Cingulum (tooth)

Abstract: It is well known that the cingulum forms a major afferent pathway for the hippocampal formation. Although earlier authors suggested that fibers went to the mtorhinal area, presubiculurn, hippocampus and dentate fascia (Beevor, 1891 ; Koelliker, 1896; E. Smith, 1896; Cajal, '01; Probst, '03) recent experimental studies in the rabbit and monkey indicate that the major supply is to the presubiculum (Krieg, '47; Adey, '51 ; ddey and M. Meyer, '52). Electrophysiologic studies (Cragg and Hamlyn, '35 ; Green and Adey, '56 ; TT71iite, Nelson and Foltz, '59) have demonstrated interconnections between the anterior thalamic nuclei, the cingulum and the hippocampus. Latency studies and evidence of recruitment (White, Nelson and Foltz, '59) suggest that tho cingulumhippocampus responses go through at least one synapse before they reach the cells of the hippocampus. An attempt to correlate the information from various authors employing diversified techniques and different species points to minor discrepancies regarding the termination of the cingulum fibers, particularly their exact termination within the various components of the hippocampal formation.

Abstract: Objective Finite-element method was used to explore the stress distribution of incisors restored with porcelain veneers. The design of the incisal palatal finish line was analyzed as a function of incisal overlap and initial tooth substance loss (coronal fractures). Method and materials The treatment of intact and fractured incisors was investigated using 8 different designs of porcelain veneer. The palatal finish line varied from butt margins to extended chamfers. The stress distribution was assessed in a 2-dimensional finite-element model, reproducing a buccolingual cross section of an incisor. A palatal 50-N horizontal force was applied to the incisal edge to simulate an extreme functional load. The palatal surface tangential stresses were calculated. Results Considerable differences were detected in the stresses at the level of the incisal-palatal restoration margin. The margins of restorations with limited incisal overlap (butt margin or minichamfer) showed low tensile stresses or even compressive stresses. Restorations with a long chamfer extending into the palatal concavity were subjected to the highest tensile stresses. In the presence of moderate crown fractures (incisal one third) or severe wear, butt margins limited the palatal extension of ceramic, thus reducing the amount of stress at the restoration interface. In the presence of severe crown fractures (incisal two thirds), the margins (either butt or chamfered) were subjected to low tensile forces when located in the smooth convex area of the cingulum. Conclusion Because of the geometry and natural elastic modulus of mineralized tooth structures, a concentration of tensile stresses is formed at the palatal concavity of teeth restored with porcelain veneers. Long chamfers extending into the palatal concavity are unfavorable because thin extensions of ceramic are generated in an area of maximum tensile stresses. Minichamfers or butt margins are generally recommended, especially in the presence of moderate crown fractures or severe wear.

Abstract: This paper describes a new mammalian tooth from the Lower Wealden bone-bed at Cliff End, near Hastings. The tooth is a lower molar. It has a trituberculate structure with a well-developed talonid. The nature of the wear on this talonid shows that a true protocone (a definite pointed cusp as against an enlarged lingual cingulum) must have been present in the upper molars. Aegialodon is the first mammal in which the existence of a definitive protocone can be inferred. A good morphological series can be traced back from Aegialodon through Peramus (Upper Jurassic), Amphitherium (Middle Jurassic) to the Upper Triassic Welsh pantotheres. This series illustrates the increasing importance of the crushing as against the shearing function of the molar teeth as their evolution progresses. The morphological expression of this development is the enlargement of the talonid and associated enlargement of the lingual cingulum of the upper molars. Finally, in the last member of the series ( Aegialodon ) the lingual cingulum becomes a true protocone. From this main line of therian evolution first the symmetrodonts and then the dryolestid pantotheres branched off by emphasizing the shearing function of the teeth and reducing their crushing function. This would have been an adaptation to an exclusively insectivorous diet. From Aegialodon can be derived the teeth of the Trinity Sands ‘ Theria ’ and Endotherium by a further increase in the size of talonid and protocone. A continuation of the same process would give rise to the teeth of marsupials and placentals.