_Experiment 5._ =Test for Fats.=--See General Introduction. _Experiment 6._ =Human Teeth.=--Study the form of teeth from every part of the mouth. Get a handful from a dentist. Break some of the teeth to make out their structure. Classify them. Draw section, enlarged. _Experiment 7._ =Study of the Teeth.= (At home.)--Sit with the back to the light and look into a mirror, with the mouth wide open. Do you see the four kinds of teeth named in text? Which are fitted for cutting? Which for grinding? Are any suited for tearing? Are any of the teeth pointed? What is the difference in the bicuspids and molars? Are there any decayed places? Are the teeth clean? Are the so-called canine teeth so long that they project beyond the line of the other teeth, as they do in a dog? Do the edges of the upper and lower incisors meet when the mouth is closed, or do they miss each other like the blades of scissors? How many roots has each lower tooth? (See Fig. 92.). Which tooth has the longest root? _Experiment 8._ =Structure of Mammalian Stomach.=--Get a piece of tripe from the market. Study its several coats. The velvety inner coat is covered with mucous membrane. (Photomicrograph, Fig. 95.) _Experiment 9._ =Model of Human Food Tube.=--Make a model of the food tube out of yellow cambric, giving to each organ its correct size. Follow the dimensions given in text. =Necessity for Foods.=--Growing plants and growing animals need new material to enable them to _increase in size or grow_. Plants never cease to grow while they live; most mammals attain their full size in one fifth of the time occupied by their whole lives. (By this rule how long ought man to live?) Animals, moreover, _move from place to place_, and _work_ with their muscles. The energy for this comes from the food they eat. Plants do not use food for this purpose. Another need for food comes from the _necessity for heat_ in all living things. The activities of animals cause the tissues to wear out, or break down, and food furnishes material with which new living matter is built up by the cells and the _tissues repaired_. We have already stated the rôle of oxygen in setting free energy in the living substance of the cell by oxidizing it. There is no furnace in the body as in an engine, but the oxidation occurs in the cells themselves and the fuel is built up into living matter by the cells before it is oxidized. Plants must lift mineral from the inorganic to the organic world before it can be food for animals. Plants can assimilate minerals; animals cannot. The body cannot make bone out of limewater. The iron in iron tonics cannot be used. Iron makes the grain brown, and the peach red. There is ten times as much iron in our food as the body needs.