48 ANLMAL DENTISTRY. 



teeth several openings are formed, one for each root. The 

 cement is ossified connective tissue, and is the last of the 

 three hard tissues to form. It develops Ijetween the perios- 

 teum of the tooth cavity and the dental substance and is 

 influenced in its evolution through life by both these ele- 

 ments. 



During these evolutionary changes each tooth is en- 

 closed within a separate sac ordinarily designated as a den- 

 tal follicle. During the same period the jaw is undergoing 

 great changes. It is growing upward to further enclose the 

 follicles within itself, and is gradually separating them one 

 from another by the formation of bony partitions (the inter- 

 dental cancellated tissue) until each tooth occupies a deep 

 individual compartirxnt — the alveolar cavity — which the 

 tooth fills completely at every stage of its evolution and ex- 

 istence and which entirely encloses both until by sheer 

 force of its expansion it forces its way into the mouth and 

 takes its place at the level of the dental arcade. This event 

 marks the beginning of the second period of dental evolu- 

 tion. 



The permanent teeth are developed much in the same 

 manner. About the sixteenth week of foetal life a stalk or 

 neck becomes disconnected from the common enamel germ 

 and takes its place first on the side, and secondly, at the root 

 of the temporary tooth. It undergoes the same steps of de- 

 velopment, and b}' its upward growth absorbs the fang of 

 the temporary tooth until only a mere shell remains. The 

 permanent teeth that arc not represented by temporary 

 predecessors are formed by the backward extension of the 

 common enamel germ. The first permanent tooth of this 

 class — the fourth molar — has its origin in embryo, while the 

 others which erupt later in life may not begin to form until 

 some time after birth. The enamel germ, therefore, is not 

 entirelv a foetal structure, but exists until the last tooth is 



