290 



SPLANCHNOLOGY. 



ENAMEL. 



In appearance enamel is distinguished by its peculiar white- 

 ness. It is the hardest animal texture, containing about 96 per 

 cent, of earthy salts. It consists of hexagonal prisms, which are 

 arranged side by side, with one extremity resting on the surface 



of the dentine, whence they 

 are disposed in gently wav- 

 ing lines towards the surface 

 of the tooth. They vary 

 somewhat in diameter, but 

 average -ginnr of an inch. 

 No nutrient vessels or nerves 

 have been traced into ena- 

 mel, and when destroyed it 

 is not reproduced. 



This tissue is protective, 

 sometimes covering the entire exposed surface of the tooth, as in 

 the human subject and the dog, and it also furnishes the rough 

 projections and cutting edges found in the teeth of some classes 

 of animals, the herbivora affording a good example of this. 



FIG. 94. 



A, Transverse section of enamel, showing the cut 

 ends of its hexagonal prisms ; B, Prisms separated. 



CRUSTA PETROSA. 



Crusta Petrosa, or cementum, the third constituent, completely 

 covers the imbedded portion of the tooth, and occupies the cavities 

 on the free portion, where such exist. It is thickest towards the 

 roots, most abundant on the molar teeth, and is distinguished by 

 its brownish-yellow colour. It is the softest dental texture, 

 closely resembling true bone in structure, having tubuli and cells 

 resembling the canaliculi and lacunae, and where it is thick it 

 may be traversed by vascular tubes analogous to the Haversian 

 canals. The proportion of earthy matter, the same as in bone, 

 is about 67 per cent. 



ARRANGEMENT AND KINDS OF TEETH. 



Teeth may be simple or compound. Simple as in the dog, 

 where the entire exposed surface is covered by enamel, which 

 alone is in wear ; compound or complex, as in the horse, where 

 various tissues are in wear. 



