The act of respiration. The organs concerned in breathing are the nostrils, the wind-pipe and the lungs. The wind-pipe is a stout tube, divided below into two tubes, one of which goes to the right and the other to the left lung. When the ribs are elevated and the diaphragm is depressed, there is a tendency to produce a vacuum between the lungs and the wall-chest. The air forces its way into the air-passages of the lungs, and expands the lung tissue so that it fills the enlarged space within the chest. This is inspiration. When the ribs and diaphragm return to their passive condition, the pressure of the air ceases, and the elastic tissue contracts, forcing the air out. This is called expiration, and the whole act respiration. At every inspiration we draw into the lungs rather more than half a pint of cool, fresh air. At every expiration we send out the same quantity of hot, foul air. Air that has been breathed once is found to have lost about one-twentieth of its oxygen, and to have gained as much carbonic acid gas. Such air is not fit to be breathed again. While we are in the open air there is little fear of our being compelled to breathe the same air twice ; but in rooms it is necessary to see that there are openings for the impure air to pass out, and other openings to allow fresh air to get in. If we reckon that we breathe fifteen times per minute, it can be readily calculated that an ordinary adult takes into his body from the air, by means of his lungs, 11/2 lbs. of oxygen daily, and gives to the air a rather greater amount of carbonic acid gas. The frog has no ribs, but simply closes its lips and swallows the air which is in its mouth. Turtles swallow the air in the same way as frogs. Fishes get all the air they need from the water, which enters freely at the mouth and passes over the gills, and escapes at the gill slit - the oxygen from the water being absorbed by the blood of the gills. A fish out of water dies for want of oxygen, which it can take from water but cannot take from air, and so it is suffocated by air. Lobsters breathe only by gills, which are situated in a cavity under the body and attached to the legs, the action of the legs and of a spoon-shaped appendage causing a current of water to pass over the gills.which absorb the needed oxygen. Insects breathe by air-tubes that pass through every part of their body, and open on the surface of the body in small holes, which exclude water or dust, but admit air. (See Lungs.)

AIR CELLS AND CAPILLARIES OF A HUMAN LUNG (MAGNIFIED).

AIR-CELLS AND CAPILLARIES OF A HUMAN LUNG (MAGNIFIED).