30 6 Mr. Bell on the nerves which associate the muscles 
muscles of respiration, so as to throw out the air with a force 
capable of removing the offending body. But if the irritation 
be on the membrane of the nose, the stream of air is directed 
differently, and, by the action of sneezing, the irritating par- 
ticles are removed from these surfaces. By the consideration 
of how many little muscles require adjustment to produce 
this change in the direction of the stream of air, we may know, 
that the action is instinctive, ordered with the utmost accu- 
racy, and very different from convulsion. 
We may notice another office of these nerves ; in smiling, 
laughing, and weeping, the influence is solely propagated 
through them. The face we have seen is dead to all changes 
of the kind when the nerve of this class which goes to it 
is destroyed, whether it be by division of the nerve, or from 
its being surrounded with inflammation or suppuration. When 
we consider that all the respiratory nerves depart from the 
same source, and participate in the same functions ; and more 
especially when we see the respiratory organs so very dis- 
tinctly affected in the conditions of the mind, which give rise 
to these affections, it is not too much to suppose, that what is 
proved in regard to one of these nerves, is true of the whole 
class, and that they alone are influenced in laughter. Physio- 
logists who have not investigated the cause, are yet agreed in 
describing laughter to be a condition of the respiratory mus- 
cles, where the air is drawn in rapidly, and thrown out in short 
spasmodic motions of these muscles ; that crying is nearly the 
reverse, the inspiration being cut by spasmodic actions of the 
muscles of inspiration. By these considerations are explained 
the subrisus which arises from abdominal irritation, and the 
sardonic retraction of the muscles of the face produced by 
