224 DIGESTION. 



liquids would almost certainly pass around the borders in quantity sufficient to moisten 

 the mucous membrane below. It must be remembered, however, that the sensibility of 

 the air-passages is very unequal in different persons, and that it may be considerably 

 modified by education of the parts. This should make us hesitate to accept the view 

 that, in gargling, the larynx receives a quantity of liquid, and that an alimentary bolus 

 may be tolerated in the trachea for many minutes without coughing. 



To sum up the mechanism by which the opening of the larynx is protected during the 

 deglutition of solids and liquids, we have 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 contraction 

 of the muscles which 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 its intrinsic muscles. If the 

 food be tolerably consistent and united into a single bolus, it slips easily from the back 

 of the tongue along the membrane covering the anterior and inferior part of the phar- 

 ynx ; but if it be liquid or of little consistence, a portion takes this course, while another 

 portion passes over the epiglottis, being directed by it into the two grooves or gutters 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 involves merely contrac- 

 tions of the muscular walls of the oesophagus, by which the food is forced into the stom- 

 ach. The longitudinal fibres shorten the tube and slip the mucous membrane, lubricated 

 by its glairy secretion, above the bolus ; while the circular fibres, by a progressive peri- 

 staltic contraction from above downward, propel the food into the stomach. The pas- 

 sage of food down the oesophagus was for the first time closely studied by Magendie, 

 who noted, in this connection, many curious and important facts. In numerous experi- 

 ments on the lower animals, he observed that, while the peristaltic contractions of the 

 upper two-thirds of the tube were immediately followed by a relaxation, which contin- 

 ued till the next act of deglutition, the lower third remained contracted generally for 

 about thirty seconds after the passage of the food into the stomach. During its contrac- 

 tion, this part of the oesophagus was hard, like a cord firmly stretched. This was fol- 

 lowed by relaxation ; and this alternate contraction and relaxation continued constantly, 

 even when the stomach was empty, although, during digestion, the contractions were fre- 

 quent in proportion to the quantity of food in the stomach. The contraction was always 

 increased by pressing the stomach and attempting to pass some of its contents into the 

 oesophagus. This provision is undoubtedly important in preventing regurgitation of the 

 contents of the stomach, especially when the organ is exposed to pressure, as in urina- 

 tion or defsecation. We have already noted the action of the crura of the diaphragm, 

 which has a tendency to close the cesophageal opening during inspiration. 



The length of time occupied in the third period of deglutition was noted by Magendie 

 in the inferior animals, but we have been unable to find any definite observations on this 

 point in the human subject, although this would have been easy in the cases of gastric 

 fistula which, from time to time, have come under the observation of physiologists. Ma- 

 gendie found that the alimentary bolus sometimes occupied two or three minutes in its 

 passage, and that it was often momentarily arrested in its course. It frequently seems as 

 though we were ourselves conscious of a very slow passage of food down the oesophagus, 

 and not infrequently a piece of bread or a mouthful of liquid is taken to hasten it ; but 

 it is not probable that every alimentary bolus remains for two or three minutes in the 

 oesophagus, and liquids undoubtedly are swallowed with considerable rapidity, as they 



