ESSAY ON SECRETION. 
7 
that by providing an outlet for the quantity of blood which 
during the period of utero-gestation had been employed in 
providing directly for the growth of the foetus, a most im¬ 
portant purpose is accomplished by this secretion, and that 
its suppression by any accidental circumstance is likely to 
lead to the production of much mischief. 
In fact, we shall find, if we look closely into the function 
of secretion, that even those secretions -which w r e have 
named as principally mechanical in their uses, do exert, to 
a greater or less extent, a purifying influence upon the 
blood, whence they are formed; especially the secretion 
of the mucous membranes, which seems to possess the 
power of withdrawing from, the blood many matters for 
which there is no use in the animal economy. 
This fact is of moment in considering the modus operandi 
of purgative medicines, many of which seem to be first 
absorbed into the blood, and afterwards are separated 
therefrom by the agency of the mucous membrane of the 
intestines. 
We now come to the consideration of one of the most im¬ 
portant purposes of the secreting processes ; and it is neces¬ 
sary to inquire into the origin of those substances which 
have to be removed from the blood, in order to its main¬ 
tenance in a fit condition for the support of the life of an 
organised fabric. 
They have many and various sources ; first of all, w r e may 
name the continual tendency of all complex bodies to form 
simpler compounds. This tendency exists among the com¬ 
ponent parts of the animal fabric, and the exciting agent to 
effect it is the oxygen, which is carried from the lungs to 
every part of the body in or by the arterial blood. 
Acting as it does, both upon the tissues and also the 
materials contained w ith itself in the circulating current, we 
have a continual formation of substances unfit for retention 
in the organism, and a necessity for the provision of some 
means by which they may be removed. 
Another source of these materials is to be found in the 
changes which are consequent upon every exertion of mus¬ 
cular or nervous force. Whenever muscular or nervous 
tissue is brought into use, there is an alteration effected in 
some of the tissues which are thus called into action, the 
amount of change being proportionate to the amount of 
force generated; in fact, the generation of force appears to 
be dependent upon a change in the tissue, so that we may 
view the development of the force as consequent upon the 
chemical change. 
