the fact, but it can, with care, be demonstrated that each of these compartments is lined by a delicate membrane, which is continuous with the lung, and is actually a kind of bubble, as it were, blown out of the lung; these delicate sacs are the air-sacs. There are altogether nine of them, but all these sacs do not lie within the cavity bounded by the oblique septa. The largest pair of all the abdominal air-sacs project into the body cavity far behind the gizzard. Now these sacs are fairly easy to see in a dissection; but it is not so easy to make out that they are all of them, except the middle two, connected with a system of ramified air-spaces which, as already said, permeates the body generally, lying among the viscera, between the muscles below the skin, and deep into the actual interior of the bones. But though it is difficult to see this by a dissection, it is easy enough to prove it by inflating them. If a syringe is passed down the windpipe and tied carefully into it, so that no air can escape at the sides, and air is blown down the tube, the passage of the air into the skin and other parts can be followed; if a bone be cut across, the air can be noticed to issue from the cut surface; and if the experiment be varied by using a coloured fluid instead of air—which is pumped in by a syringe—the fluid can be seen to ooze from the end of any bone or muscle that has been cut across. A bird, therefore, when it takes in a deep breath, not only supplies its lungs with fresh air, but fills its whole body with the superfluous air. It has been proved that a bird can continue to breathe if it be held under water, and only the end of a broken limb allowed above the surface; for, as all the spaces of air are in communication with the lungs, they (the lungs) can obviously be as conveniently filled from one end as from the other. When you are bathing, and take a very deep breath as you are swimming, you can detect a sensible increase in the buoyancy of the body; in a bird, of course, the difference is enormous, after the sacs are filled, from a condition of comparative emptiness. The way in which a bird breathes is different from the way in which a human being breathes. There is, of course, the essential resemblance that is shown between all animals that have definite organs which are set apart for respiration: the feathery gills of the marine worms, the closely set branchiæ of the fish, the lungs of the bird and of the mammal, are all constructed upon one plan, so far as essentials are concerned. In all of them blood-vessels are brought into close relation, though not into actual contact, with water or air containing oxygen. The blood-vessels are separated from the water or air by the thin membranes of the lungs or gills, through which the oxygen can pass in to the blood, and the carbonic acid and effete gases can pass out; it is this exchange which is the essential act of respiration. We cannot, however, in this book pretend to go into general matters of this kind, which would take us too far from the subject at hand; but anyone who would pursue this further can consult Professor Huxley’s ‘Elementary Physiology,’ or any other elementary text-book upon physiology. When a mammal—a human being, for example—breathes certain muscles are called into play. If a person is watched, it will be seen that the chest expands during inspiration, and that its calibre diminishes during expiration. What happens is this. The lungs are contained in a cavity which contains no air. This cavity can be increased in size in two directions. When the ribs are moved out—which they can be by the movements of the muscles called intercostal, which lie between them—the cavity of the chest from before backwards is evidently enlarged. On the other hand there is the diaphragm, which we have already spoken of as bounding the chest cavity below. Now this diaphragm is muscular, with a tendinous centre. When the muscles contract, like all muscles do, the surface of the diaphragm, which was before rather convex towards the chest cavity, becomes more flat; hence the cavity lying above it, i.e. the chest cavity, becomes larger in a downward direction also. When it is increased in this way by the action of the two separate sets of muscles, some space—more space than before—is left between its walls and the lungs which lie within it; it follows, therefore, that, as there is no air in the cavity, the pressure of air outside the body forces more air into the lungs, because there is no counterbalancing pressure to prevent this. The principle is the same in the bird, but the details are different. If you will turn again to the bird’s skeleton, you will see that the backbone and ribs and sternum form a bony box, which is jointed in the middle; this acts precisely like a pair of bellows: the bones at top and bottom represent the wood, and the soft intervening leather of the bellows is represented by the muscles which lie between, and which connect the sternum with the abdomen and with the ribs. When these muscles contract, the sternum is obviously brought nearer to the backbone, and air is expelled from the inside; when they are relaxed, a vacuum is created and air rushes in. The air-spaces, then, are really ramified tags of lung which have no blood-vessels in their walls, and are therefore not meant for respiration, but serve as reservoirs of air, lightening the body of the creature. It is curious that birds are not the only animals which possess expansions of lung that are apparently useless for breathing purposes. The lungs of the Chameleon have quite similar sacs appended to them. There is, it is true, no such complicated a ramification as that which we find in the bird, but still there is no doubt that the structure is of the same nature. It looks almost like a first step in the path towards a bird. Very possibly the extinct Pterodactyles, which flew through the woods of the middle ages of the earth, had bodies lightened in the same or a similar way; for we know that their bones have thin walls, the large cavity of which in all probability contained air-sacs. Even some of the jumping Dinosaurs, to which reference has already been made, seem to have possibly had lungs constructed on the bird type. We see, therefore, that even where a bird is, so to speak, most characteristically a bird—in the subsidiary mechanisms of flight—it betrays a likeness to the comparatively grovelling reptile, letting alone the aërial and more bird-like Pterodactyles.
Brain.
The brain of birds is large in proportion to the body, thus contrasting with that of the unintelligent reptile. From some tables on the matter which have been published, it appears that, if weight of brain goes for anything, the goldfinch is one of the most intelligent of birds. The weight of its brain is one-fourteenth of the entire weight of the body. The most unintelligent of all is the domestic fowl, whose body is 412 times heavier than its brain. The size of brain, however, seems to be largely a matter of the size of the bird: generally speaking, the smaller birds have heavier brains, and vice versâ. One might have expected something from the apparently intelligent Parrot; but the brain of the ‘Amazon’ is only one forty-second part of the weight of its body. Even the cruel and bloodthirsty Hawk, which one associates with brutality and ignorance, has a brain which is but little heavier.
The front part of the organ, known as the cerebral hemispheres, or, more briefly, as the cerebrum, is that part of the brain which is associated with intelligence. Now among the mammals this part of the brain is generally much furrowed, the brain surface being, therefore, increased without any actual increase in the skull-space required. This furrowing is met with in most mammals, but not always in the smaller and in the less intelligent kinds. But in the bird’s brain there are no convolutions: the surface is as smooth as in the reptile. Not even in the artful Raven, which some hold as the most highly developed of birds, is there a trace of the furrowing which one rightly associates, so far as the mammalia are concerned, with a high position in the series. The hinder part of the brain is known as the cerebellum; between this and the cerebrum are the optic lobes, of which there are only two, the mammals having four. From the brain arises the spinal cord, or marrow, which runs in the canal formed by the vertebræ, just as the brain lies in the brain-case. The nerves of the body come off either from the brain or the marrow, but it is not important to enumerate them. They show no difference in different kinds of birds.
The Muscles.
The muscles of a bird are what is popularly known as its flesh. When the skin is removed, the bones are seen to be covered by a mass of this flesh, which is of a red colour, darker in some birds than in others. For instance, in a Duck the colour is a dark red; in a Pigeon, quite a pale brown. The flesh is not, however, merely a thick sheet covering the bones: it can be separated into layers which are themselves made up of a number of separate pieces of muscle. These individual muscles are very commonly of a spindle-like shape, being thickest in the middle and dwindling towards both ends, where they often end in a tough substance called the tendon, which has a glistening and very characteristic appearance. All muscles are not of this form—sometimes they are strap-shaped; and not all of them end in tendons. As the most important act of the bird’s life that depends upon its muscles is flying, it is not surprising to find that the muscle which effects the downward stroke of the wing is the largest. This muscle is known as the great pectoral, and it is said to be almost as large as all the other muscles of the body put together. The way in which a muscle effects the movements of the bones to which it is attached is by contracting. All muscles are able to contract; they shorten, and, accordingly, the ends, with whatever they happen to be attached to are brought closer together. The contraction is governed by the nerves, and it has been discovered that the nerves actually end in communication with the fibres of which the muscle is composed. This pectoral muscle lies on the breast-bone, and nearly completely covers it; indeed, only the edge of the keel appears, and a very little tract at the sides. When this muscle is dissected away another muscle, not nearly so large, comes into view underneath it; this is called the pectoralis secundus, or the second pectoral. Its action is precisely the reverse of that of the great pectoral: it pulls the wing up instead of down. Between them, these two muscles do most of the work in flying. Naturally, in the ostrich tribe, which do not fly, they are much reduced in bulk. But they are never absent altogether, even in the Apteryx, which is, perhaps, further removed from the possibilities of flight than any other bird.
A very curious muscle runs into the patagium of the wing, which is that fold of skin which lies between the shoulder and the hand. This muscle is called the patagial muscle. It starts from the shoulder as a fleshy band, but soon ends in two long tendons: one of these follows the upper margin of the patagium, and finally ends in the wrist; the other passes down over the patagium, and ends below in connection with some of the muscles of the arm, and also by being attached in a fan-shaped way to the skin itself. The function of this muscle is to assist in the folding up of the wing when it is, so to speak, put away after use. The tendons in which the latter part of this muscle ends often show a most complicated branching in the patagium; they frequently offer characteristic differences in different birds, and are made some use of by the systematist. The bird has got a biceps to its arm just as we have. It sometimes happens that this biceps gives off a muscular slip, which runs into the patagium and becomes attached to the upper of the two tendons of the patagial muscle. A good deal of stress is laid by certain ornithologists as to whether this biceps slip is absent or present. Several of the common British birds will afford material to the beginner to ascertain for himself some of the chief variations in these and the other muscles of the body. It will be a good exercise to get a few birds, and to carefully dissect two of them, belonging to as widely different kinds as possible, side by side. You might select, for instance, a Crow and a Pigeon, which are fairly extreme types. To revert to our account of the muscular anatomy of a bird, it will be impossible to attempt any comprehensive account of this branch of the subject, because the facts are so appallingly numerous. We shall content ourselves, therefore, with the mention of a highly characteristic bird muscle which occurs in the leg. This muscle is known as the ambiens. This muscle is thin and ribbon-like. It takes its origin from a little process of the pubic bone usually called the prepubic process. From this point it runs along the inside of the thigh until it reaches the knee; it then bends over the knee and comes out on the other side, where it runs down the leg to join the deep flexor muscle of the foot. When this ambiens muscle contracts it pulls upon the flexor muscle, already referred to; the effect of this is that the toes are brought together by the tendons in which the last-mentioned muscle ends. The ambiens is far from being universally present among birds. It is notably absent from the passerine birds (the Sparrows, Crows, Rooks, and small perching birds generally), and from the Hornbills, Toucans, Woodpeckers, and that varied assemblage known as picarian birds. On the other hand, the Storks, Hawks, and most of the larger birds, have the muscle. But among some of these it is absent; thus, the Owls on the one hand, and the Herons on the other, have no ambiens; but from their general resemblance in other particulars to birds which have an ambiens, it was thought by Professor Garrod that the loss in them was a recent event, and that they might be fairly placed in one great group of birds with an ambiens which he termed, somewhat lengthily, the ‘homalogonatæ,’ or normal-kneed birds, reserving the name ‘anomalogonatæ,’ or abnormal-kneed birds, for the passerines, &c., without an ambiens.