There is a large amount of elastic tissue in the lungs, so that by virtue of their elasticity they can expel a large part of the air which they contain when inflated. A certain amount, however, always remains. As the heart is enclosed in a sort of sack called the pericardium, so are the lungs enclosed in a sack, the pleura, the inner part of which passes over the outside of the lungs and the outer part lines the inside of the chest. In health there is nothing between these two surfaces but a little moisture which helps them to slip easily on each other; a matter of importance, as the lungs have to keep in constant motion and follow the rising and falling of the ribs. If this sack becomes inflamed we have the disease called pleurisy.

In ordinary easy breathing most of the work is done by the action of the diaphragm, a large muscle which separates the abdomen from the chest. It is fastened to the ribs and the back bone and it arches upward; when it contracts it flattens, and by pulling on the base of the chest, makes it larger. This enlargement of the chest is also performed by little muscles between intercostals. They raise the ribs from their oblique position. In ordinary healthy respiration the current of air passes in and out of the lungs through the nose, not through the mouth. It is a serious fact that men breathe more by the action of the diaphragm than women do, in whom the intercostals appear to do a large share of the work of respiration, so that the upper part of the chest rises and falls more in the ordinary breathing of a woman than it does in that of a man.

Hutchingson examined 1897 males in the sitting position and found that in 1753, the respiration varied from 16 to 24 a minute. The proportion of respiratory acts to heart beats is about one respiration to four beats. It is a disorder of this pulse respiration ratio that sometimes causes the distress which men feel when undergoing some great physical exertion. The heart is beating out of proportion to the respirations and the distress continues till the respiratory acts overtake the heart beats and the normal ratio becomes established. A man has then his second wind, as it is called. Digestion has an effect on respiration, the increase being most marked after the largest meal of the day.

Vital capacity is the volume of air which a man can expel from his lungs after making the deepest inspiration of which he is capable. The vital capacity of a men 5 feet 8 inches tall is 230 cubic inches, a variation of 16 per cent. either way is the limit of health.

Hiccough is caused by a spasm of the diaphragm and results sometimes from irritation of the mucous membrane of the stomach, as by alcohol.

The air which we breathe is filled with floating particles of matter of every description; of course various circumstances regulate the quantity and kind. When we breathe air which has once been breathed, we are taking into our lungs air from which the necessary oxygen has been withdrawn and the poisonous carbonic acid substituted. oxygen we can get best from out-door air and from that of the country rather than that of the city.