COMPARATIVE ANATOMY. 
The permament teeth are generally form- 
ed in cavities near the roots of the tempo- 
raiy ones ; and they succeed to the vacan- 
cies left by the discharge of the latter. A 
different mode of succession obtains, how- 
ever, in some instances. The adult mo- 
lares of the human subject are formed in 
the back of the two jaws, from which situ- 
ation they advance successively towards the 
front, in proportion as the maxillary bones 
are lengthened in that direction. A simi- 
lar, but much more remarkable species of 
succession is observed in the grinders of the 
elephant, where it was ascertained by the 
labours of Mr. Corse, who has explained 
and illustrated the subject in a series of 
beautifid engravings. See “ Observations 
on the different Species of Asiatic Ele- 
phants, and their Mode of Dentition,” ' 
Phil. Trans. 1799, Part II. 
We never see more than one grinder, and 
part of another, through the gum in this 
animal. The anterior one is gradually worn 
away by mastication : its fangs and alveoli 
are then absorbed ; the posterior tooth com- 
ing forwards to supply its place. As this 
goes through the same stages as the preced- 
ing grinder, a third tooth, which was con- 
tained in the back of tlie jaw, appears 
through the gum, and advances, in propor- 
tion as the destruction emd absorption of 
the other proceed. The same process is 
repeated at least eight times ; and each new 
grinder is larger than that which came be- 
fore it. The first, or milk grinder, is com- 
posed of four transverse plates or denticuli, 
and cuts the gum soon after birth. The 2d, 
which has eight or nine plates, has com- 
pletely appeared at the age of two years. 
The 3d, formed of twelve or, thirteen, at 
six years. From the 4th to the 8th grinder 
the number of plates varies from fifteen to 
twenty-three, which is the largest liitherto 
ascertained. The exact age at which each 
of these is completed, has not yet been made 
out. But it appears, that every new one 
takes at least a year more for its formatioi) 
than its predecessor. 
From the gradual manner in which the 
tooth advances, it is manifest, that a small 
portion of it only can penetrate the gum at 
once. A grinder, consisting of twelve or 
fourteen plates, has two or three of these 
through the gmn, whilst the others are im- 
bedded in the jaw. The formation of the 
tooth is complete, therefore, first, at its 
anterior part, which is employed in masti- 
cation, while the back part is very incom- 
plete ■, as the succeeding laminae advance 
through the gum, their formation is succes- 
sively perfected. But the posterior layers 
of the tooth are not employed in mastica- 
tion, until the anterior ones have been worn 
down to the very fang, which begins to 
be absorbed. One of these grinders can 
never, therefore, be procured iii a perfect 
state : for if its anterior part has not been 
at all worn, the back is not completely 
formed, and the fangs in particular are 
wanting ; while the structure of the back 
of the tooth is not completed, until the an- 
terior portion has disappeared. 
A similar kind of succession, but to a 
less extent, has been ascertained by Mr. 
Home, 'in the teeth of the sus Aithiopi- 
cus. “ Observations on the Structure of 
the Teeth of graminivorous Quadrupeds ; 
particularly those of the Elephant and sus 
iEthiopicus,” Phil. Trans. 1799, Part II. 
The researches of the same gentleman 
have also proved it to exist in the wild boar 
to a certain degree ; and have rendered it 
probable, that it occurred likewise in the 
animal incognitum (mammoth). “ Observa- 
tions on the Strueture and Mode of Growth 
of the Wild Boar and animal Incognitum.” 
Phil. Trans. 1801, Part II. 
It is remarkable, that the nuihber of cer- 
vical vertebrae in the mammalia should be 
constantly seven, although the animals of 
tills class differ so much in the length of the 
neck. A single exception occurs in the 
three-toed sloth, which has nine. 
The lumbar vertebral vary much in num- 
ber ; the elephant has three, the camel se- 
ven, the horse six, and tlie ass five. Mules 
have generally six. The os coccygis is pro- 
longed .so as to form the tail of quadru- 
peds. 
The cavity of the pelvis is so narrow in 
the mole, that it cannot hold the parts of 
generation, and the neighbouring viscera, 
which lie, therefore, externally to the ossa 
pubis. 
In the kangaroo, and in other marsu- 
pial animals, the anterior margin of the 
ossa pubis is furnished with a peculiar pair 
of small bones for supporting the abdominal 
pouch of the female. 
Cetaceous animals, having no hind feet 
have, consequently, no pelvis : but there 
is a pair of small bones at the lower part of 
the belly, which may be compared to tlie 
ossa pubis. 
In a very few mammalia, as some bats 
and armadillos, there is a pair of ribs less 
than in man; but in most of the class these 
bones are more numerous. The horse has 
