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A TEXTBOOK OF PHYSIOLOGY 



surfaces until it is thoroughly chewed. In man, the duration of the 

 process of chewing varies with the nature of the food and the tempera- 

 ment of the individual. Some people, generally young, chew their 

 food much less than others. Great value is attached to thorough 

 mastication by some, but manj 7 continue to bolt their food, as doe& 

 a dog, often apparently without harm. 



The degree of chewing varies with the nature of the food, a hard, 

 dry food requiring considerably more chewing than a soft, pappy 

 food. Generally speaking, the food is chewed for twenty to thirty 

 seconds, and in this time about 1 to 1 grammes of saliva may be 

 added to a mass which generally varies from 3 to 6 grammes. The 

 pressure exerted during chewing may be as great as 270 pounds as 

 measured by a spring dynamometer. Such great pressures are, how- 

 ever, not usually employed, since a side-to-side grind is more effective 

 than the direct thrust. Thus, the crushing-point of cooked meat to 



Naso-pharyr::'. 



Soft palate 



^Oesophagus 



A B 



FIG. 201. To SHOW THE MECHANISM OF THE FIRST STAGE OF SWALLOWING. 

 A, at rest; B, swallowing. 



a direct thrust varies from 15 to 80 pounds, with a grinding move- 

 ment but 1 to 2 pounds pressure is required for cooked tongue, 

 and but 40 pounds pressure for tough beef. The softening effect of 

 saliva upon the pressure required in chewing is also very marked. 

 Soft crumb bread, for example, requires more than 60 pounds direct 

 pressure, but when softened with a little saliva it can be masticated 

 with a pressure of 3 pounds. The chewing of agreeable foodstuffs is 

 of value in reflexly promoting a flow of gastric juice, and perhaps 

 causing a tonic contraction of the circular muscles of the stomach, 

 thus regulating the stomach movements. 



The Mechanism of Swallowing. After a proper degree of mastica- 

 tion, the food is gathered as a bolus at the back of the tongue. Then 

 follows the complex sequence of events which constitute the act of 

 swallowing. Forward movement of the bolus is prevented by the 

 pressure of the tip and sides of the tongue against the hard palate 

 and the teeth. It is impossible to swallow with the tongue relaxed. 



