178 A MANUAL OF DENTAL ANATOMY. 



growth of the upper jaw. The different parts of the lower 

 jaw answer for different purposes ; one division of its body 

 having a very close and intimate relation with the teeth , 

 the other serving a distinct purpose, and being only secon- 

 darily connected with the teeth. 



The alveolar portion of the jaw, that which lies above the 

 level of the inferior dental canal, is developed around the milk 

 teeth : when they are lost, it disappears, to be re-formed 

 again for the second set of teeth, and is finally wholly 

 removed after the loss of the teeth in old age. 



The portion of jaw below this line, which is essential to 

 deglutition and respiration, is late in acquiring any con- 

 siderable development. Once formed it is never removed, 

 save that when in advanced old age the muscles of mastica- 

 tion are no longer in full use, it becomes, to a slight extent 

 only, wasted. 



In order to understand the drift of the following descrip- 

 tion, it is essential to keep in view the different life histories 

 of those two parts of the jaw just alluded to. 



In an early foetus, long before the necessity for respiratory 

 movement or deglutition has become imminent, a thin lamina 

 of bone has begun to be formed beneath the tooth germs, 

 forming, as it were, a semicircular gutter running round the 

 jaw, in which the developing tooth sacs are lodged. The 

 thin gutter of bone thus formed is above and outside 

 Meckel's cartilage, and intervenes between the rudimentary 

 inferior maxillary vessels and nerves, and the teeth. The 

 sides of the bony furrow rise as high as the top of the tooth 

 germs, but they do not arch over and cover them in, in such 

 manner as the permanent tooth germs are arched in, for the 

 long furrow is widely open at the top. 



Passing on to the condition of the mandibles at the time 

 of birth, the two halves are as yet not anchylosed, but are 

 united only by fibre-cart ilage. " The alveolar margins are 

 deeply indented with large open crypts, more or less per- 



