Now repeat Experiment 45, 1, but instead of starch solution use honey dissolved in water. To show that some of the honey-sugar has passed through the membrane, take about a teaspoonful of the water in the beaker, and put it in a test tube with twice as much Fehling’s solution. Heat as before, and notice the red turbidity. If table sugar is used it may be recognised by the sweet taste of the water after the experiment. 3. The action of saliva on starch.—(a) Chew a piece of india-rubber to induce a free flow of saliva, and collect the liquid. In one test tube put half a teaspoonful of starch solution; in a second tube put a mixture of equal quantities of starch solution and saliva; in a third put saliva alone. Keep the tubes at blood heat for twenty minutes. Then add a little water to each tube and divide its contents into two parts. Test one part of each for starch with iodine solution, and the other part for sugar with Fehling’s solution. The first tube contains only starch. The second now contains no starch, but shows the presence of sugar. The third contains neither starch nor sugar. Evidently the saliva has changed the starch of the second tube into sugar. (b) Repeat the experiment, but keep the mixture of starch and saliva in a cold place. No sugar is produced. (c) Repeat the experiment as in (a), but use saliva which has been heated to boiling in a test tube. No sugar is formed. The necessity for food.—It is common knowledge that a rabbit, [Pg 231] like every other animal, must have a regular supply of food if it is to continue healthy, and that it would soon die outright if food were withheld. The reason for this is that the living substance of the animal’s body is incessantly wasting away. The rabbit cannot move a muscle except at the expense of the living substance of the muscle, and the more active is its life, the more rapidly does its body waste. It is to counteract this continual waste by continual formation of new living substance that food is taken. But the succulent plants which the rabbit eats are not suddenly transformed into animal muscle and bone, and so forth, when they are swallowed. They have first of all to undergo a process which is called digestion. This takes place in a tube—the digestive canal—which runs from end to end of the body. The digestive canal of the rabbit is coiled in a somewhat intricate manner. That of the frog is, however, much simpler and more typical of vertebrates (p. 220) generally, and will serve equally well in so elementary a consideration of digestion as the present.[11]