In order to investigate on the fusing process in the mandibular symphysis, both roentgenological and histological studies were made by using 15 human autopsy cases from stillborn of 40 weeks to infant of 9 months, and the results obtained were as follows:
The mandibular symphysis from stillborn of 40 weeks to infant of 13 days was roentgenologically shown as a transparent, conical, band-like or ampullar shadow same as the fetuses over 30cm long, which became dotty in infant of 6 months and no longer visible in infant of 9 months.
Histological study showed that the formation of transitional and osteopoieticlayer was generally scanty in newborn and infant compared with fetus cases and the trabecular bone formation was inactive. Osseous fusion in the mandibular symphysis was found still incomplete in infant of 6 months, but became complete in infants of 9 months or more. The starting site of such fusion did not show any definite pattern.
Developmental relationship between fusion of the mandibular symphysis and eruption of the deciduous central incisor (a|a) was seen.
As far as the mode of fusion in the mandibular symphysis is concerned, each layer forming the mandibular symphysis developed remarkable in fetus cases, but in infant cases changed as follows: first, the trabecular bone layer approached each others, secondly, the connective tissue disappeared and the transitional layer fused by overlapping each other, thirdly, the osteopoietic layer was ossified in infant age and at last the osteopoietic layer was connected by osseous fusion.

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