THE MECHANISM OF RESPIRATION. 
605 
raising of the piston in the barrel of the pump, or the elastic dila¬ 
tion of the heart, a vacuum will be formed, or there will be an 
irregularity of atmospheric pressure ,* there will be diminished re¬ 
sistance to the enormous pressure of the air; and the air will 
rush down the windpipe, and fill the cells, and expand the 
lungs, until they once more completely fill the chest, and the 
equilibrium of pressure is restored. 
The intercostals act in an indirect manner, very little assisting 
in respiration in natural breathing, but preserving the ribs from 
closing by the action of the diaphragm. They are antagonists to 
the diaphragm, and, all together, are stronger than the diaphragm. 
The first rib is the fixed point; the intercostal between it and the 
second contracting brings it nearer to the first, and the third fol¬ 
lows the second, and they are brought somewhat forward ; and, in 
being brought forward, they are necessarily thrown outward, and 
the cavity of the thorax is increased. Thus, while acting as anta¬ 
gonists to the diaphragm, the intercostals curiously assist in pro¬ 
ducing the expansion of the chest. 
The air-cells being thus filled, the phrenic nerve ceases to be¬ 
stow its influence ; and the diaphragm, by the inherent elasticity 
of its fibre, resumes its former shape, and protrudes again towards 
the chest, and the intercostals cease to act, and, by the elasticity 
of the cartilages, the ribs fall, and the cavity of the chest is dimi¬ 
nished, and the lungs are pressed on every side, and a portion of 
the air is forced out. The abdominal muscles occasionally assist 
powerfully in this, and always to some degree; but in the usual 
healthy breathing their aid is not required or lent to any consi¬ 
derable extent. Presently, but from what cause we know not, the 
influence of the phrenic nerve is felt again, and the diaphragm 
once more contracts, and the ribs are thrown outward, and the 
cavity of the chest is enlarged, and a vacuum would be formed, 
and the external air rushes in to restore the equilibrium of pres¬ 
sure ; and during life the chest continues to expand and to con¬ 
tract, and the atmospheric air to rush in and to be expelled. 
It is judiciously remarked by Dr. Bostock, that inspiration is 
the vital act, and that expiration is the mechanical return to 
quietude by the elasticity of the parts displaced, whether we con¬ 
sider the cartilages, the diaphragm, or the lungs themselves. 
The act of respiration is effected by nerves that, although not 
of cerebral origin, are partly under the control of the will. The in¬ 
spiration may be deepened or quickened at pleasure ; and in hur¬ 
ried, or laborious, or diseased respiration, we are enabled to avail 
ourselves of the aid of the pectorales, and the serrati, and theleva- 
tores, and the transversales costarum, and many other muscles in 
inspiration; and more forcibly to employ the abdominals in expi- 
