158 



THE DISSECTION OF PIG EMBRYOS 



to erupt, the roots of the milk teeth undergo partial resolution, their dental pulp 

 dies, and they are eventually shed. Toward the sixth year, before the shedding 



Permanent second mola 



Deciduous molars 

 Ma ndibular canal 



Permanent firs; molar 



Permdueiil premuldrs ^.^^^ _ _^ .^ ,. -.»>--. ,^ 



^' ^'''^■- "iiiii^-^iTT^fif'Mii ' — Menial jaramen 



Permanent canine „ „ _ < • ,v„»^ 



Permanent incisors 



Fig. 166.— The skull of a five-year-old child showing positions of the decidual and permanent teeth 



(Sobotta-McMurrich) . 



of the deciduous teeth begins, each jaw may contain twenty-six teeth. The 

 permanent teeth are "cut" as follows (McMurrich in Keibel and Mall, vol. 2) : 



First Molars seventh year. 



Median Incisors eighth year. 



Lateral Incisors ninth year. 



First Premolars tenth year. 



Second Premolars eleventh year. 



^™^ I thirteenth to fourteenth year. 



Second Molars J 



Third Molars (Wisdom Teeth) seventeenth to fortieth year. 



The teeth of vertebrates are homologues of the placoid scales of elasmobranch fishes 

 (sharks and skates). The teeth of the shark resemble enlarged scales, and many generations 

 of teeth are produced in the adult fish. In some mammalian embryos three or even four 

 dentitions are present. The primitive teeth of mammals are of the canine type, and from 

 this conical tooth the incisors and molars have been differentiated. 



Anomalies. — Dental anomalies are frequent and may consist in the congenital absence 

 of some or all of the teeth, or in the production of more than the normal number. Defective 

 teeth are frequently associated with hare lip. Cases have been noted in which, owing to 

 defect of the enamel organ, the enamel was entirely wanting. Many cases in which a third 

 dentition occurred have been recorded and occasionally fourth molars may be developed 

 behind the wisdom teeth. 



