398 PHYSIOLOGY CHAP. 



vagus contained in the superior laryngeal. The special sensory 

 and motor functions of the two laryngeal nerves, the nerves of 

 phonation, are dealt with in Chap. III. of this volume. 



(e) The mucous membrane and plain muscle fibres of the 

 trachea, bronchi, and pulmonary alveoli are innervated by the 

 pulmonary branches of the vagus, which form the pulmonary 

 plexus. The important part played by the afferent and efferent 

 fibres of the pulmonary branches of the vagus in the innervation 

 of the respiratory apparatus is discussed in Vol. I. Chap. XIII. 



(/) The function of the branches of the vagus that form the 

 cardiac plexus (inhibitory fibres, depressor nerve, etc.) in control- 

 ling the action of the heart has been dealt with in Vol. I. Chap. IX. 



(g) The importance of the oesophageal, gastric, and caeliac 

 plexuses of the vagus has already been discussed in other chapters, 

 particularly in Chap. III. 4, 9, and 11, and IV 7, of Vol. II. (also 

 in Chap. VI. of this volume). 



In order to obtain a clear and accurate idea of the vital 

 importance of the vagi, in regulating circulation, respiration, and 

 digestion, we need only examine the effects of dividing them on 

 both sides in the neck. This study was inaugurated by Valsalva, 

 Morgagni (1740), Legallois (1812); continued more particularly 

 by Traube (1846), Cl. Bernard (1858), M. Schiff (186*7); and 

 resumed more recently by Vanlair (1893), A. Herzen (1894), 

 Pawlow (1910), Nicolaides (1901), Gomez Ocaiia (1903). The 

 results can be briefly summarised. Section of both vagi in the 

 neck produces many disturbances, which lead more or less rapidly 

 to the death of the animal. Babbits generally die in twenty 

 to thirty-six hours, dogs in four to five days, fowls in six to 

 seven hours. In young animals death takes place in thirty to 

 sixty minutes after vagotomy with symptoms of acute asphyxia, 

 owing to total paralysis of the laryngeal muscles, and almost 

 complete closure of the glottis by the passive adduction of 

 the vocal cords. This is due to paralysis of the posterior thyro- 

 arytenoid muscles which dilate the glottis, and to the fact that the 

 pars interarytaenoidea of the glottis is incompletely developed, 

 and its lips are almost entirely destitute of membrane. Legallois 

 found that in young animals simple section of the recurrent nerves 

 suffices to produce death by asphyxia ; but if, after cutting the 

 vagi free, pulmonary ventilation is supplied by tracheotomy, young 

 animals, like adults, are capable of surviving longer. 



The cause of death of adult animals after bilateral section of 

 the vagi is very complex. 



It is generally accepted that the section or ligation of one 

 vagus only in mammals or man is well borne in the majority of 

 cases, the disturbances of cardiac rhythm, respiratory rhythm, and 

 digestive functions being readily and speedily compensated. But 

 if the other vagus be simultaneously divided, the consequent trachy- 



