964 THE DIGESTIVE SYSTEM. 
embraces the back of the tuberculum impar, but it is separated from it in part by the 
anterior division of the sinus arcuatus, which persists even in the adult as the foramen 
cecum, with the sulcus terminalis running forwards and outwards on each side from it. 
Finally, the two rudiments of the tongue—the tuberculum impar and the semilunar ridge 
—become blended, the only indication of the original separation being the foramen cecum, 
the sulcus terminalis, and the different characters which the mucous membrane presents 
on the two divisions of the organ. At the foramen cecum the downgrowth which gives 
rise to the isthmus of the thyroid gland takes place, and a part of the thyro-glossal duct 
which in the early condition connects the two may sometimes be found in the adult in 
connexion with the foramen. 
THE TEETH. 
The teeth are highly modified portions of the mucous membrane of the mouth, 
specially developed to perform the important function of mastication, that is, the 
division and trituration of the food which takes place in the mouth before the bolus, 
as the resulting 
mass is called, can 
be swallowed. Each 
tooth is a calcified 
papilla of the mucous 
membrane of the 
Permanent’ mouth, and consists 
incisor ike that membrane 
Permanent of two chief portions 
‘incisor  —namely, the den- 
tine derived from the 
connective tissue, 
and the enamel from 
the epithelial layer 
of the mucous mem- 
brane. The dentine 
constitutes the chief 
mass of the tooth, 
whilst the enamel 
forms a cap for 
the portion which 
projects above the 
cum. There is also 
found in the teeth 
By removing the bony outer wall of the alveoli, the roots of the teeth which have an other special 
been erupted, and the permanent teeth which are still embedded in the tissue—the cemen- 
mandible and maxilla, Daye been ee eee ei a Net tum orcrusta petrosa, ; 
ER ereEy Fs ee ea ae eal Sie aeiers have eo replaced by the a form of modified 
corresponding permanent teeth in the upper jaw, but the milk canine and bone—which en- 
molars pave spears: Peet penne an Rees ae cases the roots, these 
mene ee amano iteeeee up to the surface oh its lingual side. latter being formed 
In addition, the canine and two molars of the milk set persist. The position chiefly of dentine. 
of the crowns of the permanent teeth between the roots of the milk molars, Dentine and 
and the deep situation occupied by the permanent canines, should be noted. Wie» 
Observe also the absorption of the root of the lower lateral incisor enamel, but particu 
larly the latter, are 
the hardest and most resistant structures in the body, and are thus specially fitted 
for the functions which they have to perform. ; 
Temporary and Permanent Teeth.—The mouth of the infant at birth contains 
no teeth, although a number, partly developed, lie embedded in the jaws beneath 
the gum. Some six months later teeth begin to appear, and by the end of the 
second year a set, known as the milk teeth, twenty in number, has been “ @ulitea 
‘Then follows a pause. of about four years, during which no visible change takes 
place in the mouth, although in reality an active preparation for further develop- 
ment 1s going on beneath the gum. 
A WN i 
Permanent canine [ il | 
Ist permanent premolar \ 
\ 
2nd permanent premolar \ 
\ \ 
Ist permanent molar 
2nd permanent 
molar 
Inferior 
dental canal 
2nd milk molar 
Ist milk molar 
Mental foramen 
Fig, 648,—TEETH OF A CHILD OVER SEVEN YEARS OLD (modified from Testut). 
