190 A MANUAL OF DENTAL ANATOMY. 



trines promulgated in the foregoing pages have the support 

 of ct, priori probability, there are some rather paradoxical 

 facts to be met with in comparative odontology. Never- 

 theless, there can be no doubt, that backward elongation as 

 teeth are successively added, &c., is sufficiently near the 

 truth in the case of human and most mammalian jaws for 

 practical purposes. 



It remains to notice the changes in form which the 

 ascending ramus and the angle of the jaw undergo. In 

 the foetus the ramus is but little out of the line of the body 

 of the jaw, and the condyle little raised above the alveolar 

 border. 



Gradually the line of development, as is indicated even in 

 the adult jaw by the course of the inferior dental canal, takes 

 a more upward direction ; copious additions of bone are 

 made on the posterior border and about the angle, so that in 

 an adult the ramus ascends nearly at right angles to the body 

 of the jaw. 



In old age, concomitantly with the diminution of muscular 

 energy, the bone about the angle wastes, so that once more 

 the ramus appears to meet the body at an obtuse angle. 

 But all the changes which mark an aged jaw are the simple 

 results of a superficial and not an interstitial absorption, cor- 

 responding with a wasting of the muscles, of the pterygoid 

 plates of the sphenoid bone, &c. 



ERUPTION OF THE TEETH. 



THE mechanism by which teeth, at the date of eruption, 

 are pushed upwards into place, is far from being perfectly 

 understood. The simplest theory would appear to be that 

 they rise up, in consequence of the addition of dentine to 

 their base ; in fact, that their eruption is due to the elonga- 

 tion of their fangs. 



