By the time a child is five or five and a half years old its baby teeth should be well spread apart in front to make room for the permanent teeth, which will soon begin to erupt. If the child's diet and care have been proper, so that his or her development has been normal, this will be so. But how often do we see it otherwise? The vault of the mouth is so often high rather than broad and flat, as it should be. There is not room in the jaws for the permanent teeth. As a result these are crowded or overlapped, malposed and irregular. They must also be defective, for the same developmental failure which resulted in a faulty dental arch, also produces defective teeth.

Dr. Howe says that: "Under favorable conditions, the child develops proper cranial and facial proportions and a broad dental arch, and at the proper age the deciduous arch voluntarily widens to form the anterior portion of the permanent dental arch" * * * "under unfavorable conditions, facial development in the child may be retarded so that when it is time for the permanent teeth to erupt the arch may not be wide enough to accommodate them and they will be malposed. Such a physical deficiency may arise from any of many causes acting either singly or together, such as poor heredity, lack of sunlight, illness, a deficient diet, and perhaps others."

Prof. E. Mellanby has shown that the structure and arrangement in the jaws of teeth of animals depend largely upon their nutrition during the period of development, so that it is now possible to produce almost any degree of imperfection of the teeth by supervising the diet of puppies.