Respiration is carried on by means of the

Mouth.                                    Trachea.

Nose.                                       Bronchi.

Larynx.                                   Lungs.

An adult breathes from sixteen to eighteen times per minute, and breathes more frequently when moving or even standing than when lying down. At every inspiration the cavity of the chest is enlarged in every direction. From above downwards by the contraction and depression of the diaphragm, and transversely by the raising of the ribs. The air passes in through the trachea to the lungs, expands them, and keeps their sides in contact with the walls of the chest, and their bases in contact with the diaphragm.

Expiration by expulsion of air diminishes the cavity of the thorax. The expired air is warmer and more moist than the inspired air. It contains carbonic acid and other effete matter.

The lungs are never empty ; after the most forced expiration some air is still left, and is called residual or stationary air. īn ordinary breathing a man takes in about a pint, or 20 to 30 cubic inches of air. This is called tidal air. In forced inspiration more can be taken in, which is called complemental air. The tidal air carries oxygen into the residual air, and takes from it carbonic acid, etc. The air in the vesicles and the blood in the capillaries are separated only by the thin walls of each, so that carbonic acid and other waste matter pass through these thin walls, and are carried off by expiration. Oxygen from the air in the vesicles also passes through these walls, and is taken up by the red corpuscles and carried to the tissues.

Deep breathing aids the flow of blood and of lymph by pressure on the large blood vessels and thoracic duct.

Muscles which assist the diaphragm in respiration :

Inspiration.

Ordinary:
Intercostals.
Levators costorum.
Scaleni.

Deep:
Trapezius.
Pectoralis minor.
Serrati.
Rhomboidei.
Sterno-mastoid.
Ilio-costalis.
Quadratus lumborum.

Expiration

Ordinary:
Return of the walls of the thorax by their own elasticity and to that of the lungs.

Forced:
Abdominal muscles.
Intercostals.
Triangularis sterni.