SUCCESSION OF TEETH.—WHALEBONE. 
173 
only present in the latter, but are largely developed, serving as 
weapons of attack and defence; as in the Boar (fig. 100). 
184. In the Mammalia in general, as in Man, the teeth are 
not much developed at the time of birth, that they may not 
interfere with the act of sucking; and they do not make 
their appearance above the gum, until the time approaches 
when the young animal has to prepare its own food, instead 
of simply receiving that which has been prepared by its 
parent. The teeth which are first formed are destined to be 
shed after a certain period, and to be replaced by others. 
They are called milk-teeth ; and in Man they are twenty in 
number,—namely, four incisors in the front of each jaw, and 
two canines and four molars on each side. These begin to fall 
out at about the age of seven years; previously to which, 
however, the first of the permanent molars appears above the 
gum, behind those of the first set. The incisors and canines 
of the first set are replaced by incisors and canines respec¬ 
tively ; but the molars of the first set are replaced by teeth 
like small molars, having only two fangs; these are called 
false molars, or, more properly, bicuspid teeth (fig. 93). The 
second of the true molars does not make its appearance until 
all the milk-teeth have been shed; since it is only then that 
the jaw becomes long enough to hold any additional teeth. 
The third does not usually come up until the growth of the 
jaw is completed; and as this time corresponds with that at 
which the mind as well as the body is matured, they are 
commonly known as wise or wisdom teeth. There are then 
thirty-two teeth in all, or sixteen in each jaw;—namely, four 
incisors, two canines, four bicuspid, and six true molars.—In 
extreme old age, these teeth fall out like those of the first 
set; but they are not replaced by others, and their sockets 
are gradually obliterated. 
185. There are a few Mammalia which do not possess teeth. 
This is the case with the common Whale, in which they are 
replaced by an entirely different structure. From the upper 
jaw (fig. 102) there hang down into the mouth a number of 
plates of a fibrous substance (fig. 101), to which we give the 
name of whalebone , though its is really analogous to the gum 
of other animals. The fibres of these plates are separate at 
their free extremities, and are matted (as it were) together, so 
as to form a kind of sieve. Through this sieve the Whale 
