_The tongue._--If the tongue is not properly exercised in childhood and youth, we find it imperfectly developed; hence in inefficient masticators it is generally small. It must not be forgotten in this connection that this organ is considerably exercised when the infant is at the breast, from which the milk is obtained, not by suction, but by a vigorous tugging and squeezing of the nipple (in which the tongue takes considerable part), whereby the gland is reflexly excited to secrete. When, on the other hand, the child feeds at the bottle he obtains his milk by actual suction, and generally through an orifice of such ample dimensions as to allow the bottle to be rapidly emptied with comparatively little exercise of the lips and tongue. In short, the breast-fed infant has to do some work for his living, and that of a sort calculated to promote the health of the jaws and their appendages, while the bottle-fed child can glut himself by doing very little more than opening his mouth; wherefore we find the tongue and adjacent parts less developed in the latter than in the former. It may be thought to be a matter of indifference whether the tongue develops to its normal proportions or remains small, but such is by no means the case, for, as Dr. J. Sim Wallace has shown, the pressure of this structure against the teeth promotes the normal development of the jaws, especially of the mandible, and when it is small they are apt to be so too. _The salivary glands._--Just as mastication increases the functional activity of the salivary glands and buccal glands and favours their normal development, so, contrariwise, inefficient mastication during early life fails to call forth their normal functional activity and to secure their adequate development. Thus we find that a child who has been brought up on hard, starchy foods, necessitating abundant mastication has much larger and more active salivary glands than one who has been fed on soft foods which slip down into the stomach before they have had the chance of being properly masticated, and it is needless to say that the more efficient these glands are the more likely is digestion to be carried out satisfactorily. _The jaw-bones._--If the jaws are not adequately exercised in youth by mastication they fail to grow to their normal size and shape, and there is apt in consequence to be overcrowding of the teeth. The main defect of the jaws in such cases is their smallness; they do not present that pronounced lateral compression and anterior protrusion which characterise the mouth-breather’s jaw, nor such extreme dental irregularity, the most common being overlapping of the incisors, displacement of the canines, and difficulties in regard to the eruption of the wisdom teeth from shortness of the alveolar ridge. I have already referred to the progressive shortening in the post-molar ridge, which has been taking place during man’s evolution from the anthropoid in correspondence with the alteration in his diet.