564: THE CRANIAL NERVES. 



and collapses in expiration ; after the section of the nerves, it is nar- 

 rowed in inspiration and passively opened in expiration by the forcible 

 expulsion of the air. The effects thus produced on the glottis, by division 

 of the inferior laryngeal nerves, are the same with those which take place 

 in the nostrils after division of the facial nerves. Both these sets of 

 movements are connected with the mechanism of respiration, and both 

 are influenced in a similar manner by division of their motor nerves. 



As the laryngeal muscles are necessarily paralyzed by division of the 

 pneumogastric nerves in the middle of the neck, the effects of this mus- 

 cular paralysis are necessarily added to those which result from in- 

 terruption of the sensitive function of the pneumogastric branches in 

 the lungs. In very young animals, as mentioned above, the effects due 

 simply to laryngeal paralysis are more marked than m adults ; and in 

 order to determine the extent of its influence upon the lungs we have 

 performed a comparative experiment, in the following manner. Two 

 pups were taken belonging to the same litter, and of the same size and 

 vigor, about two weeks old. In one of them (No. 1) a section was made 

 of both pneumogastric nerves in the middle of the neck ; in the other 

 (No. 2), the inferior laryngeal nerves only were divided, the pneumogas- 

 trics being left untouched. In No. 1, therefore, the natural stimulus to 

 respiration was diminished at the same time that the muscles of the 

 larynx were paralyzed ; in No. 2, there was laryngeal paralysis alone, 

 the sensibility to the demand for respiration remaining the same. For 

 the first few seconds after the operation there was but little difference in 

 the condition of the two animals, the laryngeal symptoms being most 

 prominent in both. There was the same obstruction at the glottis owing 

 to paralysis of the lar} r ngeal muscles, the same difficulty of inspiration, 

 and the same frothing at the mouth. Very soon, however, in No. 1, the 

 respiratory movements became quiescent, and at the same time much 

 reduced in frequency, falling to ten, eight, and five respirations per 

 minute, as usual after section of the pneumogastrics ; while in No. 2 

 the respiration continued frequent as well as laborious, and the general 

 signs of agitation and discomfort were kept up for one or two hours, after 

 which there followed diminished excitability of the nervous centres, and 

 the animal became exhausted, cool, and partially insensible, like the 

 other. They both died between thirt} 7 and forty hours after the opera- 

 tion. On post-mortem inspection it was found that congestion and 

 solidification of the lungs existed to a similar extent in each instance; 

 and the only appreciable difference between the two bodies was that in 

 No. 1 the blood was coagulated, and the abdominal organs natural, while 

 in No. 2 the blood was fluid and the abdominal organs congested. The 

 alteration in the tissue of the lungs, therefore, after the pneumogastric 

 nerves have been divided, is not a direct effect, produced by cutting off 

 the influence of these nerves upon the pulmonary tissue, but results 

 indirectly from the diminished activity of respiration and imperfect 

 aeration of the blood. 



