The Respiratory System 155 



Between each neighboring set of ribs (Fig. 2)2,A) are two sets of 

 muscles : one, the external intercostals, slant downward and forward ; 

 the other, the internal intercostals, slant downward and backward. The 

 diaphragm is a large dome-shaped muscle which separates the abdominal 

 and thoracic cavities. Its origin is along the body wall just below the 

 ribs, with the insertion on a central tendon. 



During inspiration, the diaphragm flattens and lowers by contrac- 

 tion of its fibers, making the thoracic cavity longer. At the same time, 

 the external intercostals contract and pull the ribs upward, the sternum 

 pushes forward, and the lateral diameter of the chest cavity is thus 

 increased. These movements result in an enlargement of the chest 

 cavity, decreasing the air pressure in the lungs and permitting air to 

 move rapidly into the lungs. The lungs themselves play no active 

 part in this process. They are simply elastic bags which follow the 

 movements of the thoracic walls. 



Expiration of air is usually a passive process although it may be 

 active during a forced expiration. Most often, expiration occurs due 

 to a relaxation of the diaphragm and the external intercostals. The 

 chest cavity capacity is thus reduced, and the air is forced outward. 

 During a forced expiration, the diaphragm moves upward due to the 

 contraction of the muscles of the abdominal wall, and the internal 

 intercostals pull the sternum down and in and also pull the ribs down. 

 These two actions combine to reduce the size of the thoracic cavity. 



Physical measurements of air pressures within the lungs demonstrate 

 that this interpretation of the mechanics of respiration is correct. At sea 

 level, air pressure is ordinarily 760 mm. of mercury. At the end of 

 an inspiration, the pressure within the lungs is 751 mm., sufficient to 

 cause air to move inward ; at the end of expiration, the air pressure 

 within the lungs is 754 mm. This is a difference of but 3 mm. of 

 pressure, and is typical of quiet breathing. During forced inspiration, 

 the pressure within the lungs may fall from 5 to 10 mm. 



The air as it is expired has the percentages of the various gases 

 different from those of inspired air. The nitrogen remains the same 

 (78 per cent), but the oxygen forms but 16 per cent and the carbon 

 dioxide rises to 4 per cent from 0.04 per cent. Expired air usually 

 has a higher content of water vapor than does inspired air. 



Control of Rate of Respiration. — The rate of respiration is con- 

 trolled by chemical and nervous means. Both the intercostal muscles 

 and the diaphragm are innervated by nerves from the central nervous 



