THE DEVELOPMENT OF THE JAW. 183 



tion in the dimensions of the bone : thus a process situated 

 at a point one-third distant from the articular extremity of 

 a large bone, will still be found one-third distant from the 

 end, though the bone has doubled in length. The four 

 little tubercles which give attachment to the genio-hyo- 

 glossus and genio-hyoid muscles are not, however, open to 

 these objections, as they are already, so to speak, at the end 

 of the bone, or, at least, of each half of it ; and their general 

 correspondence in level with the inferior dental canal, which 

 can hardly be imagined to undergo much alteration, in- 

 dicates that their position is tolerably constant. 



The points selected as landmarks are then, the spinse 

 mentales, the inferior dental canal and its orifice, and the 

 mental foramen. The mental foramen itself does undergo 

 slight change in position, but this change can easily be 

 estimated, and may as well at once be mentioned. As the 

 jaw undergoes increase in size, large additions are made to 

 its surface by deposition of bone from the periosteum, neces- 

 sarily lengthening the canal. The additions to the canal 

 do not, however, take place quite in the line of its original 

 course, but in this added portion it is bent a little outwards 

 and upwards. If we rasp off the bone of an adult jaw down 

 to the level of this bend, a process which nature in great 

 part performs for us in an aged jaw, or if instead we make 

 due allowance for the alteration, the mental foramen becomes 

 an available fixed point for measurement. 



The mental foramen, which undergoes most of its total 

 change of position within a few months after birth, comes 

 to correspond with the centre of the socket of the first tem- 

 porary molar ; later on it corresponds with the root of the 

 first bicuspid, which is thus shown to succeed, in exact 

 vertical position, the first temporary molar. 



On the inner surface of the jaw the tubercles for the 

 attachment of the genio-hyo-glossus and genio-hyoid muscles 

 are in the foetus, opposite to, and very little below the base 



