332 THE ANATOMY OF THE WEDDELL SEAL. 
requires the minimum of muscular effort. The differences between the adult human 
male and female types of breathing are adaptations due to the avoidance of severe 
muscular effort so long as a smaller effort will serve the purpose, and in my opinion 
they result from the normal differences in the lower or diaphragmatic diameters of the 
thoracic cavity in the two sexes. To a large extent these differences may be accounted 
for by the width of the female false pelvis as compared with that of the male. Since 
the abdomen proper (?.e. excluding the true pelvis) contains no organs which are not 
common. to both sexes, it follows that a wide false pelvis, by providing increased 
accommodation in the lower abdominal regions, is naturally associated with a reduction 
in the dimensions of the upper or diaphragmatic end of the abdomen. These conditions 
make elevation of the sternal ribs more necessary in the female than in the male, whose 
larger diaphragmatic thorax permits of ordinary breathing without the pronounced or 
visible elevation of his sternal ribs, although their movement may become visible when- 
ever the supply of air required calls for an extension of the elevating movement. 
In either sex, a change from the erect attitude to the horizontal (e.g. to the supine) 
position is usually followed by the introduction of more or less of the respiratory 
features of the opposite sex, owing to the temporary interference with the amount of 
rib movement in common use when the ribs are unobstructed. 
