and prominent; the brain begins to acquire consistence; and, a little later, the beak opens for the first time. The additional growth of a few days shows the wings and the body covered with short feathers; and that which was a little time before a shapeless mass, has now form and proportion. If an egg be opened between the twelfth and thirteenth days, the pulses of the numerous blood-vessels will be seen propelling onwards their contents, and thus presenting a very remarkable and beautiful spectacle. As, on the eighteenth day, the faint piping of the little tenant of the shell is heard, respiration has already taken place. The vesicle, which had increased with the growth of the embryo, has continued to enlarge over the surface of the membrane of the shell, which it now entirely surrounds, thus forming an external covering to the yolk. The large blood-vessels which connect the chick with this membrane -any one of which, if left open, would prove fatal to the bird-begin to be sealed and shrink, and preparation is made for its escape.

On the twenty-first day, the little bird-disengaged from all its protecting membranes, which

a Cervical Vertebra. c Thigh Bone. d Tibia. e Tarsus. Toes.

SKELETON OF A BIRD.

escape.

The first peculiarity which strikes an observer, when comparing the skulls of birds with those of mammals, is the absence of sutures in the former, the proper cranial bones being consolidated into one piece. The skull is articulated to that part of the vertebral column called the neck by a single condyle, or joint, which is situated at the front margin of the great occipital opening, through which the brain, becoming elongated, as it were, into the spinal cord, descends into the vertebral column. The following scale has been given as an example of the size of the brain in relation to that of the body:-Eagle, 1-260th; Sparrow, 1-25th; Blackbird, 1-68th; Duck, 1-257th; Goose, 1-360th; Man, 1-22nd to 1-33rd of the body.

It is this beautiful adaptation of structure to the wants of the animal that gives such a freedom of motion to the head, especially in a horizontal direction. The wryneck,† for exampleas those who have surprised the bird in the nest will readily admit-can writhe her head round so as to look the intruder in the face, hissing all the while like a snake; and, by this "terrible show," many a bird's-nesting novice is frightened away. The number of cervical vertebræ is generally very considerable. In the mammalia there are seven, but in birds there are twice that number; and in the swan there are twenty-three.

The skeleton of birds has the same constituent parts as that of other vertebrated classes. The bones of the anterior extremity, though destined exclusively to support the wing, retain the same divisions, and are composed of the usual elements. The general form of the body is the one best calculated to glide with the least resistance through the air. As birds swallow their food entire, they need no hard and solid teeth, nor the large muscles and heavy jaws, which are required by most quadrupeds: hence, the head is greatly reduced in size, whilst the pointed beak cuts the opposing air, and aids the progress of the bird in its flight. In oirds the ribs, and the viscera which they protect, are placed as far back along the spinal column as possible; and a long and flexible neck extends from the trunk to the head, which is thus carried considerably forwards.

The inferior limbs consist of the thigh-bone or femur (c), which lies close to the body enveloped + Yunx torquilla.

Foramen magnum.

in muscles, covered with the skin, and concealed beneath the feathers. The next part is the tibia (d), consisting of a single elongated bone, and to this succeeds the tarsus (e), covered only with scaly skin, having the toes articulated to the extremity. In most birds, the toes (f) are three before and one behind, but here great variety prevails.

A man would soon be tired who was accustomed to stand on stilts, but a stork does this without any fatigue. This bird, like others, has a provision, by which the legs are kept extended without any exertion of the muscles, in the manner of curtain strings-a structure which enables it to pass whole days and nights on one foot, without the slightest weariness. If the cook be visited the next time she trusses a fowl, the nature and utility of this structure will be seen at once. On bending the legs and thighs up towards the body, it will be observed that the claws close of their own accord: now, this is the position of the limbs in which the bird rests upon its perch, and in this position it sleeps in safety; for the claws do their office in keeping hold of the support, not by any voluntary exertion, but by the weight of the body drawing the strings tight.

In the structure of the body, also, we discover all that could contribute to give it lightness. The general size of birds is considerably smaller than quadrupeds of corresponding habits. Pegasus, that winged steed which sprang forth from the neck of Medusa, after her head had been severed by Perseus, and, flying upwards, fixed his abode on Mount Helicon, is the offspring of imagination alone, and, for anatomical reasons, never could have had a real existence. Great condensation has been given to the osseous substance in birds; ossification, indeed, not only proceeds more rapidly, but is also carried to a greater extent in birds than in any other animals; as a proof of which the tendons, especially those of the muscles of the legs, are frequently ossified. Thus it is ordered that the greatest degree of strength might be procured with the same weight of solid materials; and the mechanical advantage derived from their being disposed in the circumference, rather than in central masses, might be most fully obtained.

In like manner, the horny material of which the feathers are composed are formed into hollow cylinders, which, compared with their weight, are exceedingly strong. A similar shape has been given to the cylindrical bones, which are fashioned into tubes with dense but thin sides. Most of the other bones have likewise been made hollow, and they contain only air. No provision of this kind appears in the bat, winged as it is, for lightening the bones; they, too, contain marrow, as in other mammalia, and not air. Thus, in the case of birds, the whole skeleton is rendered remarkably light; that of the white pelican,* for instance, which is five feet in length, was found by the Parisian Academicians to weigh only twenty-three ounces, while the entire bird weighed nearly twenty-five pounds. The cavities in the bones communicate with large air cells, which are distributed in various parts of the body, and which contribute still further to diminish its specific gravity; and, by means of canals which open into the air passages of the lungs, this air finds a ready outlet, when it becomes rarefied by the ascent of the bird into the higher regions of the atmosphere.

In these creatures alone we find a perfect adaptation of structure to the purposes of rapid and extensive flight. The frame of the skeleton, the figure, position, and structure of the wings, the size of the muscles, their peculiar irritability, and even the outward form of the feathered tribes, have a direct and beautiful relation to the element in which they are designed by the Great Creator to move. Thus, in their formation, a new, and, in so far as relates to the organs of progressive motion, a more developed type is adopted, but still preserving a conformity with the general plan of the vertebral organisation, and with the general laws of its development.

That the body may be exactly balanced while the bird is flying, its centre of gravity must be brought precisely under the line connecting the articulations of the wings with the trunk; for it is at these points that the resistance of the air causes it to be supported by the wings. When the bird is resting on its legs, the centre of gravity must, in like manner, be brought immediately over the base of support formed by the toes it becomes necessary, therefore, to provide means for shifting the centre of gravity from one place to another, according to circumstances, and for adjusting its position with considerable nicety; otherwise there would be danger of the equilibrium being destroyed, and the body upsetting. The principal means of effecting these adjustments consist in the motions of the head and neck, which last is, for that purpose, rendered exceedingly long and flexible. The cervical vertebræ are

* Pelicanus onocrotalus.

joined together by articulations, generally allowing free motion in all directions; that is, laterally, as well as forwards and backwards. This unusual degree of mobility is conferred by a peculiar mechanism, which is not met with in the other classes of vertebrated animals. A cartilage is interposed between each of the vertebræ, to the surfaces of which these cartilages are curiously adapted, being inclosed between folds of the membrane lining the joint; so that each joint is, in reality, double, consisting of two cavities, with an intervening cartilage.

It should be observed, however, that, in consequence of the position of the oblique processes, the upper vertebræ of the neck bend with more facility forwards than backwards, while those in the lower half of the neck bend more readily backwards; hence, in a state of repose, the neck naturally assumes a double curvature, like that of the letter S, as is well seen in the graceful form of the swan's neck. By extending the neck in a straight line, the bird can, while flying, carry forwards the centre of gravity, so as to bring it under the wings; and when resting on its feet, or floating on the water, it can transfer that centre backwards, so as to bring it towards the middle of the body, by merely bending back the neck into the curved form which has just been described, and thus the equilibrium is, under all circumstances, preserved by movements remarkable for their elegance and grace. The great mobility of the neck, too, enables the bird to employ its beak as an organ of prehension for taking its food-an object which was more necessary, in consequence of the conversion of the fore extremities into wings, of which the structure is incompatible with any prehensile power, such as is often possessed by the anterior extremity of a quadruped.

Another advantage arising from the length and mobility of the neck is, that it facilitates the application of the head to every part of the surface of the body. Birds require this power, in order that they may be able to adjust their plumage, whenever it has, by any accident, become ruffled. In aquatic birds, it is necessary that every feather should be constantly anointed with an oily secretion, which preserves it from being wetted, and which is copiously provided for that purpose by glands situated near the tail. The flexibility of the neck alone would have been insufficient for enabling the bird to bring its bill in contact with every feather, in order to distribute this fluid equally over them; and there is, accordingly, a further provision made for the accomplishment of this object in the mode of articulation of the head with the neck. In fishes and in most reptiles this articulation consists of a ball and socket-joint, a rounded tubercle of the occipital bone being received into a hemispherical depression in the first vertebra of the neck. In the mammalia, the plan is changed, and there are two articular surfaces, one on each side of the spinal canal, formed on processes corresponding to the leaves of the first cranial vertebra, and assimilating it more to a hingejoint. In birds, however, where, as we have just seen, the most extensive lateral motions are required, the plan of the ball and socket-joint is again resorted to, and the occipital bone is made to turn upon the atlas by a single pivot. So great is the freedom of motion in this joint, that the bird can readily turn its head completely on its back, on either side.

As spinous or transverse processes of any length would have interfered with the flexions of the neck, we find scarcely a trace of them in the cervical vertebræ of birds. The spinal marrow passes down along the canal formed by the arches of the vertebræ, and any pressure applied to its tender substance would instantly paralyse the whole body, and speedily put an end to life. Some extraordinary provision was therefore required, in order to guard against the possibility of this accident occurring during the many violent contortions into which the body is liable to be thrown. This is accomplished, most simply and effectually, by enlarging the diameter of the canal at the upper and lower part of each vertebra, while, at the middle, it remains of the usual size, so that the shape of the cavity of one of the cervical vertebræ of the ostrich, for instance, resembles that of an hour-glass. Thus, a wide space is left at the junction of each successive vertebra, allowing of very considerable flexion, without reducing the diameter of the canal beyond that of the narrow portion, and therefore

without compressing the spinal marrow. It has been found that vertebræ united in this manner may be bent backwards to a right angle, and laterally to half a right angle, without injury to the inclosed nervous substance. The design of this structure is further evident from its not existing in the dorsal and lumbar portions of the spine, which admit of no motion whatever, and where there is no variation in the diameter of the spinal canal.

The sternum, or breast-bone, is of enormous size, extending over a considerable part of the abdomen, and having a large perpendicular crest descending, like the keel of a ship, from its lower surface. The object of this great development is to furnish extensive attachment to the large pectoral muscles employed to move the wings, and which, taken together, are generally heavier than the rest of the body. Considered with reference to all the other muscles, and to the weight of the body itself, these pectoral muscles are of enormous strength. The flap of a swan's wing is capable of breaking a man's leg, and a similar blow from an eagle has been known to be instantly fatal.

The scapular is, generally, a small and slender bone. The coracoid bone is largely developed, and assumes much of the appearance of a clavicle. But the real clavicles are united below, where they join the fore part of the sternum, appearing as one bone, which, from its forked shape, has been called the furcular bone. In the fowl, it is commonly known by the name of the merry-thought. This bone,

9

c The Crop. g The Gizzard.

placed at the origin of the wings, and stretching from one to the other, is of great importance, as constituting a firm basis for their support, and for securing their steadiness of action; and being, at the same time, very elastic, it tends to restore them to their proper situations, after they have been disturbed by any violent impulse.

If a feather be carefully examined, it will be found composed of three primary parts the quill, the shaft, and the vane. These, with the subordinate parts, are exhibited in the following figures:-In (s) appears the posterior surface of the solid stem, divided

t Trachea. b b Bronchial Tubes. 17 Lungs. into two parts by a longitudinal

m Crescent-shaped Membrane, surmounting

a piece of bone lying across.

groove, and from either side ORGANS OF RESPIRATION AND VOICE. of which proceeds a series of laminæ, composing, with the fibrils, what is termed the vane (v). The lines from which the lamina arise approach one another at the lower part of the stem till they meet at a point. Here the groove terminates, and there is a small orifice (0) leading to the interior of the quill. From this part the transparent, tubular portion of the quill (T) commences; and at its lower extremity (L) there exists a second, or lower orifice. The growth of a feather is exceedingly curious. When a bird leaves the egg, it is covered, except on the under side, with a downy kind of hair. On examining its little garment closely, several little tufts, of ten or twelve hairs each, will be observed, resembling the fibres of a camel-hair pencil. Each tuft grows from a bulbous root in the skin, just as the parts of the future flower arise from the round root of the tulip or crocus. Commonly, the first robe of down soon falls off; but, in the eagle and other rapacious birds, the tufts adhere for a considerable time to the feathers.