Chap. 11 FOODS AND NUTRITION 181 



As to teeth, at least, the human mouth is a middle-of-the-road type. Human 

 teeth like pigs' teeth are generalized and adapted to mixed diets. Although the 

 main kinds of teeth are moderately represented, none could be safely used to 

 nibble a cupboard door. There are 20 human milk or baby teeth, which 

 usually develop before three years of age; and ordinarily 32 teeth in the so- 

 called permanent set which begins to appear at about six years and finishes at 

 twenty-five (Fig. 11.10). Actually we have one-and-a-half complete sets of 

 teeth in a lifetime, the first set and a partial second one, since the molars of 

 the first are not shed like all the other milk teeth. 



The jav/s of modern man are shorter than those of his early ancestors who 



PERMANENT^ 

 INCISORS 



DEC IDUOUS— s; — ^-™ 

 INCISORS. |- \y 



SECOND 

 PERMANENT- 

 MOLAR. 



PERMANENT- 

 PREMOLARS 



PERMANENT- 

 CANINE. 



DEaDUOUS 

 MOUliRS. 



HRST 



PERMANENT 

 MOLAR. 



PERMANi=:^n' 

 INCIS0r^O. 



Fig. 11.10. Human teeth, one and a half natural sets in a lifetime. The teeth 

 of a five-year-old child with portions of the jaws cut away to expose the roots of 

 the milk teeth and the partially developed permanent teeth. (Courtesy, Rand: The 

 Chordates. Philadelphia, The Blakiston Co., 1950.) 



still had a fourth molar, now uncommon. Even the third molar or wisdom 

 tooth comes late and with difficulty and is little more than a nuisance. As a 

 result of the modern shortened jaws, the wisdom teeth often do not have 

 enough room, are crooked and out of position. 



Swallowing. When food is about to be swallowed, the tongue is moved back- 

 ward and pressed up against the hard palate (Fig. 11.11). One swallows 

 quickly and momentarily stops breathing. In that instant the food, now almost 

 at the crossways in the pharynx, moves obliquely toward the esophagus. 

 It might go into the nose, back into the mouth, or into the windpipe 

 were it not so well prevented. But the soft palate is automatically pulled up, 

 closing the way to the nose, and the base of the tongue shuts off the mouth. 

 At the same time, the voice box, or larynx is pulled upward against its cover, 

 the epiglottis, and this shuts off the road down the windpipe. On the instant 

 that all the ways are closed, the throat muscles contract and the food is shot 



