There are several points of interest in the anatomy of a bird that may be used to bring out general characteristics: (1) The long neck, with vertebræ so adjusted as to allow great freedom of movement, makes the head a convenient prehensile organ. (2) The skeleton is remarkably light. It has fewer bones than other vertebrates; the thin skull bones, the dorsal vertebræ, and bones of the feet being anchylosed.[11] They are also harder, of lighter material, and filled with air, thus having the greatest possible strength with the least weight. (3) To make respiration sufficient during rapid flight, an abundant supply of air not only inflates the lungs as in other animals, but fills little membranous sacs, or cells, distributed through the body, and extends even into the wing feathers. (4) The digestive apparatus differs materially from that of either fishes, reptiles, or mammals. The æsophagus[12] before reaching the sternum is dilated into a large sack, or crop, and serves as a first stomach, in which the food is softened, and prepared for other digestive organs. Below this is another slight enlargement, the walls of which are thicker, and secrete gastric juice. Still further down, the canal is enlarged into a third stomach, the muscular gizzard, in which the process is completed. (5) There are no teeth set in the bone sockets, as their weight would be inconvenient, but the mandibles are sheathed in a horny case, which has sharp edges, and terminates in the beak, or bill. [Illustration: NESTS OF BIRDS.] Both genera and species are very numerous, but any two birds that can be selected differ less in their anatomy than some reptiles of the same order differ. The leading orders only of this class have been selected. _Raptores_[13] (robbers). This is a mild term when used to indicate the ferocity of most birds of prey which not only plunder but destroy. They are almost constantly committing murderous assaults on their weaker neighbors. This is their nature, and accords with their physical structure. The strong hooked bill, powerful legs, feet armed with sharp claws to seize and hold their victims, while the murderous beak is tearing off bits of flesh, are some of their chief characteristics. These murderers have representatives in nearly all countries, but as civilization advances they become less numerous.