SECT. XVII. PHYSIOLOGY. 129 
SECTION XVII. 
Respiration. 
ccc. The thorax is an air-tight cavity, divided 
into two parts by the mediastinum, and capable 
of dilatation in every direction, except on its 
dorsal aspect. 
ccciv. The mechanical motions of the thorax 
are extremely easy to understand. From the 
oblique direction in which the ribs are articulated 
with the spine, when the intercostal muscles 
contract, the capacity of the thorax is enlarged 
forwards, upwards, and on both sides. 
cccv. But the ribs and their cartilages being 
of unequal lengths, their mobility is subject to 
considerable variation. The second rib being 
longer than the first, necessarily moves in the 
segment of a circle of larger dimensions than it, 
and consequently possesses a greater degree of 
mobility. The mobility of the ribs increases 
from the first to the seventh, and decreases from 
the seventh to the last. 
R 
