THE MECHANICAL PHENOMENA OF DIGESTION 317 



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 inter- 

 ruption may be short, the food must be rapidly passed over this 

 perilous portion of its descent. The main propelling 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 contraction 

 of the mylo-hyoids the larynx is pulled upwards and forwards by 

 the contraction of the thyro-hyoid muscle, and the elevation of the 

 hyoid bone by the muscTes which connect it to the lower jaw. 

 The base of the tongue is simultaneously drawn backwards by the 

 stylo- and palato-glossus. The lower or laryngeal portion of the 

 epiglottis is thus caused to come into contact with the upper orifice 

 of the larynx, occluding it completely, but the pharyngeal portion 

 projects beyond the larynx, and takes no share in its closure 

 (Eykman). The glottis is closed by the approximation of the vocal 

 cords and the arytenoid cartilages. The epiglottis, however, is not 

 absolutely indispensable for closing the larynx, since swallowing 

 proceeds in the ordinary way when it is absent. The morsel of 

 food, grasped by the middle and lower constrictors as it leaves the 

 back of the tongue, passes rapidly and safely over the closed larynx, 

 the process being accelerated by the pulling up of the lower portion 



