SECT. XVII. PHYSIOLOGY. 131 
diaphragm is, at the same moment, made to recoil 
upon the lungs, by the pressure of the atmosphere 
on the parietes of the abdomen. By this com- 
bined pressure on the lungs, the air is again 
expelled. 
cccxi. Mayow compares the motion of the 
lungs and the thoracic parietes, to a bladder in 
the inside of a pair of bellows; the comparison 
would have been more apt, if the bladder con- 
tained a sponge to represent the air-cells of the 
lungs, as well as their expansibility. 
cccx. The suction of the air into the lungs, 
is called inspiration, and its expulsion, expiration ; 
respiration is these two processes taken collec- 
tively. Respiration is generally carried on without 
any effort of the will; but its frequency may, for 
a short period, be modified by volition. 
- cccexin. Although the diaphragm is the prin- 
ciple muscle of respiration, it was found wanting, 
by Diemerbroeck, in a subject seven years old. 
Respiration may again be accomplished by the 
diaphragm, in certain circumstances, without 
much, or any aid from the intercostals. 
cccxiv. Expiration, though generally almost 
a passive motion, may be accelerated by contract- 
ing the muscles of the abdomen and loins, which 
are attached to the ribs. 
cccxv. As the lungs are protected, from the 
pressure of the atmosphere, by the thoracic 
