6 
ESSAY ON SECRETION. 
phosphate of lime, carbonate of soda and chloride of sodium, 
and may be almost viewed as the serum of the blood. 
But we shall find, as we proceed with the consideration 
of the secretions, that their importance is, with scarcely an 
exception, directly proportionate to the complexity of their 
chemical constitution; those which I have just named being- 
simple, and serving comparatively unimportant purposes. 
I say comparatively unimportant, because their intrinsic 
value, each in its place, is not to be lightly passed over—the 
tears, for instance—and also a secretion very similar to them 
in its composition, the aqueous and vitreous humours of the 
eye, serve most important purposes in connexion with the 
visual function; while the others I have named serve no 
less important purposes—the one in facilitating locomotion, 
and the other in connexion with the various uses to which 
mucous membranes are rendered subservient. 
The second class of secretions, are those which are intended, 
by virtue of their composition, to effect various changes in 
the food that is taken in, rendering it, by the modifying 
influences which they exert over it, fit to be absorbed and 
used for the nutrition of the animal organism. These are the 
salivary, the gastric, the biliary, and the pancreatic fluids. 
One of the most important of these secretions, however, 
viz., the bile, which serves to assist in preparing the food for 
absorption, also possesses characteristics which are identical 
■with those of the secretions of the third class ; containing, 
as it does, a material which would be injurious if it existed 
in the blood, and which, therefore, has to be carried out of 
the body. 
Secretions of the third class are sometimes called excre¬ 
tions. They consist of matters for which there is no further 
use in the organism, and which would prove more or less 
injurious if retained : the urine, and the secretions of some 
of the intestinal glands being, of this class, the most im¬ 
portant. 
The fourth class of secretions will include the milk and 
the semen. These neither serve mechanical or chemical pur¬ 
poses within the body, nor are they to be regarded as ex¬ 
cretory matters, for we cannot think that a material which, 
by reason of its composition, would be injurious if retained 
within the organism of the mother, can be fit for the nutri¬ 
tion of the offspring. 
But in thus asserting that the secretion of the mammary 
glands is not to be viewed as excretory in its character, I by 
no means intend to maintain that there is no necessity for 
the formation of that secretion ; on the contrary, I think 
