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OF THE ORGANIC SYSTEM. 91
by the process of cellular formation and dissolution ;
and all this matter, goes to mingle with the general
current of the blood. In it may be seen, by the micro
scope, the lymph globules, white, clear, but destined to
imbibe coloring matter, and become red globules.
But before we go further, let us glance a moment
at the structure and action of glands.
All vital actions
seem to be carried on by means of surfaces. The
more important the operation, the greater the surface
concerned. A simple membrane gives
of surface ; we have still
more in cells, and still more
when these cells line tubes,
and those tubes increase
their length by multiplied
convolutions. In the human
body
glandular apparatus,
we have all sorts of
from a
single follicle, or depression,
up to the immense convo
lutions of the seminal tubes
in the testicle,
or the still
more complicated nervous
tubuli in the nervous centers.
a certain extent
Fig. 29.
GLANDS IN THE COAT OF THE
STOMACn, MAGNIFIED 45
DIAMETEBS.
Wherever any vital action is to be performed, we
have, by some means, an extent of surface proportional
to its importance. In the lungs, the air-cell surface is
estimated at 1,500 square feet. In the same organ
the convolutions of capillary vessels, in which the blood
is brought to imbibe oxygen from the air and give off
carbonic acid, must make a surface many times greater.
The vessels and secreting cells of the liver contain a