IVORY OF THE TEETH. 101 
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 cover- 
ing 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 
haye 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. 
b. The wory. 
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, accord- 
ing 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. Miiller observed that the tubes projected 
beyond the intermediate substance from the fractured surface 
of thinly-ground laminz, and of lamellae 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 Miller noticed that 
when teeth are placed im 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. 
' See note to page 39. 
