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Rh the growth of the other unorganized tissues treated of in the previous class. If we suppose, for example, that the little cylinders (columnar epithelium) of the mucous membranes (which, according to Henle, are constantly being thrown off) could become ossified at the moment when they separated from the surface of the mucous membrane, we should obtain a covering to the membrane, consisting of little calcareous cylinders, each of which, however, would still have its organic basis like the enamel-fibres. Beneath this covering would be other cylinders not as yet ossified, which, when they in like manner became calcified would add to its thickness, whilst new cylinders grew forth from the mucous membrane. The quantity of the organic basis is extremely small in the teeth of adults which have been exposed for a considerable time to the action of the saliva, a circumstance which I suppose to be referrible to its undergoing a chemical solution in that fluid.

This is known to consist of a structureless substance, traversed by a great many minute canals. These canals (tubes) have for the most part a radiate course from the cavity of the tooth towards its external surface, and, according to Retzius, often give off branches as they proceed. Their peripheral terminations are extremely minute; they are thicker towards the dental cavity, and, when the pulp is removed, open freely into it. Müller observed that the tubes projected beyond the intermediate substance from the fractured surface of thinly-ground laminæ, and of lamellæ which had been macerated in hydrochloric acid, and were surrounded therefore by a special membrane; Retzius also remarked the same upon a transverse section. Purkinje and Müller noticed that when teeth are placed in ink, the fluid penetrates into the tubes; they must therefore be hollow. If any of them contain calcareous matter, it must be only the most minute ones. According to Retzius, many teeth present corpuscles which resemble those of bone, and like them send forth minute radiating canaliculi.