ALVEOLAR PERIOSTITIS. 291 



diseased than the upper. The third and fourth molars are most 

 commonly affected, which Giinther ascribes to their central position 

 in the fan-shaped arrangement (Fig. 295), and to their consequent ex- 

 posure to powerful compression, both from in front and from behind. 



Alveolar periostitis of the incisors is very rare in horses and 

 ruminants, is generally of a secondary nature, and is due to injury 

 of the interdental space or compound fracture of the alveolar process. 



Though single teeth are often affected, it is common for several 

 to become diseased together, frequently corresponding teeth, like the 

 right and left third lower molars. The cause of this symmetrical 

 occurrence is not quite clear, though it may be due to abnormal 

 development. Where several neighbouring alveoli are diseased, and 



Fig. 287.— Necrosis of a dental alveolus, with displacement of the last molar anc 7 

 extensive ostitis of the lower jaw (horse). 



especially where the teeth are very loose, a suspicion of some new 

 growth is always justified. 



Many cases remain confined to the alveolus. When an upper 

 molar is completely loosened it may fall out ; lower molars are not 

 so readily displaced, though they may be equally loose ; for this 

 reason, the loose molars which can be extracted with the hand are 

 usually found in the lower jaw. Sometimes inflammation starts in 

 the thin plate of bone which forms the alveolus and extends to the 

 medulla. Purulent osteomyelitis thus results. This irritates the 

 periosteum, producing periostitis ossificans, which enlarges the bone. 

 Finally, towards the inferior border of the jaw the pyogenic process 

 breaks through the bone, and a dental fistula results. After discharge 

 of the pus, granulations form, and the point of perforation heals up, 

 until only a narrow channel remains communicating with the alveolus 



