284 A MANUAL OF PHYSIOLOGY 



In mastication the lower jaw is moved up and down, so 

 as to alternately separate and approximate the two rows of 

 teeth. It has also a certain amount of movement from side 

 to side, and from front to back. The masseter, temporal 

 and internal pterygoid muscles raise, and the digastric, with 

 the assistance of the mylo- and genio-hyoid, depresses, the 

 lower jaw. The external pterygoids 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 in- 

 voluntary stage. During the former the anterior part of 

 the tongue is pressed against the hard palate 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 inter- 

 rupted 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 pharynx is accordingly provided 

 with rapidly-contracting striped muscle ; and that none of 

 its 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 ; 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 



