The saliva is the first digestive juice. It is alkaline and contains a ferment called ptyalin. This acts energetically on the cooked and gelatinous starch, and slowly on the raw starch. Starch is quite insoluble in water, but the first product of salivary digestion is a less complex substance called soluble-starch. When time is allowed for the action to be completed, the starch is converted into one of the sugars called maltose. In infants this property of acting on starch does not appear in effective degree until the sixth or seventh month, and starch should not be given before that time. Only a small quantity should be provided before the twelfth month, when it may be gradually increased. Dr. Sims Wallace has suggested that the eruption of the lower incisors from the seventh to the eighth month, was for the purpose of enabling the infant--in the pre-cooking stage of man's existence--to pierce the outer covering of fruits so as to permit his extracting the soluble contents by suction; and accordingly when these teeth are cut we may allow the child to bite at such vegetable substances as apples, oranges, and sugar cane. Dr. Harry Campbell says that starch should be given to the young, "not as is the custom, as liquid or pap, but in a form compelling vigorous mastication, for it is certain that early man, from the time he emerged from the ape till he discovered how to cook his vegetable food, obtained practically all his starch in such a form. If it is given as liquid or pap it will pass down as starch into the stomach, to setup disturbance in that organ; while if it is administered in a form which obliges the child to chew it properly, not only will the jaws, the teeth, and the gums obtain the exercise which they crave, and without which they cannot develop normally, but the starch will be thoroughly insalivated that much of it will be converted within the mouth into maltose. Hard well baked crusts constitute a convenient form in which to administer starch to children. A piece of crust may be put in the oven and rebaked, and spread with butter. Later, we may give hard plain biscuits." Dr. Campbell continues, that he does not say that starch in the pappy form, or as porridge, should find no place whatever in man's dietary at the present day, but we should arrange that a large proportion of our food is in a form inviting mastication.