DEGLUTITION 183 



protection of the larynx during the second period of deglutition, obser- 

 vations on the human subject only are to be relied upon. Such obser- 

 vations, in cases of loss of the epiglottis especially, show that this part 

 is necessary to the complete protection of the larynx. While loss of the 

 epiglottis may not interfere always with the perfect deglutition of solids, 

 and even of liquids, particles of food and liquids frequently find their 

 way into the larynx, and deglutition often is effected with difficulty, 

 showing that complete protection of the larynx at all times does not 

 exist unless the epiglottis is intact. 



To appreciate the mechanism by which the opening of the larynx is 

 protected during the deglutition of solids and liquids, one has only to 

 carefully follow the articles as they pass over the inclined plane formed 

 by the back of the tongue and the anterior and inferior part of the 

 pharynx. As the food is making this passage in obedience to the con- 

 traction of the muscles that carry the tongue backward, draw up the 

 larynx and constrict the pharynx, the soft base of the tongue and the 

 upper part of the larynx are applied to each other, with the epiglottis, 

 which is now inclined backward, between them. At the same time the 

 glottis is closed, in part by the action of the constrictor muscles attached 

 to the sides of the thyroid cartilages and in part by the action of the 

 intrinsic muscles. If the food is tolerably consistent and in the form of 

 a single bolus, it slips easily from the back of the tongue along the 

 membrane covering the anterior and inferior part of the pharynx ; but 

 if it is liquid or of soft consistence, a portion takes this course, while 

 another portion passes over the epiglottis, being directed by it into the 

 two grooves by the side of the larynx. It is by these means, together 

 with those by which the posterior nares are protected, that all solids 

 and liquids are passed into the oesophagus and the second period of 

 deglutition is safely accomplished. 



The third period of deglutition is the most simple of all. It merely 

 involves contractions of the muscular walls of the oesophagus, by which 

 the food is passed into the stomach. The longitudinal fibres shorten 

 the tube and slip the mucous membrane, lubricated with its glairy 

 secretion, above the bolus ; while the circular fibres, by a progressive 

 peristaltic contraction from above downward, propel the food into the 

 stomach. In experiments on the lower animals, it has been observed 

 that while the peristaltic contractions of the upper two-thirds of the 

 tube are immediately followed by a relaxation, which continues until 

 the next act of deglutition, the lower third remains contracted for 

 about thirty seconds after the passage of the food into the stomach. 

 During its contraction, this part of the oesophagus is hard, like a cord 

 firmly stretched. This is followed by relaxation; and alternate con- 



