168 C. R. BARNICOAT 



the intensity of etching to be recorded ( Hall, 1959 and unpublished 

 data ) . 



Electron micrographs of the biting edge of worn incisors indicated 

 that the dentine surface is subjected to both chemical and abrasive 

 action. In order to study more precisely the theory that organic 

 compounds in the herbage initially complex the calcium phosphate 

 ( apatite ) , experiments were made to compare the etching produced 

 by fractions of juices of pasture plants on surfaces of fresh dentine 

 polished smooth by metallographic techniques. The fractions exam- 

 ined were ( 1 ) organic acids from herbage, obtained bv ion ex- 

 change, at their original concentrations and pH (6.0-6.5) for chelat- 

 ing action on the calcium; (2) cell fluids, obtained by ether cvtolysis, 

 containing in addition sugars, inorganic and organic phosphates, 

 amino acids, and polypeptides, capable of chelating and dissolving 

 calcium; (3) freshly expressed juices of grasses and clovers contain- 

 ing enzymes in addition to substances of fractions 1 and 2. 



These studies have shown that fraction 3 produced the strongest 

 etching, and indicated that this reaches a maximum during the lush 

 spring growth. It was also evident that the central noncalcified core 

 of the odontoblast process was etched by fresh grass juice at a faster 

 rate than the dentinal matrix, and thereby released long lengths of 

 the membranelike sheaths enclosing the process. 



The results indicate that most of the etching is due to proteolysis 

 and that such weakening of the bonding material of the tooth would 

 facilitate its wear by attrition. Rouiller et al. (1952) and Rouiller 

 ( 1956 ) have observed that an organic component bonding the parti- 

 cles of dentine is digested by plant proteinases such as papain. It 

 is considered, therefore, that the digesting action of the proteinases 

 of actively metabolizing leaves on the organic compounds of dentine 

 could be an important factor contributing to the wear in teeth of 

 grazing sheep. 



Summary 



Increased wear in incisors occurs in sheep grazing "improved," 

 lush, highly productive pasture. 



Mouths with hereditary anatomical defects wear most rapidly, 



