DEGLUTITION. 



3i5 



and means must be taken to prevent any of the food escaping into the 

 air passages. We have therefore to consider the means whereby — 



(1) The bolus is propelled downwards into the grasp of the constrictors ; 



(2) the nasal passages are closed ; (3) the larynx is guarded. 



1. The chief factor in the propulsion of the bolus through the back 

 of the pharynx is the contraction of the mylohyoid muscle, which presses 

 the tongue against the palate and pushes it backwards. In this action 

 it may be aided by the contraction of the styloglossus and palatoglossus 

 muscles, which pull the base of the tongue suddenly backwards. These 

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

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



2. The shutting off of the naso-pharyngeal cavity is a somewhat 

 complicated act. As soon as, or even before, the bolus touches the soft 

 palate, this structure is raised and put on the stretch by a simultaneous 

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

 side. At the same time the contraction of the last-named muscles, which 

 form the posterior pillars of the fauces, approximates and straightens 

 the edges of these two structures, and raises to a certain extent the 

 upper part of the pharynx. The soft palate, wdiich has thus been raised, 

 comes in contact with an elevation on the posterior wall of the pharynx, 

 an elevation clue to the contraction of fibres of the superior constrictor 

 of the pharynx. The uvula, which is also raised slightly by the azygos 

 uvulae, lies between and fills up the opening between the straightened 

 posterior pillars of the fauces. The soft palate is thus formed into a 

 tense sloping roof, which guides the bolus down the pharynx. 



3. More important still is the shutting off of the lower air-passages 

 from the pharynx. The contraction of the mylohyoid muscles, which 

 accompanies the first act of deglutition, can be seen to be followed 

 almost immediately (at an interval of O07 seconds) by an elevation of 

 the larynx. Observations on animals and on men in whom the parts 

 involved have been extensively exposed by operation, have shown that 

 this elevation of the larynx is accompanied by a closure of the glottis 

 as well as of the superior opening of the larynx. That this latter is 

 important is shown by the fact that the mucous membrane of the 

 vestibule of the larynx is extremely irritable, so that any particle of 

 food falling on it will cause a spasmodic cough. It has generally 

 been supposed to be carried out by a movement of the dorsum of the 

 tongue backwards, effected by the stylo- and palato-glossal muscles. 

 In consequence of this movement, combined with contraction of the 

 muscular fibres in the aryteno-epiglottidean folds, the expanded portion 

 of the epiglottis has been supposed to be folded like a lid over the 

 opening of the larynx, thus furnishing a sloping floor for the passage of 

 the food. But observations by Anderson Stuart 1 have shown that this 

 bending over of the epiglottis probably does not occur, that the epi- 

 glottis remains erect, and that the food passes not over its upper 

 (anterior) but over its lower (posterior) surface ; the anterior resting 

 against the root of the tongue. Magendie and Schiff had already pointed 

 out that extirpation of the epiglottis caused very little disturbance to 

 an animal, which could swallow both solids and fluids as well after the 

 operation as before. 



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- 



1 Proc. Roy. Sue. London, 1891, vol. 1. \>. 323. 



