birds is indicated by comparing a humming-bird with an ostrich. Many of the bones of birds are hollow and contain air. The air-spaces in them connect with air-sacs in the body, which connect in turn with the lungs. Thus a bird's body contains a large amount of air, a condition helpful of course in flight. The breast-bone is usually provided with a marked ridge or keel for the attachment of the large and powerful muscles that move the wings, but in those birds like the ostriches, which do not fly and have only rudimentary wings, this keel is greatly reduced or wholly wanting. The fore limbs or wings are terminated by three "fingers" only; the legs have usually four, although a few birds have only three toes and the ostriches but two. As birds have no teeth with which to masticate their food, a special region of the alimentary canal, the gizzard, is provided with strong muscles and a hard and rough inner surface by means of which the food is crushed. Seed-eating birds have the gizzard especially well developed, and some birds take small stones into the gizzard to assist in the grinding. The lungs of birds are more complex than those of batrachians and reptiles, being divided into small spaces by numerous membranous partitions. They are not lobed as in mammals, and do not lie free in the body-cavity, but are fixed to the inner dorsal region of the body. Connected with the lungs are the air-sacs already referred to, which are in turn connected with the air-spaces in the hollow bones. By this arrangement the bird can fill with air not only its lungs but all the special air-sacs and spaces and thus greatly lower its specific gravity. The vocal utterances of birds are produced by the vocal cords of the syrinx or lower larynx, situated at the lower end of the trachea just where it divides into the two bronchial tubes, the tracheal rings being here modified so as to produce a voice-box containing two vocal cords controlled by five or six pairs of muscles. The air passing through the voice-box strikes against the vocal cords, the tension of which can be varied by the muscles. In mammals the voice-organ is at the upper or throat end of the trachea.