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Diagram 2 Postnatal growth in the white rat

structures. The development of the heart is also reciprocal to that of the lungs. The heart develops in time before the development of the brain and the lungs, but it is complementary to both. Likewise the liver may be reciprocal to the lungs, heart and brain in its development, and so each may be reciprocal to the other, but if we examine adjacent organs in their development, such as the head and trunk, the heart and lungs, and the liver and intestine, as well also as the upper and lower teeth, their reciprocal development makes them logical complements of each other.

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Diagram 3 Periods of growth in stature and eruption of the teeth

The postnatal development of the structures of the white rat as given by Jackson may be grouped into four periods, and I have roughly approximated these periods for man. The brain and lungs develop most rapidly soon after birth, the heart and kidneys a little later, followed by the development of the stomach, intestine and liver. The sex glands develop irregularly but their most rapid period of development immediately precedes puberty.

I have roughly approximated the periods of most rapid growth in stature and the most rapid development of the teeth ufter birth, and have placed them in diagram 3. This is a tentative scheme and awaits further observations for confirmation.

The first period of postnatal growth is the most rapid of all, and is associated with the development of the trunk and extremities. This is followed by the eruption of the temporary teeth, associated with the rapid development of the brain. This reminds us of the development of the trunk in the first month of prenatal life, followed by the rapid development of the head (brain). Between the arrival of the temporary teeth and the second acceleration of growth in stature (2 to 5 years), the heart and kidneys are apparently developing most rapidly. The eruption of the first permanent teeth, followed by the third acceleration of growth in stature, is related to the most rapid postnatal development of the liver, stomach and intestine. The third period of rapid tooth development followed by the fourth period of rapid growth in stature succeeds this and precedes puberty, after which the growth of the boys is retarded and the growth of girls almost ceases. There is need of more detailed

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Diagram 4 The order of eruption of the permanent teeth in Europeans. One side only of the mouth is represented, the left side as viewed from in front. The dots represent the individual teeth, whose symbols are placed between them. The order of eruption of the teeth is represented by numbers and by the lines connecting the dots. i. incisors; c, canines; p, premolars; m, molars.

data relating to the postnatal periods of growth in stature, in the teeth, and in the other structures of the human body. Until this is forthcoming there can be no final statement as to the exact relations of these periods, but enough is known of both prenatal and postnatal development to justify the law of alternation in development.

This law is applicable to the eruption of the individual teeth. No two adjacent teeth erupt at the same time except the median incisors, and the lower canines and median premolars. The median line represents a barrier, therefore the places filled by the median incisors are almost like remote areas. Diagram 4 represents the order of eruption of the teeth on one side of the mouth in the Germans and Americans.

The order of eruption is represented by serial numbers and followed by the line with barbs, beginning with the lower first molar and terminating with the upper third molar. The line skips from maxilla to mandible or mandible to maxilla nine times, and five times it goes from one tooth to another in the same jaw.

There are two waves of growth, one from the median line laterally, which includes the incisors, canines and premolars, 20 teeth in all, that erupt more rapidly (from 6 to 11 years) than the other wave, which includes the first, second and third molars, only 12 teeth in all, that erupt more slowly (from 6 to 20 years or later). Four molars erupt every 6 years, the first molars at 6 years of age, the second molars at 12 years of age, and the third molars at about 18 years of age. We may illustrate the

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Diagram 5 The eruption of the teeth of the Filipinos and the Eutopeans in two series. A, American: F, Filipino. Each tooth is represented by a dot. Under the dots are symbols for the tooth; i, incisor; c, canines; p, premolars; m, molars. The order of eruptionjis represented by the lines.

two waves of growth by diagram 5, showing the relative time of development of each tooth in the two waves.

When the permanent teeth begin to erupt the lower teeth erupt before the upper in the first period of acceleration in eruption .(the first molars, incisors and canines); the upper teeth erupt before the lower in the second period of acceleration in eruption (the premolars); and the lower teeth again erupt before the upper in the final stages of eruption (the second and third molars).

The alternation in development seems to apply to abnormal as well as normal development, although insufficient data are as yet available to demonstrate this conclusively. If one structure is unusually precocious in the periods of acceleration in development its complementary structure will be backward in the periods of acceleration, and vice versa. The eruption of the teeth of the Filipino boys may illustrate this. Take the lower teeth, for instance. In the median series the median incisors are precocious, hence the lateral incisors are relatively backward, hence the canines are relatively precocious, hence the median premolars are relatively backward, and hence the lateral premolars are relatively precocious. In the lateral series the first molars are relatively precocious, hence the second molars are relatively backward, and hence the third molars are relatively precocious. The alternation is also true for the lateral series of the upper teeth where the first molars are backward, the second molars are precocious and the third molars are backward, which is not only an alternation of adjacent teeth, but also of the similar teeth of opposite jaws, because the lower first molars are precocious, and the upper first molars are backward, the lower second molars are backward and the upper second molars are precocious, and the lower third molars are precocious and the upper third molars are backward.

Racial differences in growth also fall within the law of alternation in development. It is evident from this and other studies already published or at present under way that the Filipinos mature later than the Europeans in morphologic form, especially of the face, head, nose, etc., or never reach the state of the mature European, yet the Filipinos mature earlier than the Europeans (Ann Arbor Germans and Americans) in stature and in the eruption of the teeth. Does the early development of the teeth and stature in the Filipinos cause the late maturity of the head and face, and the early development of the head and face of the European cause a late development of the teeth and stature, or are the differences incidental, or caused by other factors?

A presentation of the supposed causes of alternation in development would be incomplete without suggesting the influence of mechanical factors and the internal secretions. The alternation in development of the teeth may be produced by a more rapid growth of one tooth than another due either to an initial stimulus, or to a better blood supply. The adjacent tooth may be crowded back by the precocity of the one that develops first and the substances used in the building of the latter delay the growth of the adjacent tooth. The position or size of the blood vessels may determine the precocity of the tooth, just as the greater quantity of pure blood going to the head in the fetus may account for the early precocity of that part. There may be blood vessels to the first molars and median incisors that are larger than those to the other teeth at first, or there may be an initial stimulus to these teeth that is greater than to the other teeth. Biting and chewing in the region of the median incisors and first molars is apt to be greater at first than in the region of the other teeth. In the same way the rapid growth of the lungs soon after birth and the closure of the ductus arteriosus and foramen ovale may be explained by the shunting of the blood stream from its fetal course through the foramen ovale and ductus arteriosis to its postnatal course through the right ventricle and pulmonary artery. The activity of the lungs immediately after birth sucks the blood through the pulmonary artery and right ventricle that formerly went through the foramen ovale and the ductus arteriosus to the body, and thus allows the closure of the last two channels.

The ductless glands with their internal secretions poured into the blood stream play a part in development that is little understood, but if the normal effect of the secretions may be inferred from their abnormal effect, we may know more than we think, or understand. I cannot enter here into a review of the literature of the internal secretions, which is enormous, but I wish to present a few facts that may be relevant. We may infer from recent work that the hypophysis influences the growth of the bones, and there is some indication of the antagonistic action of the sex glands and the hypophysis, which may account for the retardation of the growth of the bones (stature) after puberty.

The growth of the sex glands is irregular after birth, according to Jackson and Hatai, and if this irregularity in their development bears any relation to the periods of acceleration in stature, we may have a causal relationship.

The thyroid influences growth, because hypothyroidism produces cretinism and hyperthyroidism produces rapid differentiation with irritability of the nervous system, when either occurs

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