3. A further difficulty with the accuracy of digestive experiments, and one to which in the past too little attention has been paid, is the influence upon the digestibility of one food by the presence of others. Some foods, such as fruits, aid the digestion of other foods, while in many cases the presence of a certain article seriously hinders the digestive process of all. This emphasizes the great necessity for observing the laws of chemical harmony in combining our food at meals, and it also emphasizes the importance of limiting the diet to the fewest number of things possible, which in the opinion of the writer will lead inevitably to the mono-diet system, especially in curative or remedial feeding. [Sidenote: Difficulty of determining amount of undigested food] From the standpoint of the above difficulties, all digestive experiments thus far made are only approximately correct, and we are forced back to the conclusion that if we obey the laws of nutrition, Nature will give us her highest result expressed in endurance. If a single article of diet is taken by a man who is accustomed to large quantities of a highly varied bill of fare, the digestive process will not act in the usual way. On the other hand, if several articles such as nuts, grains, and milk are taken at one time, it will be impossible to determine what percentage of the proteid or of the fat from the three various sources remains undigested in the intestinal residue, hence no accurate results can be shown regarding the digestibility of each particular food. MECHANICS OF DIGESTION [Sidenote: Condition of food influences chemical action] Chemistry is not the only factor in the digestive function that is to be taken into consideration. The mechanical condition of food, when it is taken into the digestive organs, very greatly influences the chemical process that takes place. [Sidenote: Necessity for thorough mastication] This involves the question of masticating or subdividing the food into small particles. The greater the dissolving surface, the more rapidly will solution take place. If the substance being dissolved is a firm particle, the digestion or solution will take place only on the exterior surface, and the interior of the particle, however small, will remain practically unchanged. This is what occurs when food materials such as grains and nuts are taken in an uncooked state, as mastication does not dissolve them, but only divides them into small, distinct particles. [Sidenote: Action of enzyms during digestion]