88 THE VITAL PROCESSES 



The Diaphragm. Another means of varying the 

 thoracic space is found in an organ known as the dia- 

 phragm. This is the dome-shaped, movable partition 

 which separates the thoracic cavity from the cavity of the 

 abdomen. The edges of the diaphragm are firmly attached 

 to the walls of the trunk, and the center is supported by 

 the pericardium and the pleura. The outer margin is 

 muscular, but the central portion consists of a strong sheet 

 of connective tissue. By the contraction of its muscles 

 the diaphragm is pulled down, thereby increasing the 

 thoracic cavity. By raising the diaphragm the thoracic 

 cavity is diminished. 



The diaphragm, however, is not raised by the contraction 

 of its own muscles, but \% pushed up by the organs beneath. 

 By the elastic reaction of the abdominal walls (after their 

 having been pushed out by the lowering of the diaphragm), 

 pressure is exerted on the organs of the abdomen and 

 these in turn press against the diaphragm. This crowds 

 it into the thoracic space. In forced expirations the 

 muscles in the abdominal walls contract to push up the 

 diaphragm. 



Interchange of Gases in the Lungs. During each inspiration the 

 air from the outside fills the entire system of bronchial tubes, but the 

 alveoli are largely filled, at the same time, by the air which the last ex- 

 piratory effort has left in the passages. By the action of currents and 

 eddies and by the rapid diffusion of gas particles, the air from the out- 

 side mixes with that in the alveoli and comes in contact with the mem- 

 branous walls. Here the oxygen, after being dissolved by the moisture 

 in the membrane, diffuses into the blood. The carbon dioxide, on the 

 other hand, being in excess in the blood, diffuses toward the air in the 

 alveoli. The interchange of gases at the lungs, however, is not fully 

 understood, and it is possible that other forces than osmosis play a 

 part. 



Capacity of the Lungs. The air which passes into and from the 

 lungs in ordinary breathing, called the tidal air, is but a small part of 



