INTRODUCTORY. TEETH. 5 



II. MESOBLASTIC (mesodermal). 



Dentine or ivory generally forms the main mass of a 

 tooth. It is a hard, white substance allied to bone. When 

 examined microscopically dentine is seen to be traversed by 

 great numbers of nearly parallel branching tubules which 

 radiate outwards from the pulp-cavity. In fishes as a rule, 

 and sometimes in other animals, a variety of dentine con- 

 taining blood-vessels occurs, this is called vasodentine. 



Cement or crusta petrosa forms the outermost layer of 

 the root of the tooth. In composition and structure it is 

 practically identical with bone. In the more complicated 

 mammalian teeth, besides enveloping the root, it fills up the 

 spaces between the folds of the enamel. 



The hard parts of a tooth commonly enclose a central pulp- 

 cavity into which projects the pulp, a papilla of the dermis 

 including blood-vessels and nerves. As long as growth con- 

 tinues the outer layers of this pulp become successively calcified 

 and added to the substance of the dentine. In young growing 

 teeth the pulp-cavity remains widely open, but in mammals 

 the general rule is that as a tooth gets older and the crown 

 becomes fully formed, the remainder of the pulp becomes 

 converted into one or more tapering roots which are im- 

 bedded in the alveolar cavities of the jaws. The opening of 

 the pulp-cavity is then reduced to a minute perforation at the 

 base of each root. A tooth of this kind is called a rooted 

 tooth. 



But it is not only in young teeth that the pulp-cavity 

 sometimes remains widely open; for some teeth, such as the 

 tusks of Elephants and the incisor teeth of Rodents, form 

 no roots and continue to grow throughout the animal's 

 life. Such teeth are said to be rootless or to have persistent 

 pulps. 



An intermediate condition is seen in some teeth, such as 

 the grinding teeth of Horses These teeth grow for a very 

 long time, their crowns wearing away as fast as their bases 



