Bauhaus-Universität Weimar

The Cyclopaedia of Anatomy and Physiology, vol. 4: Pla [corr.: Ple] - Wri
Todd, Robert Bentley
spaces unoccupied by the contracted and 
divided pulp, and affords, by its periosteum, 
a surface for the adhesion of the cement or 
ossified capsule covering the completed part 
of the tooth. 
The matrix of certain teeth does not give 
rise during any period of their formation to 
the germ of a second tooth, destined to suc¬ 
ceed the first ; this, therefore, when com¬ 
pleted and worn down, is not replaced: all 
the true Cetacea are limited to this simple 
provision of teeth. In the Armadillos, Me- 
gatherioids, and Sloths, the want of germi¬ 
native power, as it may be called, in the 
matrix is compensated by the persistence of 
the matrix, and by the uninterrupted growth 
of the teeth. 
In most other Mammalia, the matrix of 
the first developed tooth gives origin to the 
germ of a second tooth, which sometimes dis¬ 
places, sometimes takes its place by the side 
of, its predecessor and parent. All those 
teeth which are displaced by their progeny 
are called temporary, deciduous, or milk 
teeth ; the mode and direction in which they 
are displaced and succeeded, — viz., from 
above downwards in the upper, from below 
upwards in the lower, jaw ; in both jaws 
vertically—are the same as in the Crocodile ; 
but the process is never repeated more than 
once in any mammiferous animal. A con¬ 
siderable proportion of the dental series is 
thus changed; the second, or permanent 
teeth, having a size and form as suitable to 
the jaws of the adult, as the displaced tem¬ 
porary teeth were adapted to those of the 
young, animal. The permanent teeth, which 
assume places not previously occupied by 
deciduous ones, are always the most poste¬ 
rior in their position, and generally the most 
complex in their form. The successors of 
the deciduous incisors and canines differ 
from them chiefly in size ; the successors of 
the deciduous molars may differ likewise in 
shape, in which case they have always less 
complex crowns than their predecessors.* 
The “ bicuspids,” in Human Anatomy, 
and the corresponding teeth, called “pre¬ 
molars,” in the lower Mammals, illustrate 
this law. 
The first true molar owes the germ of its 
matrix to a vegetation or bud, separated by 
the fissiparous process from the matrix of 
the last deciduous tooth ; but the backward 
elongation of the jaw affords space for its 
development by the side of its progenitor, 
during which process it may in like manner 
give origin to a second, and this to a third, 
molar, succeeding each other from before 
backwards or horizontally. 
In this successive germ-production, we 
find repeated the multiparous property of 
the dental matrix of the crocodile ; but the 
* “ C’est une règle générale, que les molaires de 
remplacement ont une couronne moins compliquée 
que celles auxquelles elles succèdent; mais cette 
couronne compliquée se trouve reportée sur les mo¬ 
laires permanentes qui viennent plus en arrière.” 
This generalisation was established by Cuvier, in 
his Leçons d’Anat. Comp., ed. 1805. vol. iii. p. 135. 
concomitant growth of the jaw allows the 
second, third, and sometimes fourth genera¬ 
tion of true molars to co-exist, and come 
into place side by side. In the Unguiculate, 
and most of the Ungulate, species of the 
placental division of the Mammalian class, 
the fissiparous reproduction of horizontally 
succeeding teeth stops at the third genera¬ 
tion ; in other words, they have not more 
than three true molars on each side of the 
upper and lower jaws. In the Marsupial 
series, the same process extends to a fourth 
generation of true or horizontally succeeding 
molars* ; and in most of the species, the 
four true molars are in use and place at the 
same time; but in certain Kangaroos, the 
anterior ones are shed before the posterior 
ones are developed. This successive de¬ 
cadence is still more characteristic of the 
grinding teeth of the Elephant, which are 
finally reduced to a single molar tooth on 
each side of both jaws. 
Thus the class Mammalia, m regard to the 
times of formation and the succession of 
the teeth, may be divided into two groups : 
— the “ Monophyodonts,”\ or those that 
generate a single set of teeth ; and the “ JDi- 
phyodonts,” J or those that generate two sets 
of teeth. 
The Monophyodonts include the orders 
Monotremata, JBruta (Edentata, Cuv.), and 
Cetacea (Cetacea vera, Cuv.) : all the rest of 
the order are Diphyodonts. In these, the 
first set of teeth are called the milk or de¬ 
ciduous teeth : the second set, the adult or 
permanent teeth ; although the teeth of this 
set are for the most part, like those of the 
first set, of limited growth, contracting to a 
root or roots, and being shed in greater or 
less proportion during the life-time of the 
species ; which life-time, in wild Carnivora 
and Herbivora, is dependent on, and would 
seem, indeed, to be determined by, the dura¬ 
tion of the adult teeth. 
The particulars of the Monophyodont 
dentition will be found under the Articles 
Monotremata, Vol. III. p. 387. ; Cetacea, 
Vol. I. pp. 563. 571. 573. ; (see my Odonto¬ 
graphy, p. 345. pis. 87—91.); and Eden¬ 
tata, Vol. II. p. 53. ; (see also Odontography, 
p.317. pis. 76—86.) Examples of some of the 
striking modifications of dental structure pre¬ 
sented by recent or extinct animals of the order 
Bruta, are given in figs. 548. and 574. of the 
present article. It will be observed that I 
have qualified the generalisation as regards 
the Monophyodont character of the Cetacea, 
by citing only that part of Cuvier’s order 
which he termed “ true or carnivorous Ce¬ 
tacea.” The animals of the order Sirenia 
* This characteristic extension of the reproductive 
power of the matrices of the true molars in the 
Marsupials, is an approximation to the peculiar 
activity and persistence of the same power in the 
vertically succeeding teeth of the cold-blooded Ovi- 
para, and is associated with many other instances of 
the same affinity in more important parts of the 
organization of the implacental Mammals. 
f fj-woi, once ;  and oSwi- 
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