276 RESPIRATION. 



thorax. By the simultaneous action of the diaphragm which descends, 

 and of the intercostal muscles which lift the ribs and the sternum, the 

 cavity of the chest is expanded in every direction, and the air passes 

 inward, through the trachea and bronchial tubes, by the force of aspira- 

 tion. 



The action of these two sets of respiratory muscles is indicated exter- 

 nally by two different motions, visible to the eye; namely, an expansion 

 of the chest, due to the action of the intercostals, and a protrusion of 

 the abdomen, caused by the descent of the diaphragm. In children, as 

 well as in the adult male, in the ordinary quiescent condition, the dia- 

 phragm performs most of the work in the act of inspiration ; and the 

 movements of the abdomen are the only ones which are especially 

 marked. Any muscular exertion, however, produces an increased expan- 

 sion of the chest ; and the movement of the ribs, accordingly, becomes 

 more plainly visible after walking or running. In the female the move- 

 ments of the chest, and particularly of its upper half, are habitually 

 more prominent than those caused by the action of the diaphragm; 

 and this difference in the mechanism of respiration is a characteristic 

 mark of the two sexes. 



In certain abnormal conditions the activity of either the intercostal 

 muscles or the diaphragm may be separately suspended, leaving the 

 entire work of respiration to be performed by the remaining set of 

 muscles. If the intercostal muscles be paralyzed, by disease or injury 

 of the spinal cord in the lower cervical or upper dorsal region, the 

 thorax remains quiescent in respiration, while the protrusion of the 

 abdomen is increased in extent to a corresponding degree. This mode 

 of breathing is called abdominal respiration. 



In cases of peritonitis, on the other hand, or any local inflammation 

 within the abdominal cavity, the movements of the diaphragm are some- 

 times restrained, owing to the pain which they excite in the inflamed sur- 

 faces. 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. 



Movement of Expiration. After the movement of inspiration is 

 accomplished and the lungs have been filled with air, the diaphragm 

 and intercostal muscles relax, and a movement of expiration takes place, 

 by which the chest is partially emptied, and a portion of the air con- 

 tained in the pulmonar}'' cavity is expelled. While the movement of 

 inspiration, however, is an active one, accomplished by means of mus- 

 cular contraction, that of expiration is a passive one, resulting from a 

 combination of several forces. The principal one of these forces is the 

 elastic reaction of the lungs themselves, due to the numerous fibres of 

 elastic tissue which enter into the structure of the walls of the pul- 

 monary vesicles and smaller bronchial tubes, and are disseminated gene- 

 rally between the lobules. The existence of this elastic force in the 

 pulmonary tissue is readily demonstrated by removing the lungs from 

 the chest of a recently killed animal, distending them by artificial 



