DEGLUTITION. 20T 



ative necessity of air in the system must, in a short time, overcome any vol- 

 untary 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 entirely 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 and peculiar movement of elevation of 

 the larynx, by the action of muscles which usually depress the lower jaw, but 

 which are now acting from this bone as the fixed point. The muscles which 

 produce this movement act chiefly upon the hyoid bone. They are the di- 

 gastric (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 ante- 

 rior and inferior portions 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 superior constrictor 

 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. All these complex movements are accomplished 

 with great rapidity, and the larynx and pharynx are then returned to their 

 original position. 



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

 tion. When the act of deglutition is performed with regularity, no portion 

 of the liquids and solids swallowed ever 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 alimentary mass, but the velum pend- 

 ulum palati itself, and in part, also, by contraction of the muscles which form 

 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. This fact has 

 been observed in cases in which the parts have been exposed by surgical oper- 

 ations, and its mechanism has also been observed in the human subject, by 

 Bidder and by Kobelt. 



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

 muscles, which form the posterior pillars of the soft palate, are in a con- 

 dition of contraction by which the edges of the pillars are nearly approxi- 

 mated, forming, with the uvula between them, almost a complete diaphragm 

 between the postero-superior and the antero-inferior parts of the pharynx. 

 This, with the application of the posterior wall of the pharynx to the superior 

 face of the soft palate, completes the protection of the posterior openings of 

 the nasal fossae. 



