Histological researches of teeth which have been published so far are concerned with permanent teeth in most cases. Consequently, milk teeth are not so often employed as a material, because most dental histologists have ignored the significance of deciduous teeth up to date. It is not too much to say thato present knowledge of dental histology is chiefly derived from that of permanent teeth. Of late, teeth of children have come to receive a new notice with the rapid strides in the realm of pedodontics. Also in the field of dental histology, deciduous teeth have been gradually brought into the limelight by many investigators from various points of view. Some items regarding deciduous dentition were investigated byt present authors. One of them which deals with the formation ability ofsecondary dentin isreported in this paper. In spite of the significance of formation ability of secondary dentin in the operative dentistry on children, this problem has not hitherto been given any attention on the part of dentists and dental investigators. That is the reason why most of them have followed opinions of CHURCHILL and others. These opinions maintain that the pulp tissue of deciduous teeth reacts without bringing about any formation of protective dentin to external stimuli. The authors have regarded a possibility very important whether or not the formation ability of secondarydentin takes place in deciduous teeth. Therefore, this problem had been examinedfrom the statistical viewpoint. As a result of this examination, the following conclusion is obtained that the formation ability of secondary dentin in deciduous teeth is all the same with that of permanent teeth.
In the previous reports of this serial study, the author described in detail the tissue structure of enamel lamella. Also in this research, the lamella wasfurther investigated to determine its histology. As mentioned in Part 3, lamella is one of the most highly disputed tissues in enamel. This problem should be principally considered histologically from various viewpoints. The author's following opinion was previously published: Genuine lamella can bealways found on enamel surfaces or in the inner layers of enamel of every tooth. In addition, lamella must be identified by its own peculiar so-called lamellar form. Its general direction runs from the enamel surface to the dentino-enamel junction. Lamella, of course, can be decisivelydistinguished morphologically from other hypocalcified areas often found in enamel. This genuine lamella is so commonly found in every tooth that it can be considered a normally existing structure in enamel. The tissue composition of genuine lamella is (1) hypomineralized enamel rods and (2) organic interrod substance. As a result of this research, the author has again reached the conclusion that lamellae consist of hypocalcified enamel rods and interrod substance. The observation of optic and electron micrographs are described below.