PREFACE. 

For the last six years, I have been collecting 
facts which bear forcibly upon the general 
principles of Physiology; and I have at dif- 
ferent times arranged them in various shapes, 
for the purpose of laying them before the public 
in some periodical publication; but I found 
they embraced so many parts of Physiology, 
that it was impossible to bring them into any 
reasonable compass, without a brevity which 
would render them at once obscure, entangled, 
and unconnected. For this reason, I ' pre- 
vented the publication of a paper which was 
submitted to the Royal Society in 1819, and 
which contained some of the facts related in 
the following work. Thought is the essence of 
science, and is most easily conveyed from a 
b 
