124 DIGESTION 



The changes which the food undergoes in the mouth 

 consist in a grinding up into fragments of about 2 mm. 

 These, when impregnated and lubricated by the sahva, 

 are ready for transference to the stomach. 



The function of ptyaUn is to convert starch into maltose, 

 the action of the ferment occurring almost entirely in the 

 stomach. 



Deglutition 



Deglutition is a complex process, or rather succession 

 of processes initiated by a muscular movement under 

 control of the will. The food is collected in a bolus on 

 the dorsum of the tongue. It has to be transferred to the 

 oesophagus, avoiding the nasopharynx and the larynx. 

 Return to the front part of the mouth is prevented by the 

 apposition of the upper surface of the tongue to the hard 

 palate. A quick contraction of the mylohyoid and hyo- 

 glossus muscles draws the tongue upwards and backwards. 

 At the same time the palatal muscles close the posterior 

 nares by drawing the soft palate back to the posterior wall 

 of the pharynx. The elevation of the hyoid bone, which 

 occurs simultaneously, raises the larynx, the upper opening 

 of which is closed by the descent of the epiglottis. By 

 this co-ordinated movement the bolus is pushed down past 

 the soft palate and posterior wall of the pharynx into the 

 upper end of the oesophagus, which is stretched open to 

 receive it. Coincidently with this there is an inhibition 

 of respiration. When the bolus enters the oesophagus it 

 passes out of voluntary control, and normally out of 

 consciousness. 



The way in which food passes along the oesophagus 

 depends upon its consistency. The ordinary bolus is 

 carried down by a wave of contraction, which is initiated 

 refiexly by the contact of the food with the pharyngeal 

 wall. This wave becomes slower as it courses downwards. 

 In the case of a thin fluid the propulsive force of the volun- 

 tary part of deglutition is sufficient to drive it mth great 



