Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 4: Pla [corr.: Ple] - Wri
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit29465/393/
RODENTIA. 
duces accordingly an oblique surface, sloping 
from a sharp anterior margin formed by the 
dense enamel, like that which slopes from the 
sharp edge formed by the plate of hard steel 
laid upon the back of a chisel; whence the 
name dentes scalprarii given to the incisors 
of the Rodentia. 
The varieties to which these incisors are 
subject in the different Rodents are limited 
to their proportional size, and to the colour 
and sculpturing of the anterior surface. Thus 
in the Guinea-pig, jerboa, and squirrel the 
breadth of the incisors is not half so great as 
that of the molars, whilst in the coypa they are 
as broad, and in the Cape mole rats 
(Bathyergus and Orycteromys) broader than 
the molars. 
In the coypa, beaver, agouti, and some 
other Rodents, the enamelled surface of the 
incisors is of a bright orange or reddish 
brown colour. In some genera of Rodents, 
as orycteromys, otomys, meriones, gerbilla, 
hydrochaerus, lepus, and lagomys, the anterior 
surface of the upper incisors is indented by a 
deep longitudinal groove. This character 
seems not to influence the food or habits of 
the species ; it is often present in one genus 
and absent in another of the same natural 
family; in mostRodents the anterior enamelled 
surface of the scalpriform teeth is smooth 
and uniform. 
The molar teeth are always few in number, 
obliquely implanted and obliquely abraded, 
the lateral series converging anteriorly in 
both jaws ; but they present a striking con¬ 
trast to the incisors in the range of their 
varieties, which are so numerous that they 
typify almost all the modifications of form 
and structure which are met with in the 
molar teeth of the omnivorous and her¬ 
bivorous genera of other orders of Mammalia. 
In some Rodents the molar teeth are root¬ 
less, like those of the wombat, the toxodon, 
and elasmothere; some have short roots 
tardily developed, like the molars of the horse 
and elephant ; and some soon acquire roots 
of the ordinary proportional length. 
The Rodents which have rootless molars 
comprise the families of the hares, chin¬ 
chillas, Chili rats, and cavies; most of the 
Fig. 
383 
voles, the houtias (Capromys), and the Cape 
jerboa (Helamys). 
The genera which Have molars, with short 
or incomplete roots, developed late, are 
Castor (beaver), Hystrix (porcupine), Cœlogenys 
(spotted cavy), JDasyprocta (agouti), Spalax 
(blind rat), Myopotamus (coypa), Euryotis, 
Accomys, and Aplodontia. 
The families of the squirrels, dormice, rats, 
and jerboas have rooted molars. 
The differences in the mode of implantation 
of the molar teeth relate to differences of diet. 
The Rodents, which subsist on mixed food, 
and which betray a tendency to carnivorous 
habits, as the true rats, or which subsist on 
the softer and more nutritious vegetable sub¬ 
stance, as the oily kernels of nuts, suffer less 
rapid abrasion of the molar teeth ; a minor 
depth of the crown is therefore needed to 
perform the office of mastication during the 
brief period of existence allotted to these 
active little mammals ; and as the economy 
of nature is manifested in the smallest par¬ 
ticulars as well as in her grandest operations, 
no more dental substance is developed after 
the crown is formed than is requisite for the 
firm implantation of the tooth in the jaw. 
Rodents that exclusively subsist on vege¬ 
table substance, especially the coarser and 
less nutritious kinds, as herbage, foliage, the 
bark and wood of trees, wear away more 
rapidly the grinding surface of the molar 
teeth ; the crowns are therefore larger, and 
their growth continues by'a reproduction of 
the formative matrix at their base, in propor¬ 
tion as its calcified constituents, forming the 
exposed working part of the tooth, are worn 
away. So long as this reproductive force is 
active the molar tooth is implanted, like the 
incisor, by a long undivided continuation of 
the crown ; when the force begins to be ex¬ 
hausted the matrix is simplified by the sup¬ 
pression of the enamel organ, and the dentinal 
pulp continues to be reproduced only at 
certain points of the base of the crown, which 
by their elongation constitute the fangs. The 
beaver and other Rodents in the second cate¬ 
gory of the order, according to the implanta¬ 
tion of the molar teeth, exemplify the above 
condition ; but in the capybara, dolichotis, 
Lower jaw of the Porcupine (Hystrix cristata). 
i, incisor tooth; m, the molar teeth, implanted in the jaws by means of fangs; i*, pulp at the base of 
incisor tooth ; p, anterior molar.
        

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