examining the teeth of many natives in their more or less uncivilised
state, from the Red Indians of North America, the negroes of Africa, to
the more civilised Chinese, Japanese, and Indians of the East, and I have
usually found them possessed of sound teeth, but so soon as they come
under the influence of civilised life in Washington, Montreal, London,
Paris and other cities, their teeth begin to degenerate, though their
general health may remain good." In a long article on mastication in the
_Lancet_ (1903-2, p. 84) from which we have already quoted, Dr. Harry
Campbell gives as the effect of thorough and efficient mastication, that
it increases the amount of alkaline saliva passing into the stomach, and
prolongs the period of starch digestion within that organ. That it
influences the stomach reflexly by promoting the flow of gastric juice.
That the frequent use of the jaws and the tongue, during the period of
growth, cause the jaws to expand. If the jaws are not adequately exercised
during this period, owing to the use of soft food, they do not reach their
normal size, the teeth are overcrowded, do not develop fully, and are
prone to decay. The effect of vigorous mastication is to stimulate the
circulation in the tooth pulp, which promotes nutrition and maintains a
firm dental setting. Dr. Campbell writes: "I am perfectly at one with Dr.
Wallace, in believing that the removal of the fibrous portion of food is
the main cause of the prevalence of caries among moderns."

When the food reaches the stomach, gastric juice is secreted. This juice
contains a ferment called pepsin and hydrochloric acid. Pepsin is only
active in an acid media. Starch digestion proceeds in the stomach to such
a time--stated as from 15 to 30 minutes--when the acid gastric juice has
been poured out in sufficient quantity to neutralise the alkalinity of
the saliva. The gastric juice acts upon the proteids only. After a time
the liquefied contents of the stomach are passed into the first portion
of the small intestine, called the duodenum. Here it meets with the