DIGESTIVE SYSTEM. 



143 



but clothes at least the anterior portion of the adult 

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

 toderm epithelium and 

 o are the Eustachian 

 tubes. 



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; n, pits on each side of the 

 raphe perforated with the orifices of 

 glands; 12, anterior rugosities of the 

 mucous membrane (after Testut). 



