THE DIAPHRAGM, &C. 619 
on the rhomboideus, associates almost every muscle of the neck, 
the shoulder, and the chest, in the expansion of the thorax. 
These are muscles, however, which, in undisturbed respiration, 
the animal scarcely needs; but which are necessary to him when 
the respiration is much disturbed, and to obtain the aid of which 
he will, under pneumonia, obstinately stand until he falls ex¬ 
hausted or to die. 
Atmospheric Pressure .—Well, the cavity of the chest is en¬ 
larged. But this is a closed cavity, and between its contents and 
the parietes of the chest a vacuum would now be formed; or 
rather an inequality of atmospheric pressure is produced from the 
moment the chest begins to dilate. As the diaphragm recedes, 
there is nothing to counterbalance the pressure of the atmospheric 
air communicating with the lungs through the medium of the 
nose or mouth, and it is forced into the respiratory tubes which 
have been described in formerlectures, and the lungs are expand¬ 
ed and still kept in contact with the receding walls of the chest. 
There is no sucking, no inhalent power in the act of inspiration ; 
it is the simple enlargement of the chest: and a resistance being thus 
removed, which the pressure of the air could not before overcome, 
but which once removed, air rushes in with a force proportionate 
to the rapidity or the extent of the removal. 
Act of Expiration .—From some cause, as inexplicable as that 
which produced the expansion of the chest, the respiratory nerves 
cease to act; and the diaphragm, by the inherent elasticity of its 
tendinous expansion and muscular fibres, returns to its natural 
form, projecting its convexity into the thorax; and the abdominal 
muscles, which had been put on the stretch by the forcing of the 
viscera into the posterior part of the abdomen by means of the 
straightening of the diaphragm, contract, and accelerate the return 
of that muscle to its quiescent figure; and the ribs, all armed 
with elastic cartilages, and those cartilages bent in such angles 
as to dispose them most rapidly and most perfectly to regain 
their former figure, fall again ; and the muscles of the shoulder 
and the chest relax, and the lungs are pressed on every side, and 
the air with which they were distended is partially forced out 
again. There is only one set of muscles actively employed in 
expiration, namely, the abdominal: the elasticity of the parts 
displaced in inspiration would almost alone accomplish the 
purpose. 
The Elasticity of the Lungs .—The lungs are not altogether 
passive. The bronchial tubes, so far as we have been enabled to 
trace them, are lined with cartilage, divided for the purpose of 
folding up when the lungs are compressed, but elastic enough to 
affoid a yielding resistance against both unusual expansion and 
