Breathing.--In every full breath there are two distinct movements, inspiration (taking air in) and expiration (forcing air out). In man an inspiration is produced by the contraction of the muscles between the ribs, together with the contraction of the diaphragm, the muscular wall just below the heart and lungs; this results in pulling down the diaphragm and pulling upward and outward of the ribs, thus making the space within the chest cavity larger. The lungs, which lie within this cavity, are filled by the air rushing into the larger space thus made. That this cavity is larger than it was at first may be demonstrated by a glance at the accompanying figure. An expiration is simpler than an inspiration, for it requires no muscular effort; the muscles relax, the breastbone and ribs sink into place, while the diaphragm returns to its original position. [Illustration: Apparatus to show the mechanics of breathing.] A piece of apparatus which illustrates to a degree the mechanics of breathing may be made as follows: Attach a string to the middle of a piece of sheet rubber. Tie the rubber over the large end of a bell jar. Pass a glass Y-tube through a rubber stopper. Fasten two small toy balloons to the branches of the tube. Close the small end of the jar with the stopper. Adjust the tube so that the balloons shall hang free in the jar. If now the rubber sheet is pulled down by means of the string, the air pressure in the jar is reduced and the toy balloons within expand, owing to the air pressure down the tube. When the rubber is allowed to go back to its former position, the balloons collapse. [Illustration: Diagram showing the relative amounts of tidal, complemental, reserve, and residual air. The brace shows the average lung capacity for the adult man.] Rate of Breathing and Amount of Air Breathed.--During quiet breathing, the rate of inspiration is from fifteen to eighteen times per minute; this rate largely depends on the amount of physical work performed. About 30 cubic inches of air are taken in and expelled during the ordinary quiet respiration. The air so breathed is called _tidal air_. In a "long" breath, we take in about 100 cubic inches in addition to the tidal air. This is called _complemental air_. By means of a forced expiration, it is possible to expel from 75 to 100 cubic inches more than tidal air; this air is called _reserve air_. What remains in the lungs, amounting to about 100 cubic inches, is called the _residual air_. The value of deep breathing is seen by a glance at the diagram. It is only by this means that we clear the lungs of the reserve air with its accompanying load of carbon dioxide.