760 PHYSIOLOGY 



swallowing, it passes much more rapidly, the total time taken being 

 between eight and eighteen seconds. 



It has been customary since the time of Magendie to divide the 

 act of swallowing into three stages, during the first of which the 

 bolus of food is carried past the anterior pillars of the fauces ; during 

 the second through the pharynx, past the openings of the nasal 

 cavities and of the larynx ; and during the third through the oeso- 

 phagus into the stomach. There is, however, no pause between these 

 various stages. The act of deglutition is one, and the initiation 

 of the first stage inevitably involves the completion of the third. 

 The food is masticated, and is collected as a bolus on the dorsum 

 of the tongue. A pause then takes place in the movements of mastica- 

 tion, a slight movement of the diaphragm usually occurs known as 

 ' respiration of swallowing,' and then a sudden elevation of the 

 tongue throws the bolus back through the anterior pillars of the 

 fauces. In this movement the chief factor is the contraction of 

 the mylohyoid muscle, which presses the tongue against the palate 

 and pushes it backwards. The backward movement of the tongue may 

 also be aided by the contraction of the styloglossus and palatoglossus 

 muscles which pull the base of the tongue suddenly backwards. 

 These muscles, especially the palatoglossi, serve to close the isthmus 

 faucium, thus preventing any return of the food towards the mouth. 



As the food is passing through the upper part of the pharynx it 

 traverses a region common to the respiratory as well as the digestive 

 passages. Its passage through this region is therefore rapid, and 

 is associated with a closure of the two openings of the air passages 

 into the pharynx. The nasal cavity is shut off by a simultaneous 

 contraction of the levator-palati and palato-pharyngeus muscles and 

 azygos uvulae, by which means the soft palate is raised (Fig. 331) and 

 the posterior pillars are proximated to the uvula. The upper and back 

 wall of the palate is thus formed into a tense sloping roof which guides 

 the bolus down the pharynx. 



More important is the shutting off of the lower air passages. 

 The contraction of the mylohyoid muscles which initiate deglutition 

 is followed almost immediately (at an interval of 047 sec.) by 

 an elevation of the larynx, and this elevation is accompanied by 

 closure of the glottis as well as of the superior opening of the larynx. 

 The laryngeal opening is bounded in front by the epiglottis, behind 

 by the tips of the arytenoid cartilages, and at the sides by the aryteno- 

 epiglottidean folds. When deglutition takes place the arytenoid 

 cartilages, which normally lie against the posterior wall of the pharynx, 

 are rotated and move, inwards and forwards, so that the laryngeal 

 opening assumes the form of a tri-radiate fissure, the vertical limb 

 being short, while the transverse limb is rounded owing to the pulling 



