DEGLUTITION 229 



anteriorly, on account of the presence of the choanae 

 (posterior nares), the isthmus faucium, and the laryngeal 

 aperture (Fig. 86). It is formed posteriorly and on each side 

 by the constrictor muscles, which become continuous below 

 with the muscular coat of the oesophagus. These muscles 

 play an important part in the act of deglutition, and they are 

 assisted by the muscles of the tongue and soft palate. 



The act of deglutition comprises a voluntary and an in- 

 voluntary stage, but the two overlap one another and are 

 difficult to distinguish. The mouth is closed by certain of 

 the muscles of mastication (p. 70), and the cheeks and 

 lips are pressed against the teeth and gums by the con- 

 traction of the buccinators and the orbicularis oris. The soft 

 palate is raised and drawn tense so as to cut off the communi- 

 cation between the nasal and the oral parts of the pharynx. 

 The tongue and hyoid bone are suddenly drawn upwards by 

 the mylo-hyoids, digastrics, etc., and the bolus of food is 

 forced backwards through the isthmus faucium. As it enters 

 the oral part of the pharynx, its passage is hastened by the 

 approximation of the palatine arches (pillars of the fauces), 

 which squeeze it onwards. The bolus is then acted on by 

 the constrictors, which force it downwards into the oesophagus. 



Owing to the attachments of the thyreo-hyoid membrane 

 (Fig. 121), the elevation of the hyoid bone is necessarily 

 accompanied by elevation of the larynx, and, at the same 

 time, the ary-epiglottic folds (p. 332) become shortened and 

 approximated so that the apices of the arytaenoids are brought 

 into contact with the tubercle (cushion) of the epiglottis. In 

 this way the cavity of the larynx is almost completely shut off 

 from the pharynx. As a result, the breath is held during 

 deglutition, and anything which may cause a sudden inspiration 

 re-opens the communication so that a portion of the bolus 

 may be drawn into the larynx. 



Not only does the act of deglutition demand a number of 

 intimately related and complicated movements, but it also 

 brings into action several different groups of muscles, which 



