4 
ESSAY ON SECRETION. 
to inculcate, or favourite doctrines to uphold. If, on the one 
hand,Idonotpretend to advance anythingthat is new,orto have 
obtained my materials from extraordinary sources, so on the 
other, I am not aware of having omitted anything necessary 
for the elucidation of the inquiry into the function of secre¬ 
tion, and the circumstances that modify this process. 
If it be inquired what should be the prominent object in 
the reading of essays at our hebdomadal meetings, and the 
discussions that follow thereon ? I answer, not to teach, so 
much as to cultivate a spirit of independent thought and 
investigation ; so that you, each for himself, may become 
master of the question under consideration, and thus derive 
a benefit which will not depend upon the correctness or other¬ 
wise of the opinions that may be advanced, but rather upon 
your being led to think and study for yourselves. 
With this end in view the present essay has been written; 
and, for the same reason, I have purposely chosen, as the 
subject matter thereof, a theme of almost indefinite extent, 
purposing, as I have before said, to give you merely an out¬ 
line which you may fill up for yourselves hereafter. 
At the beginning, it becomes necessary to define the limits 
within which I am to confine the application of the term 
secretion ; for if it be taken in its fullest signification, as 
meaning either a separation of certain substances from the 
blood, or a setting apart of certain materials for definite 
ends, we shall find too great a variety of processes, and too 
heterogeneous a mass of facts accumulated together,so as to 
be able to do anything like justice thereto. 
The definition which I think most expressive of the pro¬ 
cess of secretion, and which I have therefore adopted, is that 
which asserts it to be a separation from the blood, by an 
organ especially adapted to that end, of a fluid differing in 
its composition in a greater or less degree from the blood 
whence it is derived, and which is always poured out upon 
the free surface of the organ in which it is formed ; it being 
either intended to be directly carried out of the body, or else 
to be used for some purpose within it: or it may be that it 
is partly for further use in the body, and partly to be got 
rid of as being either useless or noxious if retained. 
By this definition we exclude respiration from the secreting 
processes; and I think rightly so; for, although in the 
respiratory action we have a separation from the blood of 
materials which must, if health is to be maintained, be got 
rid of, and although these materials are throwm out upon the 
free surface of the respiratory organs, yet the process being de¬ 
pendent upon purely physical laws, and differing in that 
