44 Dr. Carson on the elasticity of the lungs. 
The cause of the successive contractions of the diaphragm, 
in those cases at least in which the will is not concerned, seems 
to admit of the following explanation. A permanent and 
invariable load is sustained by its lower surface. By this load 
the relaxed muscular fibres become stretched to a degree 
which at length becomes painful and stimulating. To relieve 
itself from this irksome burden, the diaphragm is roused to 
contraction ; but this contractile power, agreeably to the 
laws of muscularity, is soon exhausted, and falling into a 
quiescent state, allows the painful and stimulating distension 
of the relaxed fibres of the diaphragm to be again renewed. 
From the irksomeness of this condition it relieves itself by 
a fresh contraction. Thus, by the alternated superiority of 
two powers, on the balancing of which life itself depends, 
the chest is successively enlarged and diminished, and air 
alternately expelled and inhaled. 
In a similar and equally effective manner, the elasticity of 
the lungs will be found to influence the movements of the 
heart and the motion of the blood. But as these movements 
are of a more complicated nature ; as the consideration of 
them would lead into a long detail ; and as my opinions re- 
specting them are already fully recorded, I will not pursue 
the inquiry farther at this time. 
In investigating the manner in which the elasticity of the 
lungs is rendered subservient to the great functions of res- 
piration and the circulation, I shall be found, I trust, to have 
opened to the views of the physiologist, a new and a bound- 
less field, which, I will venture to predict, will reward every 
skilful and assiduous cultivator with a rich harvest of dis- 
covery. 
