288 Mr. Bell on the nerves which associate the muscles 
regards the office of the lungs ; but they are not adequate 
to those additional functions which are in a manner imposed 
upon the respiratory apparatus, when they are brought to 
combine in other offices. 
Of the muscles of the trunk , which are brought in aid of the 
common respiratory muscles. 
If we look upon the frame of the body for the purpose of 
determining which are the muscles best calculated to assist 
in the motions of the chest, when there is an increased or 
excited action, we shall have little difficulty in distinguishing 
them, and we shall have as little hesitation in assigning a use 
to the nerves which supply these muscles exclusively. For 
these nerves have the same origin : they take an intricate 
course, threading and passing betwixt other nerves and other 
muscles, to be entirely given to the muscles which heave 
the chest. 
In this enquiry it is necessary to observe, that the life of 
animals is protected by a particular sense which gives rise 
to an instinctive motion of drawing the breath, and by which 
the chest is suddenly and powerfully expanded on exertion or 
alarm. The start on sudden alarm, is accompanied with a 
rapid expansion and rising of the chest, and the voice, at such 
a moment, is produced by suddenly inhaling, and not by ex- 
piration ; and this expansion of the chest combines with the 
preparation for flight or defence, since the extension of the 
muscles lying on the breast and back is produced by this 
motion, and since they are thereby rendered more powerful 
in their influence upon the arms or anterior extremities. It 
cannot escape observation, that oppression and difficulty of 
