=The Internal Organs, or Viscera= (Figs. 304 and 305).--The viscera (vis′se-ra), as in most vertebrates, _include_ the food tube and its glands; the lungs, the heart, and larger blood vessels; the kidneys and bladder and the reproductive organs. The lower part, or gullet, is enlarged into a _crop_. It is largest in grain-eating birds. It is found in the V-shaped depression at the angle of the wishbone, just before the food tube enters the thorax. The food is stored and softened in the crop. From the crop the food passes at intervals into the glandular stomach. Close to this is the muscular stomach, or gizzard. Are the places of entrance and exit on opposite sides of the gizzard, or near together? (Fig. 304.) Is the lining of the gizzard rough or smooth? Why? Is the gizzard tough or weak? Why are small stones in the gizzard? Why do not hawks and other birds of prey need a muscular gizzard? The liver and pancreas empty their secretions into the intestines by several ducts a little way beyond the gizzard. Beyond the mouths of two cæca (Fig. 305) the many-coiled intestine empties into the straight rectum, which terminates in a widened part called the cloaca. Not only the intestine, but the two ureters of the urinary system and the two genital ducts of the reproductive system all empty into the cloaca (Figs. 304, 305). [Illustration: FIG. 306.--POSITION OF LUNGS AND AIR SACS (Pigeon). _Tr_, windpipe; _P_, lungs; _Lm_, sac under clavicle with prolongation (_Lh_) into humerus; _La_, sacs in abdomen.] The =lungs= have their rear surfaces attached to the spinal column and ribs (_lu_, Fig. 304). They are connected with thin-walled, transparent _air sacs_ which aid in purifying the blood. When inflated with warm air, they probably make the body of the bird more buoyant. For the names, location, and shape of several pairs of air sacs, see Fig. 306. The connection of the air sacs with hollows in the humerus bones is also shown in the figure. Many of the _bones are hollow_; this adds to the buoyancy of the bird. The pulmonary artery, as in man, takes dark blood to the lungs to exchange its carbon dioxide for oxygen. Of two animals of the same weight, which expends more energy, the one that flies, or the one that runs the same distance? Does a bird require more oxygen or less, in proportion to its weight, than an animal that lives on the ground? Are the vocal cords of a bird higher or lower in the windpipe than those of a man? (Fig. 307.) The heart of a bird, like a man’s heart, has four chambers; hence it keeps the purified blood separate from the impure blood. Since pure blood reaches the organs of a bird, oxidation is more perfect than in the body of any animals yet studied. Birds have higher temperature than any other class of animals whatsoever. Tell how the jaws, tail, and wings of the fossil bird Archæopteryx differed from living birds (Fig. 290). [Illustration: FIG. 307.--POSITION OF VOCAL CORDS (_str_) OF MAMMAL AND BIRD.