alone would seem to necessitate many and varied instruments yet all this is made possible, and chiefly executed, by one small portion of the bird its bill or beak." 1

While the size and shape of the bill varies greatly in different kinds of birds, it always consists of two parts (mandibles) (Fig. 46), which correspond in position to the upper and lower jaws of man. When

the bill is opened, a careful examination shows that a bird has no teeth. Some of the birds that lived ages ago, however, had well-developed teeth in their jaws, as is well shown in (Fig. 47) which is a picture of a bird skeleton restored from bones found in the rocks of western Kansas.

FIG. 47. Skeleton of a fossil bird.

Near the base of the bill on either side, one can usually see an opening; these openings are the nostrils. On the sides of the head are the two eyes, and since they bulge out somewhat, the bird is afforded a wide range of vision. If the feathers below and behind the eye are pushed aside, an opening into the ear may be seen; this may be made out easily in the head of a chicken.

51. Wings. In Figure 48 are shown the bones that compose the wing of an ostrich and the arm of a man, and on comparing the two one sees a striking resemblance. In both, the upper arm has a single bone, while in the forearm there

1 Beebe, "The Bird."

are two bones. In the hand region, though the differences are more striking, the general plan of the two is the same. Unlike the bones of the human skeleton those of most birds are hollow and filled with air.

Any one who has eaten a chicken's wing knows that the bones are covered by muscles; these enable the bird to fold

FIG. 48.-A, skeleton of arm of a man; B, skeleton of wing of an ostrich. (A. E. Rueff.)

1

and unfold the parts of the wing, much as the human arm is stretched out or doubled up. On the bird's body are other powerful muscles, which cause the wing as a whole to make the upward and downward. strokes in flight.

Still another wonderful adaptation of the wing for flight is evident in the arrangement and structure of the feathers (Fig. 49). The feathers fit over each other in such

a way that in the downward and backward stroke of the wing a continuous surface is struck against the air, and this propels the bird upward and forward. In the up

1 Before assigning these paragraphs the structure of a feather and the arrangement of the feathers on the wing of some bird (e.g. a chicken) should be demonstrated to the class.

ward wing stroke, on the other hand, the resistance of the air is diminished since the feathers are separated more or

(Photographed by E. R. Sanborn, N. Y. Zoological Park.)

less like the slats of a Venetian blind, thus allowing the air to pass between them.

An examination of a single feather shows that it consists in the first place of a shaft running through its length (Fig.

1 See footnote, p. 66.

50, A). On the sides of the shaft are the two flat surfaces which make up the vane. This vane is composed of slender parts called barbs that may be easily separated from each

leg of man is likewise composed of a relatively thick shin bone, on the outer side of which is a thin bone extending down to the ankle.

A).

The ankle region of a bird is the joint half-way up the leg (Fig. 51, What is commonly regarded as the bird's foot consists often of three toes that point forward, and one that extends backward. Ordinarily the parts of the leg below the ankle are covered with scales, and the tips of the toes are provided with claws.

Secure the egg of a hen or other domestic bird, and study it as follows:

1. Describe the difference in the shape and size of the two ends of

the egg.

2. Carefully crack the shell at the larger end and remove the pieces

of shell.

a. State what you have done and describe the membrane that

lines the shell.

b. Carefully cut this membrane and note that the liquid contents of the egg do not completely fill the eggshell in this region. This cavity is called the air space. Describe the position of this air space.