94 R. F. SOGNNAES 



ing action of the saliva (an 18-\ ear-old girl with complete xero- 

 stomia was once seen whose mouth was paper drv, and who had 

 worn her teeth down to the gum line, so that she had the appearance 

 of an old Eskimo ) . 



Mention should also be made of the extremely destructive wear 

 that may occur as a result of the combination of mechanical and 

 chemical influences; that is, accelerated attrition of tooth substance 

 partly weakened by chemical dissolution. The most interesting ex- 

 ample of such combinations is the excessive wear reported in sheep's 

 teeth (Barnicoat, 1957, 1960, and this \dume, chapter 5) which 

 has been attributed not onl\ to masticatory wear, but also to the 

 action of organic acids from herbage, in combination with freshly 

 expressed juices of grasses and clovers containing enzymes capable 

 of acting on the organic portion of the teeth (proteolysis). In 

 other words, it is suggested that the rapid wear of dentin in sheep 

 mav be due in part to attrition, in part to demineralization, and in 

 part to digestive action on the organic matrices by the proteinases of 

 actively metabolizing leaves. 



Alveolar Bone Resorption 



When the process of resorption aff^ects the maxillofacial bones, 

 it differs in no conspicuous histomorphological way from the typi- 

 cal lacunar resorption seen elsewhere in the vertebrate endoskele- 

 ton. Multinucleated giant cells are characteristically found in typi- 

 cal Howship's lacunae along the resorbing bone surfaces, be it in 

 normal alveolar remodeling accompanving growth, eruption, and 

 shedding ( see chapter 12 bv Bhaskar ) , in pathological bone destruc- 

 tion of periodontal disease ( see chapter 11 by Reichborn-Kjennerud ) , 

 or in modified metabolic states such as hibernation (see chapter 10 

 by Mayer and Bernick); or in other parts of the skull, beyond the 

 dento-alveolar apparatus, e.g. in experimentally induced calvarium 

 destruction, be it in living animals (see chapter 17 by Young), in in 

 vitro explants ( see chapter 23 bv Dowse et al. ) , or in tissue culture 

 ( see chapter 24 by Goldhaber ) . 



\\'ithin the supporting tissues of the teeth, however, special 

 mention should be made of several phenomena of local significance 

 and general interest. 



