300 - Multicellular Animals, Especially Man 



MOLARS 



mMI fmii* 



grinding teeth behind the premolars (Fig. 

 16-15). However, the third molars, or wisdom 

 teeth, sometimes fail to erupt beyond the 

 gum level. In man and other mammals (p. 

 673), the permanent teeth are preceded by 

 the milk teeth, which resemble the adult 

 teeth, except that the molars are not repre- 

 sented. 



The dentition of man is generally similar 

 to that of the other Mammalia; but in dif- 

 ferent mammals the dentition is variously 

 modified according to the food habits of the 

 species. In Rodentia (squirrels, mice, etc.), 

 for example, the incisors, or gnawing teeth, 

 are very prominent. Herbivorous animals 

 (horses, cows, etc.) also have well-developed 

 incisors with which to crop the vegetation; 

 even more particularly, herbivorous animals 

 have very conspicuous molar teeth, to grind 

 and pulp the vegetation in preparation for 

 swallowing. Carnivorous animals (such as 

 wolves and tigers) have very prominent ca- 

 nine teeth, which serve for tearing flesh from 

 the victim, as well as for offensive and defen- 

 sive purposes. Man, however, is quite omniv- 

 orous in his food habits, and the dentition 

 of man is very conservatively balanced. 



As it is chewed, the food is cut and ground 

 thoroughly, which increases the surface ex- 

 posed to the action of the saliva and other 

 digestive juices. Chewing also mixes the food 

 with the saliva and converts the whole food 

 mass into a pulp that can be swallowed with- 

 out difficulty. 



The Tongue. The tongue, essentially, is a 

 mucosa-covered mass of intricately arranged 



Fig. 16-15. The dentition of man. The same 

 kinds of teeth are duplicated in the upper jaw, 



muscle fibers, which can be shaped and 

 moved in a very agile manner. The role of 

 the tongue in shaping our speech — especially 

 when we ask for food — is related indirectly 

 to nutrition, but the tongue also has three 

 direct nutritional functions: (1) the tongue 

 manipulates the food during chewing, so 

 that each morsel is kept in range of the 

 proper teeth; (2) the tongue shapes the 

 chewed food into a pulpy mass, the bolus, 

 and squeezes the bolus into the pharynx, 

 when it is time for swallowing; and (3) the 

 mucosa of the tongue provides a housing for 

 a majority of the taste buds (p. 421). 



The Salivary Glands. Man and most other 

 mammals possess three pairs of salivary 

 glands, which secrete the saliva and send it 

 into the oral cavity. The salivary glands of 

 man include: (1) the parotid glands, which 

 lie embedded in the soft tissues of the cheeks, 

 just below and behind the cheek bones; 

 (2) the submaxillary glands, which are em- 

 bedded in the floor of the mouth immedi- 

 ately in front of the angles of the lower jaws; 

 and (3) the sublingual glands, which also lie 

 in the floor of the mouth along the sides of 

 the tongue (Fig. 16-16). Each gland is a lobu- 

 lated mass of glandular tissue, weighing 

 about one ounce; but the salivary glands 

 jointly produce about two quarts of saliva 

 daily. The saliva drains from the glands via 

 ducts. A single duct from each parotid opens 

 upon the inner surface of the cheek opposite 

 the second upper molar; whereas the duct 

 from each submaxillary, and the several ducts 

 from each sublingual, drain upon the floor 



