Experiment 81.  Use olive oil or lard.  Show by experiment that they
  are soluble in ether, chloroform and hot water, but insoluble in water
  alone.

  Experiment 82.  Dissolve a few drops of oil or fat in a teaspoonful
  of ether.  Let a drop of the solution fall on a piece of tissue or rice
  paper.  Note the greasy stain, which does not disappear with the heat.

  Experiment 83.  Pour a little cod-liver oil into a test tube; add a
  few drops of a dilute solution of sodium carbonate.  The whole mass
  becomes white, making an emulsion.

  Experiment 84.  Shake up olive oil with a solution of albumen in a
  test tube.  Note that an emulsion is formed.

Chapter VII.

The Blood and Its Circulation.

177.  The Circulation.  All the tissues of the body are traversed by exceedingly minute tubes called capillaries, which receive the blood from the arteries, and convey it to the veins.  These capillaries form a great system of networks, the meshes of which are filled with the elements of the various tissues.  That is, the capillaries are closed vessels, and the tissues lie outside of them, as asbestos packing may be used to envelop hot-water pipes.  The space between the walls of the capillaries and the cells of the tissues is filled with lymph.  As the blood flows along the capillaries, certain parts of the plasma of the blood filter through their walls into the lymph, and certain parts of the lymph filter through the cell walls of the tissues and mingle with the blood current.  The lymph thus acts as a medium of exchange, in which a transfer of material takes place between the blood in the capillaries and the lymph around them.  A similar exchange of material is constantly going on between the lymph and the tissues themselves.

This, then, we must remember,—­that in every tissue, so long as the blood flows, and life lasts, this exchange takes place between the blood within the capillaries and the tissues without.

The stream of blood to the tissues carries to them the material, including the all-important oxygen, with which they build themselves up and do their work.  The stream from the tissues carries into the blood the products of certain chemical changes which have taken place in these tissues.  These products may represent simple waste matter to be cast out or material which may be of use to some other tissue.

In brief, the tissues by the help of the lymph live on the blood.  Just as our bodies, as a whole, live on the things around us, the food and the air, so do the bodily tissues live on the blood which bathes them in an unceasing current, and which is their immediate air and food.