INSPIRATORY MUSCLES. 333 



are slight when compared with those of the upper part of the 

 thorax. This difference is only well marked during quiet, un- 

 conscious breathing; any forced or voluntary respiratory effort 

 changes the typical character of man's breathing, and the costal 

 movements become more prominent. In a forced, deep inspira- 

 tion, the upper part of the chest shows the greatest increase in 

 the antero-posterior diameter in both sexes. 



This difference in type between male and female respiratory 

 movements has been ascribed to different causes. The most 

 common of these is the change brought about by the costume 

 ordinarily adopted by females. This can hardly be an adequate 

 explanation of the phenomenon, for we find the same type 

 existing when the tight garments are removed, and it is apparent 

 in those who have never been constricted by tight clothing, and 

 even in cases where no clothing at all has been used, as among 

 the inhabitants of hot countries; so that, though the corset may 

 induce an exaggeration of the costal respiration, by constricting 

 the lower ribs and interfering with the action of the diaphragm, 

 it would not seem sufficiently to account for the normal physio- 

 logical costal type of breathing found in women. 



The occasional distention of the abdomen during pregnancy 

 has also been assigned as a cause of the female type of breathing. 

 That this type of breathing should be transmitted from our female 

 ancestors is possible, but it is very unlikely that pregnancy is the 

 sole agency in producing it, since in childhood the costal type is 

 marked in both sexes. It is probable that the abdominal breath- 

 ing of the male is also acquired and increased by hereditary trans- 

 mission, and is really due to the gradual increase in the develop- 

 ment of the muscles of the upper extremity in males, causing a 

 greater fixedness of the upper ribs from which they take origin. 



INSPIRATORY MUSCLES. 



The act of inspiration is not performed by any single muscle ; 

 indeed, even the most gentle and quiet respiration requires the 

 coordinated action of many sets of muscles. Most of these mus- 

 cles have other duties to perform besides helping to produce 

 respiratory movements. 



