Doubts have been expressed as to whether function plays a part in the development of the jaws and alignment of the teeth. This paper attempts to show that function is an important factor. It is shown, first, by analogy that function affects other parts of the body; and secondly by direct examples. The more important of the latter are the breadth of the dental arch being dependent on the size of the tongue, the form of the glenoid fossa being related to the extent of the overbite of the incisors, and finally the changes in the occlusion of the deciduous teeth, between 3 and 8 years of age, so that the permanent incisors and first permanent molars may occlude correctly, such change taking place with difficulty unless the cusps of the deciduous teeth are worn. Where greater forward movement of the lower deciduous teeth compared with the upper deciduous teeth has not taken place, a degree of post-normality of the lower arch must inevitably be present.
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