THE PNKUMOGASTRIO. 567 



membrane take their shun' in exciting tlui action of its muscular coat. 

 The general sensibility of this canal, however, is very slight, as coin- 

 pared with the parts above, and is not usually sntlicient to cause a 

 perceptible impression from the food in the a.ct of swallowing. Its 

 muscular contraction takes place, as a general rule, without any ell'ect 

 on the consciousness; and it is only when the food is very cold or very 

 hot, or when it contains pungent or irritating ingredients, that it8 

 passage through the (esophagus produces a distinct sensation. 



Jt appears that the filaments of the superior laryngeal nerve, dis- 

 tributed about the anterior surface of the epiglottis and borders of the 

 larynx, take an active part in exciting the movements of deglutition. 

 In the experiments of Waller and I'revost on dogs and cats, galvaniza- 

 tion of the superior laryngeal nerve produced, in many repeated trials, 

 rhythmical movements of deglutition, consisting of contraction of the 

 pharynx and elevation of the larynx, followed by peristalt ic motion of 

 the whole length of the oesophagus. All the sensitive fibres of the 

 pneumogMsl.ric, therefore, distributed to the parts concerned in the net 

 of swallowing, undoubtedly assist in exciting the necessary muscular 

 contractions, 



1'ntlccfion of Ike Glottis in the act of Deglutition. As the larynx 

 communicates, by its superior orifice, directly with the cavity of the pha- 

 rynx, and as all solids and liquids, in the act of swallowing, necessarily 

 pass over its surface, portions of the food would be constantly liable to 

 find their way through the rima glottidis into the respiratory passages, 

 unless then' were some provision against it. The epiglottis, which 

 stands in front of the glottis ill a nearly upright position, and which 

 shuts down over its orifice like a cover when the base of the tongue is 

 drawn back at the time of deglutition, might seem to be adapted to 

 secure protection in this respect 



Kxperienee shows, however, that the epiglottis is not essential for 

 the safety of the glottis in deglutition. The entire organ may be cut 

 off in dogs, as we have verified by repeated experiments, without any dif- 

 ficulty being afterward exhibited by the animal in swallowing either 

 liquid or solid food. The epiglottis, furthermore, is an organ which 

 exists only in mammalians, being absent, in all the remaining classes of 

 vertebrate animals. In birds especially, the orifice of the glottis can IK; 

 readily seen on opening the beak, unprotected by anything similar to 

 an epiglottis, and performing the alternate movements of expansion 

 and collapse connected with respiration. Finally, the existence of the 

 epiglottis in man does not prevent foreign substances from passing into 

 the glottis whenever the other conditions of normal deglutition are sus- 

 pended or dist urbed. The protection of the glottis against the entrance 

 of solid or liquid food does not depend upon a mechanical obstacle, but 

 upon a definite association of nervous acts. 



The first requisite for the act, of swallowing is the sunjHtnuion of respi- 

 ration. This takes place, at the beginning of deglutition, by a nervous 

 influence which it is difficult to describe, but which may be designated 



