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INTRODUCTION. 



m a direction perpendicular to the superficies of the tooth, Professor 

 Retzius has more particularly observed and described the dichotomous 

 branching of the primary tubes ; the minuter ramuli sent off, throughout the 

 course of the main tubes, into the clear interspaces ; the calcigerous cells, 

 with which those fine branches communicate ; the terminal ramifications 

 of the tubuli, and their anastomoses with each other, and with calcigerous 

 cells at the superficies of the ivory or bony part of the tooth." According 

 to Professor Retzius, the function of these branched and anastomosing 

 tubes is nutritive : they convey, " by capillary attraction, a slow current 

 of nutritive, or preservative fluid, through the entire substance of the 

 tooth ; which fluid may be derived either from the superficies of the 

 pulp, in the internal cavity of the tooth, or from the corpuscles and cells 

 of the external layer of cortical substance or coementum ; with the tubes 

 radiating from which corpuscles, the fine terminal tubes of the ivory 

 anastomose." In addition to the determination of the nature and 

 arrangement of the dental tubuli, Professor Owen has proved the 

 existence of other component substances in teeth, besides those usually 

 described and admitted ; the microscopic characters of which are alike 

 distinct from those presented by ivory, enamel, cement, or true bone. 



One of the substances, he states, is " characterized by being traversed 

 throughout by numerous coarse canals, filled with a highly vascular 

 medulla, or pulp, sometimes anastomosing reticularly ; sometimes 

 diverging, and frequently branching ; sometimes disposed nearly parallel 

 with one another, and presenting more or fewer dichotomous divisions. 

 The canals, in many cases, are surrounded by concentric lamellae, and 

 thus resemble, very closely, the Haversian canals of true bone ; but the 

 calcigerous tubes, which everywhere radiate from them, are, relatively, 

 much larger. The highly-organized tooth-substance, just described, 

 differs from true osseous substance, and from the coementum, in the 

 absence of the Purkingian corpuscles, or cells. This structure is 

 exemplified in the teeth of many fishes, and in some of the edentate 

 Mammalia. 



" Another component substance of tooth more closely resembles true 

 bone and cement, inasmuch as the Purkingian cells are abundantly 

 scattered through it : it differs, however, in the greater number and 

 close parallel arrangement of the medullary canals. This structure is 

 exhibited in the teeth of the Megatherium, Mylodon, and other extinct 

 Edentata." 



An attempt will not be made to follow Professor Owen through his 

 elaborate details of the modification of the above-named dental substances 

 in various fishes, reptiles, and Mammalia ; but those, who are interested 

 in minute anatomy, are referred to the original paper. (See the Eighth 

 Report of the British Association for the Advancement of Science.) His final 



