142 NERVOUS SYSTEM. 



also pass to the liver and spleen and communicate with the solar 

 plexus. 



Mention some of its communications with other nerves. 



Soon after leaving its origin in the medulla the vagus enters into 

 so many communications with other nerves, both sensory and mo- 

 tor, that it is difficult to know the real fibres of the original root 

 and to determine what are original and what derived functions. 

 The sympathetic system sends fibres in all the branches of the 

 pneumogastric, and the pneumogastric sends branches to many of 

 the important sympathetic plexuses and ganglia ; the pharyngeal, 

 laryngeal, oesophageal, pulmonary, cardiac, and solar plexuses are 

 so made up by branches from both. The spinal accessory nerve is 

 an important contributor, in that it sends a large branch which is 

 incorporated in the vagus. There are also communications to the 

 glosso-pharyngeal and hypoglossal nerves, and it also receives 

 motor fibres from the facial and upper two cervical nerves. The 

 original nerve is probably entirely sensory, and its motor function 

 is derived from these connections with motor nerves. 



Where are the motor fibres from the spinal accessory nerve sup- 

 posed to be distributed? 

 Principally in the recurrent laryngeal nerve. 



What is the function of the vagus in connection with respira- 

 tion? 



The nerve supplies, as has been said, the motor and sensory 

 functions of the larynx, and in this is of value to the respiratory 

 function both in the prevention of foreign substances entering the 

 rima glottidis, and in the opening of that orifice for the entrance 

 of air. Besides this, it supplies sensory fibres to the pulmonary 

 plexus which transmit the reflex stimulus to the medulla, by which 

 the motor apparatus is excited to action. 



What is its influence upon the voice? 



The muscles of the larynx involved in the production of sound 

 are supplied by this nerve, and, as the approximation of the chor- 

 dse vocales is necessary for this, it follows that the voice is depend- 

 ent upon the fibres of the pneumogastric supplied by the inferior 

 or recurrent laryngeal nerve. 



What effect upon respiration follows section of the vagi ? 



The respiration is slowed immediately to about half its usual 



