EXPIRATION 317 



With regard to the relative importance of the diaphragm and the rib-lifting 

 muscles in inspiration, we find among civilized people that in the man the 

 diaphragm plays a more significant part than it does in the woman. At- 

 tempts have been made to relate this difference to the state of pregnancy and 

 the attendant growth of the uterus. But this appears to be true only to a 

 limited extent, for Sewall and Pollak have found that the respiration of 

 Indian women is plainly of the abdominal type. The respiration of growing 

 (European) girls is characterized by Gregor as a combination of the abdom- 

 inal and the thoracic types, with the diaphragmatic part predominant, and 

 with weak action of the shoulder girdle; that of boys as predominantly tho- 

 racic with strong action of the shoulder muscles. In forced respiration of boys 

 the chief auxiliary mechanism called into play is the shoulder muscles, in 

 girls it is the diaphragm. All this means that the actual cause of the feminine 

 type of respiration, often considered as normal, is to be sought in the com- 

 pression of the abdomen by clothes, especially the corset; and this has been 

 confirmed directly by the observations of Fitz. As a consequence of this 

 compression a woman gradually acquires the costal type of respiration, until 

 finally it becomes normal for her. 



We have at present but a single measurement of the absolute value of the 

 diaphragmatic as compared with the costal enlargement of the thorax; namely, 

 out of 490 c.c. of inspired air (in a man) about 320 c.c. devolved upon the eleva- 

 tion of the ribs and only 170 c.c. on the descent of the diaphragm (Hultkrantz). 



3. EXPIRATION 



In ordinary quiet respiration the thorax appears to pass into the expira- 

 tory position principally by mere cessation of the inspiratory phase. When 

 the diaphragm contracts it pushes the abdominal viscera downward and pro- 

 duces an increased tension of the abdominal wall. When it relaxes, it is 

 brought back to its position of rest by the elastic recoil due to this tension. 

 The ribs are brought back from the inspiratory position to their position 

 of rest by the force of gravity and by the elasticity of the cartilaginous con- 

 nections between the ribs and the sternum. Both in the abdominal and the 

 thoracic types of respiration, the return to the expiratory position is aided 

 by the elastic pull of the lungs (cf. page 311). 



Ordinary expiration appears, therefore, not to require any muscular 

 effort. The fact that expiration does not begin suddenly, but gradually, 

 can be explained by saying that the contraction of the inspiratory muscles 

 does not cease all at once, but rather slowly. According to some authors, 

 however, the internal intercostal muscles, which, as we have seen, tend to 

 lower the ribs, are active in ordinary expiration. 



IJnder some circumstances expiration takes place by reason of muscular 

 activity, and the volume of the thoracic cavity is diminished considerably 

 more than is ordinarily the case. This kind of expiration is described as 

 active to distinguish it from the ordinary or passive expiration. It is executed 

 chiefly by the abdominal muscles. 



By the contraction of these muscles (primarily the recti and external 

 oblique, secondarily the internal oblique, and least of all the transversi) the 



