Expiration: Upon the relaxation of the inspiratory muscles (the muscles whose contraction has brought about the thoracic expansion), the elasticity of the chest walls and lungs, aided by the contraction of certain muscles and other circumstances, causes the chest to return to its original size, or even become smaller. In consequence of this the pressure within the lungs now becomes greater than that outside, and thus air rushes out of the trachea, until equilibrium is once more established. This constitutes expiration. The inspiratory and expiratory act together form a respiration. The fresh air introduced into the upper part of the pulmonary passages by the inspiratory movement contains more oxygen and less carbonic acid than the old air previously present in the lungs. By diffusion the new or tidal air, as it is frequently called, gives up the oxygen to, and takes carbonic acid from, the old or stationary air, and thus when it leaves the chest in expiration has been the means both of introducing oxygen into and of removing carbonic acid from it. By this ebb and flow of the tidal air and the diffusion between it and the stationary air, the air in the lungs is being continually renewed, through the alternate expansion and contraction of the chest. In what may be considered normal breathing, the respiratory act is repeated about seventeen times a minute; and the duration of the inspiration as compared with that of the expiration and such pause as exists, is about as ten to twelve. When the ordinary respiratory movements prove insufficient to effect the necessary changes in the blood, their rhythm and character become changed. Normal respiration gives place to labored respiration, and this in turn to dyspnoea, which unless some restorative event occurs terminates in asphyxia. Changes of the air in respiration: 1. The temperature of the expired air is variable, but under ordinary circumstances is higher than that of the inspired air. 2. The expired air is loaded with aqueous vapor. 3. The expired air contains about 4 to 6 per cent less oxygen and about 4 per cent more carbonic acid than the inspired air, the quantity of nitrogen suffering but little change. Thus: Oxygen. Nitrogen. Carbon. Inspired air contains 20.81 79.15 .04 Expired ,,     ,,    16.033 79.557 4.380