Section 41. the Mechanism of Respiration Will Be Understood by

reference to Figure 3, Sheet 2. It will be noted, in dissecting that the lungs have shrunk away from the walls of the thorax; this collapse occurs directly an aperture is made in the thorax wall, and is in part due to their extreme elasticity. In life the cavity of the thorax forms an air-tight box, between which and the lungs is a slight space, the pleural cavity (pl.c.) lined by a moist membrane, which is also reflected, over the lungs. The thorax wall is muscular and bony, and resists the atmospheric pressure on its outer side, so that the lungs before this is cut through are kept distended to the size of the thoracic cavity by the pressure of the air within them. In inspiration (or breathing-in) the ribs are raised by the external intercostal (Anglice, between-ribs, e.i.c.m.) and other allied muscles, and the diaphragm (dia.) contracts and becomes flatter; the air is consequently sucked, in as the lungs follow the movement of the thorax wall. In expiration the intercostals and diaphragm relax and allow the elastic recoil of the lungs to come into play. The thoracic wall is simultaneously depressed by the muscles of the abdominal area, the diaphragm thrust forwards, as the result of the displacement and compression of the alimentary viscera thus brought about. (r.r.r. in the Figure mark ribs.)