43 
elasticity of the lungs. 
muscular, pliant, and of a more extensive area than that of 
the transverse section of the chest, is in consequence of the 
greater weight sustained by its outward or inferior surface, 
necessarily pressed, or, in popular language, sucked upwards 
in the form of a cone. The extent of this cone will be ne- 
cessarily regulated by the extent of the area of the diaphragm, 
compared with that of the area of the transverse section of 
the chest. But the contraction of the muscular fibres of the 
diaphragm diminishes its area, and reduces it to a nearer 
equality with the area of the transverse section of the chest, 
and thus diminishes the magnitude of the diaphragmatic cone, 
and in an inverse proportion enlarges the boundaries of the 
chest. But the diaphragm at the succeeding relaxation of its 
fibres, is restored to its former dimensions ; becomes capable 
of being swelled into a larger cone ; and, by this encroach- 
ment, reduces the boundaries of the chest to their former limits. 
Two powers are therefore concerned in regulating the 
movements and in varying the dimensions and form of the 
diaphragm, the elasticity of the lungs, and the contractile 
power of the muscular fibres of the diaphragm. Of these 
powers the one is permanent and equable, the other variable 
and exerted at intervals. The contractile power of the dia- 
phragm, when fully exerted, is evidently much stronger than 
its antagonist, the resilience of the lungs ; but the latter not 
being subject to exhaustion, takes advantage of the neces- 
sary relaxations of the former, and rebounding, like the stone 
of Sisyphus, recovers its lost ground, and renews the toil of 
its more powerful opponent. 
Breathing is in a great measure the effect of this inter- 
minable contest between the elasticity of the lungs and the 
irritability of the diaphragm. 
