thus appears to be a vital process, and not one simply controlled by physical forces like osmosis.  Here our explanation runs against what we call vital power of the ultimate elements of the body.  The consideration of this vital feature we must, of course, investigate further; but this will be done later.  At present our purpose is a general comparison of the body and a machine, and we may for a little postpone the consideration of this vital phenomenon.

[Illustration:  FIG. 4.—­Diagram of a single villus enlarged. B represents the membranous surface covering the villus; C, the blood-vessels within the villus.]

[Illustration:  FIG. 5.—­An enlarged figure of four cells of the membrane B in Fig. 4.  The free surface is at a; f shows fat droplets in process of passage through the cells.]

Circulation.—­The next piece of mechanism for us to consider in this machine is the device for distributing this fuel to the various parts of the machine where it is to be used as a source of energy, corresponding in a sense to the fireman of a locomotive.  This mechanism we call the circulatory system.  It consists of a series of tubes, or blood vessels, running to every part of the body and supplying every bit of tissue.  Within the tubes is the blood, which, from its liquid nature, is easily forced around the body through the tubes.  At the centre of the system is a pump which keeps the blood in motion.  The tubes form a closed system, such that the pump, or heart, may suck the blood in from one side to force it out into the tubes on the other side; and the blood, after passing over the body in this closed set of tubes, is finally brought back again to be forced once more over the same path.  As this blood is carried around the body it conveys from one part of the machine to another all material that needs distribution.  While in the intestine, as already noticed (Fig. 3), it receives the food, and now this food is carried by the circulation to the muscles or the other organs that need it.  While in the lungs the blood receives oxygen, and this oxygen is then carried to those parts of the body that need it.  The circulatory system is thus simply a medium by which each part of the machine may receive its proper share of the supplies needed for its action.

Now in this circulation we have again to do with chemical and physical forces.  All of its general phenomena are based upon purely mechanical principles.  The action of the heart—­leaving out of consideration for a moment its muscular power—­is that of a simple pump.  It is provided with valves whose action is as simple and as easy to understand as those of any water pump.  By the action of these valves the blood is kept circulating in one direction.  The blood vessels are elastic, and the study of the effect of a liquid pumped rhythmically into elastic tubes explains with simplicity the various phenomena associated with the circulation.  For