example, the rhythmically contracting heart forces a small quantity of blood into the arteries at short intervals.  These tubes are large near the heart, but smaller at their ends, where they flow into the veins, so that the blood does not flow out into the veins so readily as it flows in from the heart.  The jet of blood that is sent in with every beat of the heart slightly stretches the artery, and the tension thus produced causes the blood to continue to flow between the beats.  But the heart continues beating, and there is an accumulation of the blood in the arteries until it exists under some pressure—­a pressure sufficient to force it rapidly through the small ends of the arteries into the veins.  After passing into the veins the pressure is at once removed, since the veins are larger than the arteries, and there is no resistance to the flow of the blood.  Hence the blood in the arteries is under pressure, while there is little or no pressure in the veins.  Into the details of this matter we need not go, but this will be sufficient to indicate that the whole process is a mechanical one.

We must not fail to see, however, that in this problem of circulation there are two points at least where once more we meet with that class of phenomena which we still call vital.  The beating of the heart is the first of these, for this is active muscular power.  The second is a contraction of the smaller blood-vessels which regulates the blood supply.  Both of these phenomena are phases of muscular activity, and will be included under the discussion of other similar phenomena later.

[Illustration:  FIG. 6.—­A bit of muscle with its blood-vessels:  a, the muscle fibres; b, the minute blood-vessels.  The fibres and vessels are bathed in lymph (not shown in the figure), and food material passes through the walls of the blood-vessels into this lymph.]

We next notice that not only is the distribution of the blood explained upon mechanical 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.