66 PHYSIOLOGY OF THE HUMAN BODY. 
4 \ 
Secretion and Excretion. 
We have seen that the two functions of the blood are to deposit 
materials for building up and keeping in repair the different parts of the 
body, and to carry off the waste matter arising from the decay continually 
going on. The various processes by which the products of this waste are 
drawn off from the blood receive the general name of secretion ;1 and the 
organs for the purpose, all of which are constructed on the same principle, 
are called glands.2_ These secretions are of two kinds. One kind is merely 
a purification of the blood, in carrying off something which would be 
injurious to the system, while the other kind extracts from the blood , 
a fluid which is intended to serve a purpose in the economy of the system. 
The products of the former are called excretions,’ as the exhalations from 
the skin, urine, &c.; the products of the latter are secretions proper, 
as the saliva, gastric juice, &c. The general structure of a gland is a 
collection of follicles, or minute membranous bags, surrounded with a. 
network of capillaries, from which the materials of the secretion, or the 
injurious substance to be excreted, are drawn. They are divided into 
two kinds, according to the manner in which they discharge their con- 
tents. In some cases, the secreting follicles discharge their contents 
separately, as the sweat glands and sebaceous glands of the skin. The 
work of secretion is really performed by cells or exceedingly minute 
bags, lining the follicles; and another form of discharge is when these 
cells, without being contained in follicles into which they discharge the 
fluid secreted, lie on the surface of a membrane, and. discharge their 
contents upon it directly, as in the case of the mucous membranes. 
In the more complicated secreting organs, such as the liver or kidneys, 
one or more tubes which carry off the secretion ramify in the organ into 
very minute branches; and at the point of each of the smallest twigs is 
a follicle, lined with cells, which discharge their contents into the follicles, 
to be carried off by the tubes. 
The LrvzR is the organ which secretes the bile, and is a body of a dark- 
brown colour, divided into two lobes, and lying opposite the stomach, on 
the right side. Part of the bile is sent direct from the liver to the 
intestine, but there is a reserve kept in the gall-bladder, a little bag 
attached to it. The bile is drawn from the venous blood returning from 
the abdomen, which is made to pass through the liver in its passage to 
the heart. 
The Krpneys lie in the lumbar region, in front of the backbone, one 
on each side, and are imbedded in fat. The function of this organ is to 
1 From Latin secerno, to put aside or separate. — 2 Latin glans, an acorn, a gland. 
5 From Latin excerno, to sift. 
