lungs and airsacs of birds



lungs and airsacs of birds defined in 1930 year

lungs and airsacs of birds - Lungs and Airsacs of birds;
lungs and airsacs of birds - 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 aeration 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 aeration 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 vertebrae; 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 lunga 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 he 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 branchiae 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 bloodvessels 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 ├žair 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 Pterodactyls, 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 aerial and more bird-like Pterodactyls.

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