last named organs are likewise proportionally very large, arranged along on each side of the spine, and occupying the hollows between the bases of the ribs. By this great development of the respiratory system, the blood is more rapidly and effectually oxygenised, and muscular energy greatly increased for the action of flight; while by the animal heat thus evolved the air contained in the complex apparatus described is rarified, and so the body is increased in bulk, but relatively diminished in weight.

The wings of a Bird correspond to the arms and hands of man; but the hand is composed of but two fingers and a thumb, all of which are rudimentary. From the bones of the hand arise the primaries, or quill-feathers properly so called, which are ten in number; these are the largest and strongest feathers of the wing, and the character and power of flight is indicated by their form, stiffness, and relative length. From the principal bone of the fore-arm arise the secondaries, the number of which varies in different species; they are usually shorter, broader, and more flexible than the primaries; and are less removed in form from the general clothing feathers of the body. The bone of the upper-arm (humerus) gives rise to another series of feathers, called tertiaries: these, in some birds, particularly the Plovers, Curlews, &c., are greatly lengthened; they are, however, still weaker in their structure

than the secondaries. Attached to the little bone which represents the thumb are two or three short and very stiff feathers, called the winglet; they lie upon the basal part of the first primaries at the very edge of the wing. Corresponding to these series of feathers there are, both on the outer and inner surface of the wing, several rows of smaller ones, called coverts, from their office of covering the basal part of the quills.

The feathers of the wing, overlapping each other, present a continuous surface of great breadth, with which the repeated strokes upon the air are performed, which constitute flight. Each feather is concave, whether we regard it transversely or longitudinally; its stem, or midrib, is remarkably strong, though very light, and the beards, which present their edges in the direction of the stroke, are linked to each other by a series of minute hooks. All of these provisions increase the power of the wing in its downward strokes upon the resisting air.

To use these broad fans with sufficient force to impel the bird through the air, large and vigorous muscles are required. Accordingly, in Birds, particularly those of long and powerful flight, the greatest portion of the whole muscular force of the animal is concentrated upon these organs. The muscles which produce the downward stroke of the wing are enormous; and, for their attachment, the breast-bone is not only

greatly enlarged, but its surface is still further increased by its medial portion rising into a high perpendicular keel or ridge, the two faces of which, from their direction, afford a point of resistance, or purchase, of peculiar advantage.

To resist the tendency of the shoulders to be drawn together by the powerful muscular action exerted during flight, there is inserted between the two bones (coracoids) to which the shoulder-blades are attached, a singular bone of an arched form, well known as the merry-thought. In the common fowl, which flies but little and weakly, this bone is feeble, but in birds of vigorous flight, as the Hawks, the Swallows, and the Humming birds, it is very strong and elastic. On the other hand, where the bird never rises upon the wing, as in the case of the Ostrich, Emu, and kindred birds, this bone is reduced to a mere rudiment.

The posterior extremities also differ materially in structure from those of quadrupeds. The general number of toes is four, which is never exceeded; but a few birds have but three, and the Ostrich but two. For the most part, three toes are directed forwards, and one, answering to the great toe, backwards. The climbing birds, as the Parrots and Woodpeckers, have the outer toe also reversed; while the Swifts have all the four turned forwards. The feet are much more lengthened than in the Mammalia generally,

especially the tarsal portion, which is never applied to the ground in the action of walking.

Though many birds feed on hard substances, their jaws are entirely destitute of teeth; but are more or less extended into two mandibles, and incased in horny coverings of considerable density, which are exceedingly diversified in form and use. "Thus in birds of prey [the beak] well executes the office of a dissecting knife; in seedeating birds it forms a pair of seed-crackers for extricating the kernel from the husk which envelopes it; in the Swallows and Goat-suckers it is a fly-trap; in the Swans, Geese, and Ducks, it is a flattened strainer, well furnished with nerves in the inside for the detection of the food remaining after the water is strained, by that particular operation which every one must have observed a common duck perform with its bill in muddy water. In the Storks and Herons we find it a fish-spear; and in the Snipes and their allies it becomes a sensitive probe, admirably adapted for penetrating boggy ground, and giving notice of the presence of the latent worm or animalcule." The stomach in Birds consists of three parts (which are not, however, in all cases distinctly developed); the crop or craw, the membranous stomach or proventriculus, and the gizzard. The last, which is seen to advantage in the grain-eating birds, is composed of two very

Penny Cyclop.; Art. BIRDS.

dense and powerful muscles of a hemispherical form, whose flat faces, coated with a thick skin, work over each other like a pair of millstones, and by the aid of small angular stones, sand, &c., swallowed for the purpose, grind down the hardest substances in a very short time.

With the exception of the beak and the hinder extremities, every part of a Bird is, for the most part, clothed with feathers. The feet are protected by a naked, scaly skin, which in some species extends partly up the leg (tibia).

The soles of the toes are covered with a callous modification of this skin, having a granular surface. The plumage of Birds attracts universal admiration, for its beautiful fitness for the ends it answers, for its softness, its smoothness, its compactness, and for its ever varied hues. The most brilliant colours in nature are lavished upon the feathers of these tenants of the sky; embellished and set off in some instances with a peculiar reflection that rivals the lustre of burnished metals, or the radiance of precious stones.

Every one is familiar with the general form of a feather. “When a bird has just left the egg, its covering is a downy kind of hair, several little bundles taking their rise from one common bulb. This is the origin of the future feather. A dark cylinder soon makes its appearance, from the upper extremity of which the sprouting feather emerges, while the lower extremity receives the