DEGLUTITION l8l 



time, overcome any voluntary effort by which respiration has been 

 arrested. 



The second period of deglutition involves more complex and 

 important muscular action than the first. By a rapid succession of 

 movements, the food is made to pass through the pharynx into the 

 oesophagus. The movements are then beyond the control of the will 

 and belong to the kind called reflex. After the alimentary mass has 

 passed beyond the isthmus of the fauces, it is easy to observe a sudden 

 movement of elevation of the larynx, by the action of muscles which 

 usually depress the lower jaw, but are now acting from this bone as 

 the fixed point. The muscles which produce this movement act chiefly 

 on the hyoid bone. They are the digastric (particularly the anterior 

 belly), the mylo-hyoid, the genio-hyoid, the stylo-hyoid and some of the 

 fibres of the genio-glossus. It is probable, also, that the thyro-hyoid 

 acts at this time to draw the larynx toward the hyoid bone. With this 

 elevation of the larynx, there is necessarily an elevation of the anterior 

 and inferior parts of the pharynx, which are, as it were, slipped under 

 the alimentary bolus as it is held by the constrictors of the isthmus of 

 the fauces. 



Contraction of the constrictor muscles of the pharynx takes place 

 almost simultaneously with the movement of elevation ; and the supe- 

 rior constructor is so situated as to grasp the morsel of food, and with it 

 the soft palate. The muscles, the constrictors acting from the median 

 raphe, draw up the anterior and inferior walls of the pharynx and pass 

 the food rapidly into the upper part of the oesophagus. These complex 

 movements are accomplished with great rapidity, and the larynx and 

 pharynx are afterward returned to their original position. 



Protection of the Posterior Nares during the Second Period of Deglu- 

 tition. When the act of deglutition is performed with regularity, 

 no part of the liquids and solids swallowed finds its way into the air- 

 passages. The entrance of foreign substances into the posterior nares 

 is prevented in part by the action of the superior constrictors of the 

 pharynx, which embrace, during their contraction, not only the alimen- 

 tary mass, but the velum pendulum palati itself, and in part, also, by 

 contraction of the muscles forming the posterior pillars of the soft 

 palate. During the first part of the second period of deglutition, the 

 soft palate is slightly raised, being pressed upward by the morsel of 

 food. 



While the food is passing through the pharynx, the palato-pharyn- 

 geal muscles, which form the posterior pillars of the soft palate, are in 

 a condition of contraction by which the edges of the pillars are nearly 

 approximated, forming, with the uvula between them, almost a complete 



