DIGESTION 301 



mylo- and genio-hyoid, depresses, the lower jaw, but its down- 

 ward movement is mainly a passive one. The external ptery- 

 goids pull it forward when both contract, forward and to one side 

 when only one contracts. The lower fibres of the temporal muscle 

 retract the jaw. The buccinator and orbicularis oris muscles 

 prevent the food from passing between the teeth and the cheeks 

 and lips. The tongue keeps the food in motion, works it up with 

 the saliva, and finally gathers it into a bolus ready for deglutition. 



Deglutition. This act consists of a voluntary and an involun- 

 tary stage. Just before the beginning of the voluntary stage 

 mastication is suspended, and a slight contraction of the dia- 

 phragm generally takes place. The anterior part of the tongue 

 is suddenly elevated and pressed against the hard palate, and the 

 elevation travels back from the tip towards the root, as the mylo- 

 hyoid muscles in the floor of the mouth contract sharply, so as 

 to thrust the bolus through the isthmus of the fauces. As soon as 

 this has happened and the food has reached the posterior portion 

 of the tongue, it has passed beyond the control of the will, and 

 the second or involuntary stage of the process begins. 



This stage may be divided into two parts : (i) Pharyngeal, 

 (2) oesophageal both being reflex acts. During the first the 

 food has to pass through the pharynx, the upper portion of which 

 forms a part of the respiratory tract, and is in free communication 

 with the larynx during ordinary breathing. It is therefore 

 necessary that respiration should be interrupted and the larynx 

 closed while the food is being moved through the pharynx. But 

 that the interruption may be short, the food must be rapidly 

 passed over this perilous portion of its descent. The main pro- 

 pelling force under which the bolus is shot through the back of 

 the pharynx is derived from the contraction of the mylo-hyoid 

 muscles already mentioned, assisted to some extent by the stylo- 

 and palato-glossi ; and that none of the purchase may be lost, 

 the pharyngeal cavity is cut off from the nose and mouth as soon 

 as the bolus has entered it. The soft palate is raised by the 

 levator palati and palato-pharyngei muscles ; at the same time 

 the upper part of the pharynx, narrowed by the contraction of the 

 superior constrictor, comes forward to meet the soft palate, 

 closes in upon it, and so prevents the food from passing into the 

 nasal cavities. The pharynx is cut off from the mouth by the 

 closure of the fauces through the contraction of the palato- 

 pharyngeal muscles which lie in their posterior pillars. The 

 upper free end of the epiglottis (the so-called pharyngeal part) 

 aids the back of the tongue in completing a movable partition 

 across the pharynx, which keeps close to the bolus as it passes 

 down between the posterior surface of the epiglottis and the 

 posterior wall of the pharynx. Almost immediately after the 



