264 DIGESTION. 



follow eacli other rapidly. The first is performed volun- 

 tarily by the muscles of the tongue and cheeks. The second 

 also is effected with the aid of muscles which are in part 

 endued witli voluntary motion, such as the muscles of the 

 soft palate and pharynx ; but it is, nevertheless, an invo- 

 luntary act, and takes place without our being able to 

 prevent it, as soon as a morsel of food, drink, or saliva is 

 carried backwards to a certain point of the tongue's sur- 

 face. When we appear to swallow voluntarily, we only 

 convey, through the first act of deglutition, a portion 

 of food or saliva beyond the anterior arch of the palate ; 

 then the substance acts as a stimulus, which, in accordance 

 with the laws of reflex movements hereafter to be described, 

 is carried by the sensitive nerves to the medulla oblongata, 

 when it is reflected by the motor nerves, and an involuntary 

 adapted action of the muscles of the palate and pharynx 

 ensues. The third act of deglutition takes place in the 

 oesophagus, the muscular fibres of which are entirely beyond 

 the influence of the will. 



The second act of deglutition is the most complicated, 

 because the food must pass by the posterior orifice of the 

 nose and the upper opening of the larynx without touching 

 them. When it has been brought, by the first act, between 

 the anterior arches of the palate, it is moved onwards by 

 the tongue being carried backwards, and by the muscles of 

 the anterior arches contracting on it and then behind it. 

 The root of the tongue being retracted, and the larynx 

 being raised with the pharynx and carried forwards under 

 the tongue, the epiglottis is pressed over the upper opening 

 of the larynx, and the morsel glides past it ; the closure of 

 the glottis being additionally secured by the simultaneous 

 contraction of its own muscles : so that, even when the 

 epiglottis is destroyed, there is little danger of food or drink 

 passing into the larynx so long as its muscles can act freely. 

 At the same time the raising of the soft palate, so that its 

 posterior edge touches the back part of the pharynx, and 



