LETTER XXXIV.
AVES. (Birds.)
Tell me, my dear child, when you have seen birds taking their flight into the air, and going boldly to their object, without a thought of all the barriers, ditches, rivers, and mountains, which hinder man at every step in his travels, did it never strike you to wish for their wings, and imagine how you would fly off if you had them? If you ever dreamt this dream, do not apologise for it; it is one as old as the world. 'Oh that I had wings like a dove!' cried the Prophet, nearly 3,000 years ago; and the dialogue of the swallow and the prisoner, so often sung by poets, has been repeated in prose behind all the prison-bars on the globe since prisons were first invented.
Now you will not think it kind on my part, but I must undeceive you about this fancy, as you will be undeceived some day about many others. The wings of a dove or swallow would be of no use to you if you had them, any more than the formidable swords of the middle ages would be to our modern gentlemen, were any one to put such into their hands. We are not adapted for them, nor they for us.
You saw, some time ago, what an amount of muscular exertion was required for running—what a violent flow of blood, what hurried play of the lungs. Now in flying it is still worse; for the earth, at any rate, holds us up quite naturally, whereas the air will not hold up the bird unless it is beaten vigorously and unremittingly by an untiring wing. If we men, constructed as we are, had to do such work, we should be out of breath at once; the heart would cry out immediately for quarter, and the diaphragm turn red with anger. And only just imagine in what a critical position a poor wretch launched into the air on the wings of a swallow would find himself when, at the end of five minutes, his servants should refuse point-blank to go on working at a height of 500 feet above the ground!
But a bird has not these internal rebellions to fear. In the first place, it has no diaphragm; so here is another friend to whom we must say good-bye. We shall not meet with him again anywhere. The journey we are taking together, my dear, is somewhat like the journey of life. One sets off, surrounded by friends and acquaintances, but whoever travels on to the end is apt to find himself alone at last; this is what is happening to the digestive tube, which we shall see losing all its accessories, one by one, as we gradually advance in our study. Even now here is one essential fundamental difference in the internal machinery. The body has only one compartment instead of two; and the lungs, masters of the whole space, extend freely to its utmost depths. When a fowl is cut up at table, look along the body, and you will find lodged in the cavity of the ribs, a long, blackish, and spongy mass: this is the lungs. There is not, therefore, the same danger of a bird's getting out of breath as with us; that delicate board which is found in our bellows is wanting in his. His is set in action solely by the to-and-fro movement of the ribs, which is produced by muscular exertions, which are greatly increased during the action of the wings. From which it follows, that the rapidity of flight itself regulates the arrival of air, and consequently the expenditure of strength, or, if you like better, the activity of the fire, since the energy of the muscles depends, as we have seen, upon the quantity of oxygen that feeds the internal stove.
This is not all. These elongated lungs are still not sufficient to furnish the blood with all the oxygen demanded by this excessive labor of flight. They are pierced with holes, through which issue pipes which carry the air all over the body. You know what is said of spendthrifts?-that they burn the candle at both ends. It is so with the blood of birds. That fillip which in our case it receives in the lungs, and which sends it back full of vigor into the arteries, is repeated in the bird at the other end of the arteries as well. The capillaries, those delicate vessels at the end of the arteries, plunge from all sides into little reservoirs of air-lungs, therefore-where the blood renews its provision of oxygen, and relights its half-extinguished fire, so that it sends the combustion afresh into the muscles on its return back to the heart, and sets them going a second time.
The natural consequence of this prodigality of combustion is, that there must be, in proportion, much more oxygen in birds than in us; and that of all animals a bird is the one most quickly poisoned by his own carbonic acid when the air is not renewed around him. Therefore, let me beg you never to think of putting a poor little bird under a wine-glass, as a child of my acquaintance once did, that she might examine her little friend more closely. In the twinkling of an eye he would consume all the oxygen inside his prison, and you would soon see him fall upon his side and die.
On the other hand, the temperature of these flying machines, which consume so much oxygen, is very much higher than ours. It rises to 41°, 42° (centigrade), and sometimes to 44°, 7° higher than with us. If ever you have taken hold of a little bird, you will have remarked how warm it makes your hand: this is quite natural, since there is always a double fire going on within him, to meet the extraordinary expenditure of strength that is required of him whenever he takes wing. Besides, do but look at the poor little creature when you have imprisoned it in a cage! How it goes up! How it comes down! How it hops from one perch to another, with a quick sudden movement, like that of a spring when it unbends. There is no apparent cause for this state of continual agitation; and yet there is a cause, and only too serious a one. Its fire is not slackened because you have put it into a cage, and its muscles, lashed furiously on by the double-oxygenized blood, drive it hap-hazard into a thousand movements, in which it expends, as best it can, a superabundance of power, which no longer finds natural employment. Little children, who are the real singing-birds of our homes, and whose blood also drives much more energetically along than ours—little children I say—often fare no better than caged birds in those larger cages we call schools; and schoolmasters and governesses would scold rather less if they thought rather more about this. It is right, I do not deny it, that the rebellious young rogues should be taught in good time not to abandon themselves, like wild birds, to the mere animal impulses of the blood: but, in dealing with them, one must also make allowances, as they say, for the fire within, and know how to open the cage now and then. It is not for you, however, that I say this, young lady: you are no longer a little child; but it may happen that you may have some to take care of some day. Believe me, then, you must not expect too much wisdom from them, and you must allow them to change their perch every now and then. It is a law of our Almighty Father that little children, and little birds, should not stay too long in one place.
The mechanism of the circulation is here the same as with us, and does not offer any important peculiarity. Only the left ventricle of the heart has walls of extreme thickness, which enable it to launch the blood into the members with greater vigor and rapidity; and the blood itself, although it is composed of precisely the same materials as that of the mammals, differs from it nevertheless as regards the globules. In the first place, they are more numerous; secondly, they are larger; and finally, instead of being round like a plate, they are drawn out ovally, and are almost shaped like those long dishes on which fish is usually served. I shall not attempt to give you the reason of their size and form. This is hidden from us in the same mystery which envelopes all the microscopic population of the blood; but is it not a curious thing, this strange persistency of form in the globules ofall animals of one class? In all birds they are oval; in all mammals they are round. In all? Nay, I am wrong. As if the better to hide from us the key to this riddle, nature has amused herself by making an exception. Camels and llamas, I forgot to tell you, have also globules in the form of long dishes, like the hen and the chaffinch. Find out why, if you can. As to the reason of the number, it is a very simple one. Since the energy of the blood resides in the globules, it follows that the most energetic blood will contain the largest amount of globules. Looking at you, for instance, little monkey, running and jumping about the garden, I would lay a wager, without counting first, that there are, in one drop of your blood, some millions more globules than in one of mine.
Let us now go on to the digestion, with which, properly, we ought to have begun; but I preferred pointing out to you, first, the particular character which is the chief mark of distinction in the organization of the bird.
'When hens grow teeth,' says a shrewd proverb, meaning of course, never. Birds have no teeth, and in this respect there is no variety among them. All, from the first to the last, have uniformly the same tool to eat with—the bill, that is—which is, in all cases, composed of the same elements, two jawbones elongated to a point, and clothed in a horny armour, which makes their edges sharp and cutting. At the same time were we to review the birds in detail, as we have done the mammals, you would see that there are almost more modifications to be observed in this one single instrument than in our thirty-two teeth. All birds have a beak, but each has his own, organized expressly with reference to the kind of food needed by its owner. The eagle's beak, which mangles living prey, is pointed, bent, and hard as steel; the bill of the duck, which laps up water from ponds and puddles, in order to get worms and half-decomposed refuse out of it, is soft, and flattened like a shovel. The woodpecker's, which has to pierce the trunks of trees, is like a pickaxe; that of the humming-bird, which has to suck up the juice of flowers from the bottom of their corollas, is slender as a needle. The swallow feeds on flies, which it snaps up on the wing, and has a soft bill, which opens like a little oven. The stork picks up reptiles in the mud of the marshes; its beak is straight-pointed, cutting as a knife, and resembles a long pair of pincers. The sparrow feeds especially on hard grains, difficult to break; accordingly its beak is stumpy, short, and thick, and is arched on the upper side for still further solidity. But I should never end if I began to enumerate all the thousand varieties in the bills of birds. Each variety, too, corresponds with some peculiar sort of life, and consequently with a general conformation (easily ascertained) of the animal in which it appears. Give a naturalist the bill of a bird—only its bill remember—and he will tell you half its history without fear of being mistaken.
On the other hand, we must not deceive ourselves as to the real value of this complaisant bill. Let it transform itself as it pleases into all manner of forms for the better fulfilment of its task, it makes, at the best, but a very poor instrument for mastication; nay, to say the truth, it breaks, cuts, and tears, but it never masticates at all. Thus the bird's mouthful is far from undergoing as perfect a preparation as ours does. It is no sooner taken in than it is swallowed, and the salivary glands, which are still to be found under the tongue, seem only to be there as a matter of form; what little saliva they produce is thick and sticky, and has none of the qualities necessary for making that liquid paste which our tongue sweeps up from every corner of the mouth. Besides, it must be owned that a bird's tongue would be a very awkward implement in such a task. Open a hen's bill and you will see therein a very inferior sort of porter. It is merely a dry hard lance, as it were, armed with prickles at the point, as ill-qualified for tasting as for sweeping. So the hen does not waste her time in finding out the flavor of what is thrown to her. She picks up and swallows over and over again, without appearing to experience any other pleasure than that of satisfying her appetite. Birds of prey, it is true, have rather more convenient tongues, capable, moreover, of tasting up to a certain point; and the parrot, who is a complete epicure, and chews his food philosophically, has a charming-little black one, thick, fleshy, and susceptible—a true porter, in fact-who enables Polly thoroughly to enjoy her breakfast. But certain birds who live on insects surpass even the hen in the dryness and hardness of their tongues. That of the woodpecker, especially, is a model of the kind, and deserves a few words more than the others. Picture to yourselves a long pin, terminated by an iron point with barbs like those of fish-hooks. An ingenious mechanism enables the bird to dart it out with the rapidity of lightning, far beyond his bill, upon the insects to which he gives chase. The point pierces them, and the hooks retain them, without any need of assistance from the bill. I have just told you that this bill pierces the bark of trees; but it only plays the part of gamekeepers on grand sporting occasions, who beat the bushes to make the game rise. The woodpecker's bill routs up the insects by destroying their shelter; but the real sportsman is the tongue. Good-bye to any notion of a cosy little chat in such a porter's lodge as that! What could a harpoon have to say for itself?
Do not, however, let this miserable entrance-hall alarm you, at the same time, for the fate of the mouthful thus presented half-dressed to the sophagus. You will find it only so much the better treated within. In the first place, the sophagus, when half-way down to the stomach, swells out suddenly and forms a pocket, which is generally particularly well developed in birds who feed on grain; this is called the crop in English, in French jabot; whence comes the application of that word to those full shirt-frills which have sometimes been the fashion. It is the pigeon's crop that gives him the rounded chest over which he bridles so prettily. The crop is a receptacle where the food makes a halt: it is something between the pouch of the monkey and the paunch of the ox; a preparatory stomach, which does not, it is true, send back the grain to the bill, for the bill could do it no good, but in which that grain lies until there is room for it further on.
Prom thence it resumes its journey; but, before reaching the true stomach, it passes through a second enlargement of the oesophagus, whose walls are pitted with numberless little cavities, from which pour over it the juices destined to supply the place of the saliva that was wanting above.
It reaches its destination at last, but still hard, and generally whole. No matter, however. The stomach which receives it, and which is called the gizzard, is quite a different sort of thing from a useless membrane, thin and delicate like ours. It is a thick muscle of enormous power, lined inside with a kind of horny skin, so tough that nothing can break through it. You may form some idea of the prodigious strength of this organ, when I tell you that turkey-fowls have been made to swallow hollow balls of glass, so thick as not to break when dropped to the ground, and that at the end of a few days they have been found reduced almost to powder in the uninjured stomach. No fear of indigestion with such an apparatus as that. Though the grain may not have been masticated in the bill, what does it signify? There is a power here, as you see, quite equal to carrying the whole work through. Thanks, indeed, to the invaluable horn which lines it, fowls which have no teeth of their own can safely present themselves with as many and as hard ones as they please. They swallow small pebbles, which rub against the grain, during the contractions of the gizzard, and act just as effectually as if they were fixed in the jawbone. Well, this terrible gizzard performs its crushing work with such energy, that not only the grain but the pebbles themselves are ground down there, and end by being pounded into fine sand. When you rear fowls, do not forget, if you keep them shut up, to put within their reach a store of small pebbles, so that they may have teeth to run to in time of need.
You remember the pylorus—the porter down below, who keeps the door of egress from our stomach? He is as badly provided for here as his fellow-workmen up above; worse in fact. It is a gaping hole, and we cannot expect a very strict supervision from it. Birds who feed on fruits profit by this fact to carry vegetables from one country to another. With such an easy opening, seeds have a good many chances of passing from the stomach unaltered; and then they drop from the clouds, as is supposed, hap-hazard, and germinate afterwards, when circumstances prove favorable, to grow up before the astonished eyes of the natives into plants of which they have never even heard. The French Acclimatization Society, which I spoke of lately, and which, though so modern, has correspondents all over the globe, is at this moment laboring to effect an exchange between all countries of the natural productions of their soil. But here you see that nature had thought of this before, and established her acclimatization society long ago.
To complete the internal work of digestion, so feebly begun in the bill, an extremely large liver pours torrents of bile into the duodenum, and the manufacture of chyle proceeds with that wild rapidity which characterizes all the living actions of birds. But speaking of this liver, I think I ought to give you an account of a celebrated dish, considered a great dainty by epicures, called pâtés de foies gras—fat liver patties, to translate it into its meaning. Very likely you will not care to eat them after hearing my story; but that will be no great loss to you, for it is a very indigestible sort of food, and not at all good for children.
You remember my telling you about Englishmen going to India and coming back with a liver-complaint, from having eaten and drunk more than the climate allowed? By an imitation of this process, human ingenuity—occasionally so cruel—has created the pâtés de foies gras, the glory of Strasburg. I have been in the country, and can tell you how it is managed. They shut a goose up in a square box, where there is just room for his body. They open his bill at feeding-time, and cram down with the finger as much food as can be got in. This is throttling rather than feeding it. The poor beast, who can use no resistance, since it cannot move, and who is kept in the dark to prevent excitement; the poor beast is quite unable to burn all the mass of combustibles with which the blood soon finds itself loaded. This carries them to the liver to be turned into bile; but the liver is not equal to the work, becomes loaded in its turn by unemployed materials, and grows and grows, till at last, having filled up all the space around it, it stops the play of the heart and lungs. When the animal is nearly suffocated they kill it; and this is how we come to have pâtés de foies gras to eat! If they give us a fit of indigestion afterwards, it is a vengeance we richly deserve. At Toulouse, where the same trade is carried on on a large scale, they used formerly to go even beyond this. They fastened the goose by the feet before the fire-place, after having put out its eyes. The imitation of the Englishman's proceeding was still more perfect here, for the fire acted the part of the Indian sun to perfection. I do not know that part of the country well enough to tell you whether they have quite given up this piece of wicked ingenuity; all I can say is, I devoutly hope so.
The intestine of birds is much shorter than that of mammals. Here everything is done at full gallop, and the chyle has not to go far before it is absorbed. I have before me a book, in which I am told that the wagtails eaten in France can be fattened in twenty-four hours, if you only know how to set about it, and these birds are not rare; they belong to the same family as the red-breasts, the tomtits, and the nightingale. Thrushes and wheatears (ortolans) require, for the same purpose, four or five days in the same country, left to themselves to roam about, when the vine keeps open table for them.
This incredible quickness, not only in digesting, but, what is much more, in transforming food into fresh living material (assimilating it, as it is called), has often a fatal result for the bird. He is prohibited from fasting; his life is a fire of straw, which must be replenished unceasingly, or it will die out in the twinkling of an eye. Our own little birds—children—eat oftener than grown-up people, and if by any accident they are kept waiting awhile, they soon cry out with hunger. You know this, do you not? Well, then, if any one should give you a bird to keep in a cage, remember that you have undertaken a great responsibility, and that it will not do to be careless with him. To neglect feeding him for one day is to run the risk of finding him starved to death next morning. With this warning, I will conclude my chapter on birds. I hope I have not spoken in vain in behalf of those poor little captive songsters, whose fragile lives are at the mercy of their young masters and mistresses.