58 PHYSIOLOGY. 



Mastication is particularly important when solid and fibrous foods 

 are eaten, to prepare them by comminution for the fermentative 

 action of the various digestive fluids. When improperly performed 

 repeatedly, a severe form of dyspepsia ensues. 



During mastication there is performed a separate and distinct 

 act, insalivation, or the mixing of the food with saliva. By means of 

 it, the dry, hard portions of food are moistened and softened the better 

 to fit them for swallowing; at the same time the mucous membrane 

 is lubricated to allow free movement of the food over its surface, and 

 the surfaces of the teeth are freed from accumulations of food dur- 

 ing mastication, which otherwise would collect and impede its pro- 

 gress. A fever patient attempting to swallow a dry cracker affords 

 ample illustration of the mechanical value of the saliva during mas- 

 tication. 



DEGLUTITION. 



The swallowing of the food, which has been named the act of 

 deglutition, is performed by the aid of the tongue, fauces, pharynx, 

 and the oesophagus or gullet. For the purpose of description only, 

 since the process in reality admits of no lines of distinction, this act 

 is usually said to comprise three stages: first, that in which the food 

 is forced backward from the mouth, through the fauces into the 

 pharynx. This act is voluntary, though usually performed uncon- 

 sciously, being ascribed to the movements of the tongue itself. The 

 second stage is that in which the bolus is made to travel along the 

 middle and lower part of the pharynx to the oesophagus. This sec- 

 ond act is more complicated and requires quicker movements, because 

 the nasal and laryngeal orifices are open, but past which the food must 

 go without entering. The main motive power for this performance 

 is gained by the contractions of the three constrictors, aided by the 

 synchronous action of other muscles, whose duty is to occlude tem- 

 porarily the nasal and laryngeal openings. The opening into the 

 nasal cavity is closed by the elevation of the soft palate, uvula, and 

 the contraction of the posterior pillars of the fauces. Just above the 

 laryngeal opening and at the base of the tongue is a small, leaf- 

 shaped piece of cartilage, the epiglottis. It was formerly believed that 

 the laryngeal orifice was guarded during deglutition by the retraction 

 of the tongue pressing down the epiglottis to fit it firmly. But, as 

 removal of the epiglottis did not interfere with normal swallowing, it 

 was learned that the real safeguard was the contraction of the aryteno- 

 epiglottic folds. The third stage is that in which the bolus descends 



