The teeth should be examined by a good dentist twice a year, so that small cavities shall be discovered at an early stage, the tartar removed, and the teeth kept in their best condition. This will prevent the early loss of the teeth. Lost teeth must be replaced, so that the teeth shall be opposite each other, for a very important factor in mastication is that the teeth shall strike properly. =Digestion.=—The factors controlling digestion are psychic, mechanical, and chemical. Briefly stated, the process of digestion consists first in the liquefaction of the solid portions of food and the conversion of the insoluble into soluble and diffusible, for no nutriment can be assimilated until reduced to a state in which it can pass through a cell membrane. These chemical changes are carried on by a series of enzymes. [Illustration: Fig. 5.—Location of the viscera of the body and their relation to each other: D, D, Lungs with air expelled; E, E, diaphragm cut away to show, F, liver cut to show stomach; 2, gall-bladder; H, H, large intestine; K, small intestine; L, vermiform appendix (after Heath). ] _Enzymes_ are unorganized ferments which possess the power of producing chemical changes in certain substances with which they come in contact under particular conditions without themselves suffering permanent alteration. The digestion of food is largely accomplished by the specific action of these enzyme bodies, of which every digestive fluid contains one or more. But neither solubility nor diffusibility is adequate. Freely soluble substances like cane-sugar need to undergo digestive changes just as definite as those carried out in the case of fats or coagulated proteins. The changes which they undergo before absorption serve a more fundamental purpose than the mere hastening of their passage through the lining membrane of the intestine. In the light of modern chemical knowledge we can be somewhat specific in regard to the molecular aspects of the digestive processes. They are probably always cleavages, large molecules giving rise to smaller ones. When the original molecule is of extraordinary size, as with proteins and starches, these cleavages have a serial character, and a number of intermediary products must accordingly be formed; that is to say, the earlier products are in turn subjected to digestion. Such cleavages are generally, if not always, hydrolytic; that is, water enters into the reaction and its elements are found combined in the products. =The Psychic Aspect or the Effect of the Emotions on Digestion.=—The relation of the emotions to the activities of the alimentary canal are of very practical importance, because recent investigations have shown that not only are the first stages of the digestive processes normally started by the pleasurable smell, sight, and taste of food, but also the pouring out of the gastric juice.