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/lit29465/936/
926 
TEETH. 
Ungulates ; it is not a mere miniature oF the 
great molars of the mature animal, hut re- 
rains, agreeably with the period of life at 
which it is developed, a character much more 
nearly approaching that of the ordinary Pa- 
chydermal molar, manifesting the adherence 
to the more general type by the minor com¬ 
plexity of the crown, and by the form and 
relative size of the fangs. In the transverse 
divisions of the crown we perceive the affinity 
to the Tapiroid type, the different links con¬ 
necting which with the typical elephants are 
supplied by the extinct Lophiodons, Dino- 
theriums, and Mastodons. The sub-division 
of the summits of the primary plates recalls 
the character of the molars, especially the 
smaller ones, of the Phacochère in the Hog- 
tribe. As the elephant advances in age the 
molars rapidly acquire their more special and 
complex character. 
The first molars are completely in place, 
and in full use at three months, and are shed 
when the elephant is about two years old. 
The sudden increase and rapid develop¬ 
ment of the second molar may account for the 
non-existence of any vertical successor to the 
former tooth, or “premolar,” in the elephant. 
The eight or nine plates of the crown are 
formed in the closed alveolus, behind the 
first molar by the time this cuts the gum, and 
they are united with the body of the tooth, 
and most of them in use, when the first 
molar is shed. 
The average length of the second molar is 
two inches and a half; ranging from two 
inches to two inches and nine lines. The 
greatest breadth, which is behind the middle 
of the tooth, is from one inch to one inch 
three lines. There are two roots : the cavity 
of the small anterior one expands in the 
crown, and is continued into that of the three 
anterior plates. The thicker root supports 
the rest of the tooth. The second molar is 
worn out and shed before the beginning of 
the sixth year. 
The third molar has the crown divided into 
from eleven to thirteen plates ; it averages 
four inches in length, and two inches in 
breadth, and has a small anterior, and a very 
large posterior root ; it begins to appear 
above the gum about the end of the second 
year, is in its most complete state and exten¬ 
sive use during the fifth year, and is worn out 
and shed in the ninth year. The last rem¬ 
nant of the third molar is shown at m. 3, 
fig. 592. 
It is probable that the three teeth above de¬ 
scribed are homologous with the deciduous mo¬ 
lars, d. 2, d.3, and dA, in the Hyrax and horse. 
The fourth molar presents a marked supe¬ 
riority of size over the third, and a somewhat 
different form : the anterior angle is more 
obliquely abraded, giving a pentagonal figure 
to the tooth in the upper jaw {fig. 592. m. 4). 
The number of plates in the crown of this 
tooth is fifteen or sixteen : its length between 
seven and eight inches ; its breadth three 
inches. It has an anterior simple and slender 
root supporting the three first plates ; a 
second of larger size and bifid, supporting the 
four next plates ; and a large contracting base 
for the remainder. The fore-part of the 
grinding surface of this tooth begins to pro¬ 
trude through the gum at the sixth year : the 
tooth is worn away, and its last remnant shed, 
about the twentieth or twenty-fifth year. It 
may be regarded as the homologue of the first 
true molar of ordinary Pachyderms (m. 1). 
The fifth molar, with a crown of from 
seventeen to twenty plates, measures between 
nine and ten inches in length, and about 
three inches and a half in breadth. The 
second root is more distinctly separated from 
the first simple root than from the large mass 
behind. It begins to appear above the gum 
about the twentieth year : its duration has 
not been ascertained by observation ; but it 
probably is not shed before the sixtieth year. 
The sixth molar appears to be the last, and 
has from twenty-two to twenty-seven plates ; 
-its length, or antero-posterior extent, following 
the curvature, is from twelve to fifteen inches : 
the breadth of the grinding surface rarely 
exceeds three inches and a half. 
The reproductive power of the matrix in 
some cases surpasses that of the formative 
development of the cavity for lodging the 
tooth, and the last lamellae are obliged to be 
folded from behind forwards upon the side of 
the tooth. Fig. 99, p. 233. of my “ History of 
British Fossil Mammals,” shows this condition 
in the last lower molar of the Mammoth. 
One may reasonably conjecture that the 
sixth molar of the Indian elephant, if it make 
its appearance about the fiftieth year, would, 
from its superior depth and length, continue 
to do the work of mastication until the pon¬ 
derous Pachyderm had passed the century of 
its existence. 
Mr. Corse has figured the sixth molar, 
(which he calls the seventh or eighth,) with 
twenty-three plates, in tab. x. of his Memoir, 
and a small cavity, c, is marked as an in¬ 
cipient alveolus for a succeeding grinder. 
Had it actually been such, it might have been 
expected to contain some calcified portions 
of the anterior plates of such succeeding 
grinder. 
The molar teeth, in all the species of 
elephant, succeed each other from behind 
forwards, moving, not in a right line, but in 
the arc of a circle, shown by the curved line 
in fig. 592. The position of the growing 
tooth in the closed alveolus (m. 5) is almost 
at right angles with that in use, the grinding 
surface being at first directed backwards in 
the upper jaw, and forwards in the lower jaw', 
and brought, by the revolving course, into a 
horizontal line in both jaws, so that they 
oppose each other, when developed for use. 
The imaginary pivot on which the grinders 
revolve is next their root in the upper jaw, 
and is next the grinding surface in the lower 
jaw ; in both, towards the frontal surface of 
the skull. Viewing both upper and lower 
molars as one complex whole, subject to the 
same revolving movement, the section dividing 
such whole into upper and lower portion runs
        

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