DIGESTIVE SYSTEM. 



but clothes at least the anterior portion of the adult 

 pharynx. The tongue, however, is invested with en- 

 toderm epithelium and 



so are the 

 tubes. 



Eustachian 



TEETH. 



Morphologically, 

 teeth are appendages 

 of the skin, and are to 

 be compared with such 

 structures as hair and 

 nails. They are thus 

 a part of the exoskel- 

 eton and their relation 

 to the bones or the en- 

 doskeleton is entirely 

 a secondary process, 

 for the purpose of 

 strength and support. 

 In many of the fishes 

 the mouth generally 

 is lined by simple cone- 

 shaped teeth that serve 

 the purpose of seizing 

 and holding the ani- 

 mal's prey. In man 

 the additional function of masticating the food has 

 greatly modified the form and structure of teeth. 



Dentition. There are twenty deciduous or tem- 

 porary teeth that erupt between the ages of six 

 months and two and one-half years. In each jaw 

 these are, incisors 4, cuspids 2, molars 4, the dental 



Fig. 103. The palate and supe- 

 rior dental arch (right side): i, Me- 

 dian incisors; 2, lateral incisors; 3, 

 canine; 4, first bicuspid; 5, second 

 bicuspid; 6, first molar; 7, second 

 molar; 8, wisdom-tooth; 9, mucous 

 membrane of the hard palate con- 

 tinuous behind with that of the soft 

 palate; 10, the anteroposterior raphe 

 of palate; u, pits on each side of the 

 raphe perforated with the orifices of 

 glands; 12, anterior rugosities of the 

 mucous membrane (after Testut). 



