THE CRANIAL NERVES. 143 



rate, and soon drops to five or six to the minute, and even slower. 

 The respiration is easy inspiration slow and full, expiration harsh 

 and sudden. Death follows this operation in a short time (one to 

 six days), and the animal during the time is sluggish and appa- 

 rently suffers from slow carbonic-oxide narcosis. It is inferred 

 from this that the vagus is the nerve which carries to the auto- 

 matic centre the stimuli which are needed to keep up the automat- 

 ism, and that the medulla is incapable of originating the motor 

 impulses unless controlled by afferent stimuli. 



What is the function of the pneumogastric in deglutition ? 



Deglutition both in the pharynx and oesophagus is under the influ- 

 ence of the vagus, which gives innervation directly to the thoracic 

 part of the latter and through the inferior laryngeal branch to the 

 cervical part. The sensory fibres act as conductors of the stimulus 

 which results in the reflex peristalsis by which the food is carried 

 on through the oesophagus. The sensory distribution to the larynx 

 must not be forgotten in this connection, for by it food is kept from 

 the respiratory organs. Section of the vagi causes paralysis of 

 swallowing, and food is apt to pass the glottis on an attempt to 

 swallow, not even a cough being excited by such an accident. The 

 closure of the glottis in swallowing is caused by a reflex action 

 known as the " action of arrest," and is derived from the sensory 

 fibres of the vagus. 



What is the physiological relation of this nerve to the stomach ? 



Both sensory and motor. The stomach receives its warning of 

 the presence of food through the sensory fibres, and the muscular 

 fibres excite the organ to contract upon it and '' churn " it about 

 during digestion. There is also a vaso-motor influence derived 

 from the vagus. When the nerve is cut but little food can reach 

 the stomach because of the paralysis of the oesophagus, and what 

 food does enter is digested very slowly, so that the function of the 

 pneumogastric may be considered essential to stomach digestion. 

 The connection with the solar plexus also involves the intestines in 

 the action of the vagus. 



What important excito-motor reflexes depend upon the pneumo- 

 gastric for one or both stimuli? 



Coughing and vomiting, as well as many other less essential 

 reflexes, such as sighing, hiccoughing, and the like. 



