The service of air to the bones in most parts of the body by the air-sacs, as just shown, is in special cases otherwise rendered. The Eustachian tubes furnish air to the bones of the cranium and to the upper jaw; while the lower jaw receives air from the pneumatic foramen situated upon each ramus behind the tympanic articulation, and from an air cell which surrounds the joint. The cavities of the embryonic bones, which afterward become pneumatic, are filled with marrow. Selenka states that the invasion of the bones by the air is a late development, and that in the humerus this invasion occurs after the twenty-second day in the life of the chick. Hunter and Compar, who have made extensive researches, consider the function of the air-sacs as threefold. First, the air-sacs are subsidiary respiratory organs, which aid in ridding the blood of waste products and in taking in oxygen. Second, they aid mechanically the actions of respiration in birds. During the act of inspiration the sternum is depressed, the angle between the vertebral and the sternal ribs is made less acute, and the thoracic cavity proportionately enlarged; the air then rushes into the lungs and into the thoracic receptacles, while those of the abdomen become flaccid. When the sternum is raised, or approximated toward the spine, part of the air is expelled from the lungs and the thoracic air-sac through the trachea, and part is driven into the abdominal receptacles, which are thus alternately enlarged and diminished with the expansion and the contraction of the thorax. Hence the lungs, notwithstanding their fixed condition, are subject to due compression through the medium of the contiguous air receptacles, and are affected equally and regularly by every motion of the sternum and of the ribs. Third, they reduce decidedly the specific gravity of the whole body. This must necessarily follow from the large spaces filled with air as well as from the absence in the bones of marrow and other fluids. The air-sacs by their position also render equilibrium more stable. ANGIOLOGY =The Circulatory Apparatus.=—The circulatory apparatus consists of two tubular systems: the blood vascular system and the lymphatic system. The blood vascular system consists of the heart, the arteries, the veins, and the capillaries. The heart is the central, propelling organ. The arteries form a series of efferent tubules, which, by branching, constantly increase in number and decrease in caliber, and which serve to carry the blood from the heart to the tissues. The capillaries are extensions from these latter tubules into which the arteries empty, and through the walls of which the interchange of elements between the blood and the other tissues takes place. The veins form a system of converging tubules which receive the blood from the capillaries, decrease in number and increase in size as they approach the heart, and return the blood to that organ.