RESPIRATION. 237 



walking or running. In the female, the movements of the chest, par- 

 ticularly of its upper half, are habitually more prominent than those of 

 the abdomen ; and this difference in the mechanism of respiration is 

 characteristic of the sexes. 



In certain abnormal conditions the activity of either the intercostal 

 muscles or the diaphragm may be separately suspended, leaving the 

 work of respiration to be performed by the remaining set of muscles. 

 If the intercostals be paralyzed by injury of the spinal cord in the 

 lower cervical or upper dorsal region, the thorax remains quiescent, 

 while the protrusion of the abdomen is increased to a corresponding 

 degree. This mode of breathing is called abdominal respiration. 



In cases of peritonitis, on the other hand, the movements of the 

 diaphragm are restrained, owing to the tenderness of the inflamed sur- 

 face. This is known as thoracic respiration ; since the expansion of 

 the chest becomes more active than usual, and is the only visible move- 

 ment performed. 



Jlovement of Expiration. After inspiration is accomplished and 

 the lungs are filled with air, the diaphragm and intercostal muscles 

 relax, and a passive movement of expiration takes place, by which the 

 pulmonary cavity is partially emptied. It is mainly accomplished by 

 the elastic reaction of the lung tissue, which compresses the pulmonary 

 lobules and vesicles, and expels a portion of the contained air. This 

 elasticity is readily shown by removing the lungs from a recently killed 

 animal, distending them by insufflation through the trachea, and then 

 allowing them to collapse. They react, under these circumstances, with 

 sufficient power to expel the larger portion of the injected air. Other 

 organs, during life, aid in the process. The elastic costal cartilages, 

 slightly twisted in inspiration by the elevation of the ribs, resume 

 their original form in expiration, and, by drawing the ribs downward, 

 compress the thorax. Lastly, the abdominal organs, displaced by the 

 descent of the diaphragm, are forced backward by the elasticity of the 

 abdominal walls and of their own fibrous attachments, carrying the 

 relaxed diaphragm before them. By the recurrence of these two move- 

 ments, of inspiration* and expiration, fresh portions of air are alternately 

 introduced into and expelled from the pulmonary cavity. 



All the air in the lungs, however, is not changed at each movement. 

 A considerable quantity remains behind after the most complete expira- 

 tion ; and even when the lungs have been removed from the chest, 

 they still contain a certain volume of air, which cannot be displaced by 

 any violence short of disintegrating the pulmonary tissue. Only a 

 comparatively small portion of the air, therefore, passes in and out 

 with each respiratory movement ; and its complete renewal will require 

 several successive respirations. The relation in quantity between the 

 air changed at each respiration and that contained in the chest varies 

 with different conditions ; but the average results obtained by different 

 observers show that, in general, the volume of the inspired and expired 

 air is from 10 to 13 per cent, of the whole contents of the pulmonary 



