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HANDBOOK OF PHYSIOLOGY. 



more so in the upper, part of the chest (superior costal type). (See 

 Figs. 158, 159.) 



B. Expiration. From the enlargement produced in inspiration, 

 the chest and lungs return in ordinary tranquil expiration, by their elas- 

 ticity; the force employed by the inspiratory muscles in distending the 

 chest and overcoming the elastic resistance of the lungs and chest-walls, 

 being returned as an expiratory effort when the muscles are relaxed. 

 This elastic recoil of the chest and lungs is sufficient, in ordinary quiet 

 breathing, to expel air from the lungs in the intervals of inspiration, 

 and no muscular power is required. In all voluntary expiratory efforts, 

 however, as in speaking, singing, blowing, and the like, and in many 

 involuntary actions also, as sneezing, coughing, etc., something more 

 than merely passive elastic power is necessary, and the proper expiratory 



Fia. 158. 



FIG. 159. 



Fm. 158. The changes of the thoracic and abdominal walls of the male during respiration. 

 The back is supposed to be fixed, in order to throw forward the respiratory movement as much as 

 possible. The outer black continuous line in front represents the ordinary breathing movement: 

 the anterior margin of it being the boundary of inspiration, the posterior margin the limit of expi- 

 ration. The line is thicker over the abdomen, since the ordinary respiratory movement is chiefly ab- 

 dominal: thin over the chest, for there is less movement over that region The dotted line indicates 

 the movement on deep inspiration, during which the sternum advances while the abdomen recedes. 



FIG. 159. The respiratory movement in the female. The lines indicate the same changes as in 

 the last figure. The thickness of the continuous line over the sternum shows the larger extent of 

 the ordinary breathing movement over that region in the female than in the male. (John Hutchin- 

 son.) 



The posterior continuous line represents in both figures the limit of forced expiration. 



muscles are brought into action. By far the chief of these are the ab- 

 dominal muscles, which, by pressing on the viscera of the abdomen, 

 push up the floor of the chest formed by the diaphragm, and by thus 

 making pressure on the lungs, expel air from them through the trachea 

 and larynx. All muscles, however, which depress the ribs, must act also 

 as muscles of expiration, and therefore we must conclude that the abdom- 

 inal muscles are assisted in their action by the greater part of the internal 



