the horizon. Should the bird begin to fall it can easily steer itself upward by means of its tail until the motion it had acquired is nearly spent, when by a few more strokes of the wings the impetus is renewed. When alighting a bird expands its wings and tail fully against the air, just as a ship, in tacking round, backs her sails in order that they may meet with the maximum of resistance.
The construction of Birds shows that their eyes are peculiarly adapted to the requirements of their environment. As a defence against external injury from the thickets and hedges in which they pass a great part of their life, and also as a protection against the effects of the light when they are flying in the face of the sun, their eyes are provided with a nictating or winking membrane, or third eyelid, placed below and within the ordinary lids, and moved by two little muscles on the back of the eyeball; this lid is kept moist by a gland which secretes a fluid, and it can be drawn at pleasure over the whole eye like a curtain. This covering is neither opaque nor wholly pellucid, but is somewhat transparent; and it is by its means that the eagle is said to be able to gaze at the sun. "In birds," says a writer on this subject, "we find that the sight is much more piercing, extensive and exact than in the other orders of animals. The eye is much larger in proportion to the bulk of the head than in any of these. This is a superiority conferred upon them not without a corresponding utility; it seems even indispensable to their safety and subsistence. Were this organ in birds dull, or in the least degree opaque, they would be in danger, from the rapidity of their motion, of striking against various objects in their flight. In this case their celerity, instead of being an advantage, would become an evil, and their flight be restrained by the danger resulting from it. Indeed, we may consider the velocity with which an animal moves as a sure indication of the perfection of its vision. Among the quadrupeds, the sloth has its sight greatly limited; whilst the hawk, as it hovers in in the air, can espy a lark sitting on a clod, perhaps at twenty times the distance at which a man or a dog could perceive it."
Respiration.—Of the many peculiarities in the construction of birds, not the least is the means by which they breathe. They do so by the aid of air-vessels extending throughout the body and adhering to the under surface of the bones; these by their motion force the air through the true lungs, which are very small and placed in the uppermost part of the chest, and closely braced down to the back and ribs; the blood is oxidized in the lungs. The arterial circulation of birds is similar to that of mammals, and consists of two auricles and two ventricles; of these vessels, those of the right send the venous, or impure blood, into the lungs for purification; those of the left send the arterial, or pure blood, out for circulation in the body; the blood of birds has a higher temperature than that of mammals, averaging 103° Fahr.
