After an ordinary expiration, about 200 cubic inches are left in the lungs, but by forced expiration about one-half of this may be driven out.  This is known as supplemental air.  The lungs can never be entirely emptied of air, about 75 to 100 cubic inches always remaining.  This is known as the residual air.

The air that the lungs of an adult man are capable of containing is thus composed: 

Complemental air 100 cubic inches. 
Tidal " 30 " "
Supplemental " 100 " "
Residual " 100 " "
——­
Total capacity of lungs 330 " "

If, then, a person proceeds, after taking the deepest possible breath, to breath out as much as he can, he expels: 

Complemental air 100 cubic inches. 
Tidal " 30 " "
Supplemental " 100 " "
——­
230

This total of 230 cubic inches forms what is called the vital capacity of the chest (Fig. 90).

209.  The Movements of Breathing.  The act of breathing consists of a series of rhythmical movements, succeeding one another in regular order.  In the first movement, inspiration, the chest rises, and there is an inrush of fresh air; this is at once followed by expiration, the falling of the chest walls, and the output of air.  A pause now occurs, and the same breathing movements are repeated.

The entrance and the exit of air into the respiratory passages are accompanied with peculiar sounds which are readily heard on placing the ear at the chest wall.  These sounds are greatly modified in various pulmonary diseases, and hence are of great value to the physician in making a correct diagnosis.

In a healthy adult, the number of respirations should be from 16 to 18 per minute, but they vary with age, that of a newly born child being 44 for the same time.  Exercise increases the number, while rest diminishes it.  In standing, the rate is more than when lying at rest.  Mental emotion and excitement quicken the rate.  The number is smallest during sleep.  Disease has a notable effect upon the frequency of respirations.  In diseases involving the lungs, bronchial tubes, and the pleura, the rate may be alarmingly increased, and the pulse is quickened in proportion.

210.  The Mechanism of Breathing.  The chest is a chamber with bony walls, the ribs connecting in front with the breastbone, and behind with the spine.  The spaces between the ribs are occupied by the intercostal muscles, while large muscles clothe the entire chest.  The diaphragm serves as a movable floor to the chest, which is an air-tight chamber with movable walls and floor.  In this chamber are suspended the lungs, the air cells of which communicate with the outside through the bronchial passages, but have no connection with the chest cavity.  The thin space between the lungs and the rib walls, called the pleural cavity, is in health a vacuum.