must be
removed. A household might be almost as much embarrassed by the
accumulation of garbage and ashes as by the absence of food and coal.
The food, which is taken into the alimentary canal and converted by
the digestive fluids into material more directly adapted to the uses
of cells, must be conveyed to them. A supply of oxygen is essential
for the life of the cells, and the supply which is given by
respiration must be carried from the lungs to every cell of the body.
All this is effected by the circulation of the blood, which takes
place in the system of branching closed tubes in which the blood
remains (Fig. 11). Certain of these tubes, the arteries, have strong
and elastic walls and serve to convey and distribute the blood to the
different organs and tissues. From the ultimate branches of the
arteries the blood passes into a close network of tubes, the
capillaries, which in enormous numbers are distributed in the tissues
and have walls so thin that they allow fluid and gaseous interchange
between their contents and the fluid around them to take place. The
blood from the capillaries is then collected into a series of tubes,
the veins, by which it is returned to the heart. This circulation is
maintained by means of a pumping organ or heart, which receives the
blood from the veins and by the contraction of its powerful walls
forces this into the arteries, the direction of flow being determined
as in a pump, by a system of valves. The waste products of cell life
pass from the cells into the fluid about them, and are in part
directly returned into the blood, but for the greater part pass into
it indirectly through another set of vessels, the lymphatics. These
are thin-walled tubes which originate in the tissues, and in which
there is a constant flow towards the heart, maintained by the constant
but varying pressure of the tissue around them, the direction of flow
being maintained by numerous valves. The colorless fluid within these
vessels is termed "lymph." At intervals along these tubes are small
structures termed the lymph nodes, which essentially are filters, and
strain out from the fluid substances which might work great injury if
they passed into the blood. Between the capillary vessels and the
lymphatics is the tissue fluid, in which all the exchange takes place.
It is constantly added to by the blood, and returns fluid to the blood
and lymph; it gives material to the cells and receives material from
them.

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