Anatomy and Physiology of the Larynx. 95 



expiration is easily effected. After a few days, however, the 

 animals experimented upon (carnivora) become tranquil, 

 respire with less effort, and the passive vibratory movements 

 of the vocal cords diminish. But even after a considerable 

 time, if the creatures are excited, they are seized with 

 severe dyspnoea, which disappears only when they have 

 become quiet again. 



In this condition, also, owing to paralysis of the larjTigeal 

 muscles, foreign matters are likely to enter the trachea, 

 because of the paralysis rendering the initial stage in the act 

 of swallowing more difficult in the oesophageal region. 

 Hence broncho-pneumonia may ensue.^ 



It is, therefore, a well-established fact that division of 

 both pneumogastric nerves, or their recurrent branches, is 

 often very quickly followed by fatal results, especially in 

 young animals. But in old ones, section of the recurrent 

 is not generally fatal, and even that of both pneumogastric 

 nerves is not always so ; when it does occur, it only ensues 

 slowly. This difference is probably because the laryngeal 

 cartilages are very elastic in young creatures, and in yield- 

 ing permit the glottis to be closed by the excess of external 

 atmospheric pressure in inspiration, over that of the rarified 

 air within the trachea ; and though in expiration the glottis 

 returns to its original size, yet these animals will quickly die 

 of suffocation, unless tracheotomy be performed upon them. 



In old animals, on the contrary, the rigidity and pro- 

 minence of the arytaiuoid cartilages prevents the glottis 

 from becoming completely closed by the atmospheric 

 pressure ; for even when all the muscles are paralysed, 

 the posterior portion of the pars respiratoria remains open, 

 and through this sufficient air can be drawn so long as 

 the creatures remain tranquil. 



Schech's experiments on dogs, made in 1873,^ show that 



1 " Landois and Stirling : " A Text-Book of Human Physiology," 

 2nd edition, vol. ii., p. 874. 



2 " Experimentelle Untersuchungen fiber die Functionen der Nerven 

 und Muskeln des KehlkofEs ;' Wurzburg, 1873. 



