It is well known that the size of a bone is largely determined by the degree to which the muscles attached to it are exercised. That the jaws do not grow to their normal size, if not adequately exercised during their period of growth, is strikingly shown by the overcrowding of the teeth, which takes place in those brought up on soft foods, and this even though there be no contraction of the jaws resulting from mouth-breathing. The dependence of the size of the jaws upon the degree to which they are exercised is also shown by the smallness of the modern jaw, as compared with that of primitive peoples, a difference which, as we shall see, is in part congenital and in part due to the comparative disuse of the former. Mastication influences not only the size but also the shape of the jaws (a), through its influence on the size of the tongue, which by pressing against the teeth tends, as Sim Wallace has shown, to expand the jaws; (b) by the pressure of opposing teeth against one another, which has a similar effect; and (c) by the outward pull of the pterygoids, which tends to widen the maxilla posteriorly and to broaden the posterior nares.