of the chest in breathing , speaking , and expression. 307 
wounds of vital parts, and particularly of the diaphragm. It 
explains also the successive convulsive lifting of the shoulders 
in wounds of the diaphragm. 
That a system of nerves so intimately combined as this is 
with the other parts of the general system, should suffer in 
hysterical disorders, cannot surprise us ; and admitting that 
irritation reaches to the respiratory system, we may perceive 
how rapidly the change may be produced, from the convul- 
sions of laughter to those of crying ; and where, if there be a 
corresponding condition of the mind, it rather follows than 
precedes the expression of the frame. 
These respiratory nerves are organs of expression. 
It would have been extraordinary if we had arrived at any 
- satisfactory theory of expression, before it was known through 
what instruments the mind influenced the body, during emo- 
tion or passion. But since we know that the division of the 
respiratory nerve of the face, deprives an animal of all ex- 
pression ; and that the expressive smile of the human face is 
lost by an injury of this nerve : since it is equally apparent, 
that the convulsions of laughter arise from an influence 
extended over this class of nerves ; it comes to be in some 
sort a duty in pursuing this matter, to examine farther into 
the subject of expression. We may be at the same time 
assured of this, that whatever serves to explain the constant 
and natural operations of the frame, will also exhibit to us 
the symptoms of disease with more precision. 
In terror, we can readily conceive, why a man stands with 
eyes intently fixed on the object of his fears : the eyebrows 
elevated, and the eye balls largely uncovered ; or why. 
