200. The amount of muscular power required for flight in the air is not commonly appreciated. If we look at the breadth of wing in a bird, as compared with the size of the animal when stripped of its feathers, we can have some idea of the extent of wing which a man would need to enable him to fly. And to work efficiently such enormous wings as he would require, he must have enormous muscles. Those which move the arms of the most broadchested and brawny man are far from being large enough to enable him to fly, even if he had wings. To do this, he must have the keel on the breast-bone, like the bird, to afford an attachment for a thick mass of muscle. We see, then, why it is that all the attempts which men have made to fly have proved failures. It is not that the wings have not been properly made, but that there was not suf ficient muscle to work them.

201. As flying requires such strong exertion, it is important that the Bird should be as light as possible. There is a singular contrivance for this purpose. The air taken into the lungs does not all stop there, but some of it passes thence into cells or sacs in different parts of the body, and also into many of the bones, which are hollow for this purpose. This air apparatus is in extent proportionate to the powers of flight. Thus, in the Eagle, the air goes into all the bones, while in the Ostrich and the Penguin it goes only into the thigh-bones.

202. The digestive organs of the Bird are very peculiar. They are the only animals that have a gizzard. This organ is a stomach, which has on its inside a lining as tough and hard as leather. This is for the purpose of bruising and rubbing the food, which is done by the action of very stout muscles. These constitute the bulk of the gizzard; and they are so arranged that they squeeze and rub two opposite surfaces of the inside lining against each other. The food is therefore ground in the same manner as grain is between the millstones of a flour-mill. The power of this grinding apparatus is made still more

effectual by sand and small stones, which the Bird swallows with its food. In Fig. 99 you see the gizzard of a Turkey cut open. You observe the two semi-globular

a

b

Fig. 99.-Gizzard of the Turkey.

masses of muscle, and the lining covering them on the inside of the organ. While these grind the food, the gastric juice which digests it is all the time trickling down upon it from the gullet at a, where it oozes out from a great many little openings.

203. This grinding operation of the gizzard takes the place of the mastication which is done by those animals

that have teeth; the Bird using its bill only for gathering its food, and not for masticating it. The arrangement described does not exist in full in all birds, but only in those that live on grains, termed granivorous birds. In other birds it varies according to the nature of the food. In those that live altogether on flesh, or on fishes, there is no real gizzard, but a thin and membranous stomach, for there is no need in them of any grinding and crushing process.

a

b

204. There is one part of the digestive apparatus of birds yet to be noticed. Before the food is subjected to the grinding of the gizzard, it is macerated or soaked for some time in the crop, as it is called, a sac or pouch which opens into the gullet. When the grains are first swallowed, they are passed into the crop; and when they are sufficiently macerated, they are forced out of the crop, down the gullet, into the gizzard to be ground. The crop, you see, is to the Bird what the paunch is to the Ruminant quadruped (§ 154), a convenient receptacle for the food, and a place for its maceration. In Fig. 100 you have a representation of the parts mentioned, a being the gullet, b the crop, c that part of the gullet where the gastric juice is made, and d

d

C......

Fig. 100.

the gizzard.

205. The incubation, or hatching of eggs, requires different periods in different species of birds. In the Humming-birds it is but twelve days, in the Canaries fifteen to eighteen, Fowls twenty-one, Ducks twenty-five, and Swans forty to forty-five. The object of sitting on the eggs is simply to provide the requisite amount of heat. The same degree provided in any other way will answer, and eggs have often been hatched by steam. The heat of the sun is sufficient to hatch the eggs of some birds

living in the tropics, as the Ostrich. The popular story about this bird is not true. There is no neglect on her part when she leaves her eggs in the sand, for when she is in a temperate climate, where the heat of the sun is not sufficient to hatch them, she sits on them. The Mound birds of Australia have a singular way of providing for heat in hatching their eggs. Instead of sitting on them, they place them in mounds of decaying vegetable matter, which they heap up for this purpose. The process of decay produces all the heat that is requisite. Most birds make nests, not to live in, but to hatch their eggs, lining them commonly with some soft material. The Eider-duck lines her nest with down which she strips from her own breast.

206. The formation of a feathered animal from the simple contents of an egg by the stimulus of heat is one of the most wonderful things in nature. When the bird is fully formed, it cuts its way out of the shell with an instrument furnished it for this purpose, a pointed scale fastened to the end of its beak. Any one can readily see this on the upper bill of the newly-born chicken. Soon after its birth this scale drops off, as the chicken has no farther use for it.

207. The senses which are most developed in birds are the sight, smell, and hearing. The sense of touch in most of them is very slight; but some, as the Duck tribe, have quite an acute sense of touch in their bills, guiding them in their search for food. The sense of taste is also, in most birds at least, very slight. The sight is generally acute, especially in birds of prey. Birds have a kind of third eyelid inside of the others, called the nictitating or winking membrane. It is very thin, and is commonly folded up in the corner of the eye out of sight, but it can be drawn over the whole front of the eye when it is needed. The bird can see through it, and the object of it is to diminish the light that enters the eye when it is very intense. It is this which

F

enables the Eagle, and other birds also, to look directly at the sun. The sense of smell is very acute in all birds in which it can be of service in searching for food, as, for example, in those that live on carrion. While all birds have ears, there is only one kind, the Owl tribe, that has any external ear. In all others there is merely an opening to the passage leading to the internal apparatus of hearing, and even this is concealed among the feathers of the head.

208. Birds are digitigrade, § 92. You can see this to be true in the case of the Ostrich, Fig. 4, if, comparing the bones of the leg with the same bones in man, Fig. 1, you begin at the thigh-bone and go downward. In Fig. 101 you have the bones of a bird's leg, a being the thigh

bone, b the bones of the leg proper, c the heel-bone, long and extending upward, and d the bones of the foot. In Fig. 102 is the outline of the leg of a man, with letters to correspond with those of Fig. 101, that you may readily make the comparison. In Fig. 103 you have the perching apparatus of birds represented, and you can see how it is that they can sleep on their perches without falling off. There is, you observe, a large muscle in front of the thigh-bone; from this a long tendon or cord, A, extends down the leg, and in the foot it divides into branches, which go to all the toes. When the muscle pulls on this the toes will all be bent, as every body