Normally, the first or "milk teeth," twenty in number, are cut in groups, each effort being succeeded by a pause or period of rest. The diagram and table following show the grouping, the date of eruption, and the duration of the pauses, the numbers, 1 to 5, indicating the groups to which the individual teeth belong and their order of appearance, and the letters a and b the precedence of eruption in the different groups (Fig. 5).

Fig. 5. - Diagram Showing Eruption of Milk Teeth.

1 1, Between the 4th and 7th months. Pause of 3 to 9 weeks. 2 2 2 2, Between the 8th and 10th months. Pause of 6 to 12 weeks. 3 3 3 3 3 3, Between the 12th and 15th months. Pause until the 18th month. 4 4 4 4, Between the 18th and 24th months. Pause of 2 to 3 months. 5 5 5 5, Between the 20th and 30th months.

The pauses are, to say the least, most helpful, giving the infant's system an opportunity to rest after each effort, to recover from any coincident illness, and to prepare for the next strain.

The dates here given show the time within which the different teeth naturally may be expected. In regard to the period given for the eruption of the lower central incisors, 1 would state that the fourth month, although an early is not a very rare time for their appearance. For example, I have seen during one winter five cases in which these teeth pierced the gum at this age.

Often the teeth appear without the production of any symptoms. Sometimes the edges of the gums lose their sharpness and become swollen, rounded, and reddened as the teeth approach the surface. At the same time the saliva is increased in quantity, and the mouth is unnaturally warm and the seat of abnormal sensations, evidenced by the tendency to bite upon any object that comes to hand - in other words, there is a condition of mild catarrhal stomatitis. The consequent discomfort is not sufficient to interfere with the child's appetite, good humor, or sleep, and when, after a few days, the margin of the tooth is free, all the local symptoms vanish.

Examples of Variations. - Abnormal dentition is manifested by departures from the laws if development already stated. The standard rules may be departed from in three ways:

1. The appearance of the teeth may be premature. Children may be born with one or more of their teeth already cut; these are usually imperfect, and soon fall out, to be replaced, at the proper age, by well-formed milk teeth. Sometimes, however, they remain permanently, as in a case that came under my own observation. Natal teeth are always incisors. Instances of the lower central incisors being cut in the third month are not uncommon. Girls are more apt than boys to cut their teeth early, and, as an early dentition is likely to be an easy one, the occurrence is to be looked upon as fortunate.

2. Dentition may be delayed. This deviation is more frequently seen and of more consequence than the first. Bottle-fed babies, as a class, are more tardy in cutting their teeth than those reared at the breast. With such, though healthy in every respect, a delay of one or two months is a common and not at all serious event. On the contrary, whatever the method of feeding, if no teeth have appeared by the end of a year, it may be assumed that the child's general nutrition is faulty, or that rachitis is present. Delay does not necessarily imply difficulty in cutting the teeth, although the two conditions are often associated.

3. The teeth may appear out of their regular order. Bottle-fed infants are most likely to show this irregularity, which is of some importance as an indication of general feebleness. In other instances, however, it is merely a family peculiarity, and, as such, bears no special significance.

The order of eruption of the permanent teeth is as follows:

The two central incisors of lower jaw, from 6th to 8th year.

The two central incisors of upper jaw, from 7 th to 8th year.

The four lateral incisors, from 8th to 9th year.

The four first bicuspids, from 9th to 10th year.

The four canines, from 10th to 11th year.

The four second bicuspids, from 12th to 13th year.

These replace the temporary teeth; those which are developed de novo appear thus:

The four first molars, from 5 th to 7th year. The four second molars, from 12th to 13th year. The four third molars, from 17th to 21st year.

There are, therefore, twelve more permanent teeth, making thirty-two in all, sixteen in each jaw.

The diagram, Fig. 6, will aid in explaining the process.

As these teeth approach the surface, absorption begins in the alveoli and at the roots of the deciduous teeth, and this continues until the latter are loosened and readily extracted, or if this be not done, until little is left but their crowns.

When the first and second molars approach the surface, the gums, just as in primary dentition, may become red, swollen, rounded, and tender. The salivary secretion is increased, the mouth is hot, the patient complains of aching in the gum, and, on account of tenderness, refuses food requiring mastication. With the other sets there is a gradual loosening of the superimposed temporary teeth, pain on mastication, redness and tumefaction of the gum, and augmented flow of saliva. As there is no impairment of the general health, these trifling symptoms must be regarded merely as manifestations of the progress of a physiological process.

Fig. 6. - Diagram Showing Relation Between Permanent and Temporary Teeth. The figures 1, 2, 3, etc., indicate the groups of teeth and the order of their appearance.

In concluding this subject it is important to emphasize the fact that many diseases occurring in infancy were formerly attributed to dentition, but as the affections of early life have been more carefully studied and better understood, one disorder after another has been relegated to its proper etiological class, and teething is now regarded as a purely physiological process, unproductive of symptoms. All that can be said is that the interval between the fourth and thirtieth months of an infant's life - the period of primary dentition - is an era of great and widely extended physical progress. The teeth are advancing; the follicular apparatus of the stomach and intestinal canal is undergoing development in preparation for the digestion and absorption of mixed food; the cerebrospinal system is rapidly growing and functionally very active, and the organs and tissues of the whole body are in a state of active change. This period of normal transition must also be one in which there is great susceptibility to abnormal change, or disease, provided there be a causal influence at work. Such an influence usually originates outside of the body, as when there is exposure to cold or to contagion.

Second dentition has perhaps a greater but still a merely indirect effect upon the health. During this process the loss of teeth and the local irritation of the gums interfere materially with mastication of the older child's solid food, and digestion and nutrition being consequently impaired, the general health suffers and there is less resistance to the various external causes of disease. The approach of puberty at this time also puts an additional strain on the system.