Male thorax, frontal view

Chest in mid-inspiration (fig.1), state of near maximal in- (fig. 3) and near maximal expiration (fig. 4). Note the difference of heights of the nipples in-between the different states. The lung-volume in-between maximal in- and expiration may vary as much as 6 liters.

Fig. 2: Normal state of inspiration with marked increase of the muscle tone. Note the distinct bundles of muscle fibers of the pectoralis arising from near the sternum and of the serratus anterior from the right side of the thorax. Their shadows are easily mistaken as due to indentation of the intercostal space. Other sources of shadows are  or "bumps" of subcutaneous fat tissue.

The thoraco-abdominal transition is effaced by contraction of parts of the rectus abdominis arising from near the lower part of the sternum.

Fig. 3:  Note that the sub-costal arch extends up to nearly the highs of the nipples.