THE DENTAL TISSUES OF THE ORDER RODENTIA. 
531 
but is generally present towards the outer surface of the tooth. The tubes in the 
anterior half gradually diminish in calibre from their commencement, while those in 
the posterior half retain their dimensions until they arrive near the surface, where 
they break up into branches. 
At and near the apex of the pulp-cavity the development of dentinal tubes is sus- 
pended. The tooth at this point is rendered solid by the conversion of the pulp into 
a clear laminated subgranular mass, into which very few, if any tubes are continued ; 
and the few that are sometimes found are usually small in size, irregular in form and 
direction, and never reach the centre. The ends of the dentinal tubes are perfectly 
sealed up, and their connection with the pulp-cavity and its vascular contents com- 
pletely cut off in the manner shown in Plate XLIII. fig. 1. The perfected part of the 
tooth is rendered by this process in the fullest sense of the word extra-vascular. A 
similar condition may be seen in the molars of persistent growth, and offers a striking’ 
difference to the condition of the rooted teeth, in which the dentinal tubes retain their 
connection with the pulp-cavity until the tooth becomes diseased or dead, or the 
crown worn down by mastication. In the latter case, as the crown wears down, the 
pulp is converted into secondary dentine, in which but few tubes exist, and these do 
not reach a vascular surface like that which lies in contact with the surface of the 
pulp-cavity in the normal state of the tooth. 
Professor Owen*, after describing the manner in which the incisor teeth of rodents 
are developed, says, “The tooth thence projecting consists of a body of compact dentine, 
sometimes with a few short medullary canals continued into it from the persistent 
pulp-cavity, with a plate of enamel laid on its anterior surface, and a general invest- 
ment of cement, which is very thin upon the enamel, but less thin, in some rodents, 
upon the posterior and lateral parts of the incisor.” 
The medullary canals described by Professor Owen, pursue a course parallel with 
the dentinal tubes, form a narrow loop, and return to the pulp-cavity. The dentinal 
tubes never radiate from them, but enter through the medium of lateral branches only 
(fig. 5). Hence the teeth so constituted do not form an exception to the law, that 
the incisors of rodents are formed of a single denticle-i-, which, exclusive of the enamel, 
is comparable to an Haversian system. 
* Odontography, page 399. 
t It is proposed to restrict the term dentinal system to a canal from which dentinal tubes radiate (fig. 2), 
and denticle to a dentinal system coated with enamel or cementum ; and that a tooth com.posed of a series of 
dentinal systems, each coated with enamel and united into one tooth by cementum, shall be described as a tooth 
composed of denticles. The molars of the Capybara afford an excellent example of a tooth constituted in the latter 
manner, while the Orycteropus affords an equally good example of a tooth compounded of a series of dentinal 
systems — a tooth in which we have a number of medullary canals from which dentinal tubes radiate, the terminal 
branches of which inosculate with the terminal branches from neighbouring systems, either by confluence or the 
intervention of small cells (fig. 2). Teeth composed of a series of dentinal systems mayor may riot have an external 
investment of enamel ; in the Orycteropus enamel is absent, while in the Labyrinthodon, Varanus and Lepidosteus, 
it is present near the upper extremity of the tooth. The dentinal systems commonly run into each other at some 
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