THE PNEUMOGASTRIC. 563 



thus separate the vocal chords from each other and largely increase the 

 transverse diameter of the orifice of the glottis. When they relax at 

 the time of expiration, the arytenoid cartilages return to their former 

 position, and the opening of the glottis is again narrowed by the passive 

 approximation of the vocal chords. As the movements of expansion 

 are accomplished by the action of the laryngeal muscles, they depend 

 upon the influence of the pneumogastric nerve and its inferior laryngeal 

 branch. 



Both the movements of the glottis in respiration and their dependence 

 upon nervous influence may be seen in the dog by means of an operation 

 which consists in making a dissection along the side of the neck, in 

 such a way as to expose the pharynx and a considerable portion of the 

 oesophagus. The superior laryngeal nerve on that side is necessarily 

 cut across, but the inferior laryngeal, as well as the trunk of the pneu- 

 mogastric, are left uninjured. By a longitudinal incision through the 

 pharynx and oesophagus, the upper and posterior surfaces of the larynx 

 are then exposed, and, notwithstanding the previous division of the 

 superior laryngeal nerve, the alternate movements of expansion and 

 collapse of the glottis are seen going on in their natural order, and 

 keeping pace with the corresponding respiratory movements of the 

 chest. If now the inferior laryngeal nerve be divided upon either the 

 right or the left side, the vocal chord of that side becomes motionless, 

 while that of the opposite side continues to move as before. If the re- 

 maining laryngeal nerve be divided, all movements of expansion in the 

 vocal chords instantly cease ; and the same effect is produced by section 

 of both pneumogastric nerves in the middle of the neck, since the in- 

 ferior laryngeals are given off as branches below that point. 



If the section of both pneumogastric nerves, or of their inferior laryn- 

 geal branches, be made simultaneously under these circumstances while 

 the breathing is tolerably rapid, the injurious effect of laryngeal paralysis 

 upon respiration at once becomes manifest. Both vocal chords being 

 then deprived of the active control of their muscles, the borders of the 

 rima glottidis are left in a condition of passive flexibility. They have 

 not only lost the power of separating from each other and thus opening 

 the glottis at the time of inspiration, but they are also drawn downward 

 and inward by the current of air passing into the trachea, and thus, like 

 a double membranous valve, they occlude more or less completely the 

 orifice of the glottis, and offer a physical obstacle to the free entrance 

 of the air. In very young animals, where there is but little rigidity of 

 the laryngeal cartilages, the occlusion of the glottis thus produced after 

 section of the inferior laryngeal nerves, may be so complete as to produce 

 immediate death by suffocation ; in adult animals the occlusion is only 

 partial, but is still sufficient to diminish perceptibly the capacity of res- 

 piration. 



The natural movements of the glottis in breathing are therefore 

 reversed after section of the inferior laryngeal nerves. Before this 

 operation, in the normal condition, the glottis is opened at inspiration 



