110 HISTOLOGY 



The contact between the dentine and enamel is usually quite smooth. 

 Each enamel prism rests in a shallow socket on the dentinal surface, 

 and in places the dentinal canals extend into basal clefts in the enamel 

 cement. A short distance beneath the enamel the dentine exhibits a 

 layer of spaces, which in ground sections are filled with air and appear 

 black (Fig. 94, B, i.s.). They occur along the contour lines, and 

 are due to imperfect calcification of the cement in that region of the 

 matrix which was the first to form. Each space is bounded by spherules of 

 calcified matrix which project into it from all sides, and the cavities are 

 therefore known as inter globular spaces (Fig. 99). Toward the root of the 

 tooth they are smaller and more numerous than in the crown. They are 

 said to be particularly abundant in poorly developed teeth. 



The pulp consists of a fine network of reticular tissue together with the 

 peripheral layer of odontoblasts already described. The odontoblasts 

 persist throughout life, and may continue to produce dentine so that the 

 root canals may become nearly or quite obliterated. They are also active 

 in repairing injuries. Some of the late-formed dentine contains blood 

 vessels and resembles bone, so that it has been called osteo-dentine. The 

 odontoblasts connect with one another and with the rest of the pulp by 

 protoplasmic processes. The pulp tissue is free from elastic fibers and 

 from bundles of white fibers. It is very vascular. The small arteries 

 entering the apical foramina send capillaries close to the odontoblasts, but 

 normally they do not enter the dentine. Lymphatic vessels, according to 

 Schweitzer, are found by injection to begin as a tuft of branches in the 

 pulp of the crown; they empty into one or a few very wide vessels passing 

 through the root (Arch. f. mikr. Anat., 1907, vol. 69, pp. 807-908). The 

 nerves of the pulp are the medullated dental branches of the alveolar nerves, 

 which enter through the apical foramina, lose their sheaths and form a loose 

 plexus beneath the odontoblasts, between which they terminate in free 

 endings. 



DENTAL SAC, CEMENT, AND PERIODONTAL TISSUE. 



Each embryonic tooth, consisting of its enamel organ and papilla, is 

 completely surrounded by mesenchyma, which extends from the oral 

 epithelium to the bony trabeculae of the developing jaw (Fig. 101). This 

 mesenchyma gives rise to the dental sacs enclosing the teeth; each sac 

 consists of a dense outer layer and a loose inner layer of young connective 

 tissue (Fig. 102). Toward the base of the dental papilla the tissue of the 

 sac is separated from the dentine by the epithelial sheath, which is a part 

 of the enamel organ. After the crown of the tooth is well developed, the 

 epithelial sheath disintegrates or becomes penetrated by cells of the dental 

 sac, which are then transformed into osteoblasts and deposit bone directly 



