4 MICROSCOPIC 1 ANATOMY OF THE TEETH 



structure to their several purposes ; the pointed, more or 

 less conical teeth being employed for the seizure of prey, 

 both the sharp pointed teeth of many fishes and the more 

 powerful canines of Carnivora. The incisors are modified for 

 dividing and tearing, and the molars both for this purpose, 

 and also, in many animals, for the grinding and trituration 

 of the food. 



In all these modifications they are most perfectly adapted 

 to their chief purpose, the seizure and preparation of the 

 food. 



As resisting power and a certain degree of hardness are 

 essential to the functions performed by the teeth, they are 

 usually calcified, that is, permeated by, or impregnated 

 with, inorganic salts. Some teeth, however, persist through- 

 out life as horny structures, produced from the stratum 

 corneum of the oral epithelium, as the teeth of the Cyclo- 

 stomata (Petromyzon, Myxine, and Bdellostoma) in fishes, 

 and the adult structures in Ornithorhynchus which take the 

 place of teeth, although more correctly described as horny 

 plates. 



While confined in the higher forms to the maxillary, 

 premaxillary, and mandibular bones, teeth are found, 

 especially in fish, in many other positions, as on the pre- 

 mandibular, vomer, pterygoid, and pharyngeal bones, and 

 upon the branchial arches. In many fish the whole mouth 

 bristles with teeth, and they are even found upon the tongue 

 in the parasitic Myxine. Horny structures are also present 

 on the tongue of Ornithorhynchus. 



Three chief tissues constitute the structure of the com- 

 pleted tooth : dentine, enamel, and cement. 



The bulk of the tooth is made up of dentine, a hard 

 (falcified substance penetrated in different degrees by 

 channels ; an exceedingly hard external layer, the enamel, 

 is present in most teeth and covers the exposed surface of 

 the crown and the roots, and in some instances the crowns 

 of the tooth are coated with cement, which, as its name 

 indicates, serves in many teeth to bind together the other 

 two tissues, as shown in the Ungulates, where the cement 

 in some forms in early stages a complete investment of the 

 crown, and when subjected to wear, occupies the intervals 



