principles, but the supplying of the active parts of the
body with food is in the same way intelligible. As we have seen, the
blood coming from the intestine contains the food material received from
the digested food. Now when this blood in its circulation flows through
the active tissues--for instance, the muscles--it is again placed under
conditions where osmosis is sure to occur. In the muscles the
thin-walled blood-vessels are surrounded and bathed by a liquid called
lymph. Figure 6 shows a bit of muscle tissue, with its blood-vessels,
which are surrounded by lymph. The lymph, which is not shown, fills all
the space outside the blood-vessels, thus bathing both muscles and
blood-vessels. Here again we have a membrane (i.e., the wall of the
blood-vessel) separating two liquids, and since the lymph is of a
different composition from the blood, dialysis between them is sure to
occur, and the materials which passed into the blood in the intestine
through the influence of the osmotic force, now pass out into the lymph
under the influence of the same force. The food is thus brought into the
lymph; and since the lymph lies in actual contact with the living muscle
fibres, these fibres are now able to take directly from the lymph the
material needed for their use. The power which enables the muscle fibre
to take the material it needs, discarding the rest, is, again, one of
the _vital_ processes which we defer for a moment.

_Respiration_.--Pursuing the same line of study, we turn for a moment to
the relation of the circulatory system to the function of supplying the
body with oxygen gas. Oxygen is absolutely needed to carry on the
functions of life; for these, like those of the engine, are based upon
the oxidation of the fuel. The oxygen is derived from the air in the
simplest manner. During its circulation the blood is brought for a
fraction of a second into practical contact with air. This occurs in the
lungs, where there are great numbers of air cells, in the walls of which
the blood-vessels are distributed in great profusion. While the blood is
in these vessels it is not indeed in actual contact with the air, but is
separated from it by only a very thin membrane--so thin that it forms no
hindrance to the interchange of gases. These air-cells are kept filled
with air by simple muscular action. By the contraction of the muscles of
the thorax the thoracic cavity is enlarged, and as a result air is
sucked in in exactly the sam