ELEMENTARY EXPERIMENTAL PHYSIOLOGY 199 



CHAPTER LV. 

 THE INFLUENCE OF THE VAGUS UPON RESPIRATION. 



THE lungs are supplied with motor and sensory fibres from the vagus 

 nerve ; the motor fibres pass to the involuntary muscle fibres of the 

 bronchioles and control their contraction ; the sensory fibres carry 

 impulses from the lungs to the respiratory centre to co-ordinate the 

 respiratory movements. The latter group of fibres can be stimulated 

 by the degree of distension of the alveoli, and may be divided into 

 expiratory and inspiratory fibres. 



DEMONSTRATION. A rabbit is anaesthetised with chloral, urethane, 

 or ether. The trachea and vagi nerves are exposed and a cannula is 

 placed in the trachea. The respiratory movements can be recorded and 

 rendered visible to a large class by fastening one end of a thread to a 

 tuft of fur in the epigastric region and the other end to a lever. The 

 rate and depth of respiration are observed, then the effect of blowing 

 air into the lungs, and, after an interval, the effect of sucking air out 

 of the lungs. Positive ventilation produces expiratory apnoea, 

 negative ventilation a condition of continued contraction of the 

 diaphragm. 



One vagus is now divided ; little or no change will be observed in 

 the rate and depth of respiration, but when the other vagus is cut the 

 breathing becomes very slow and deep. The experiments of positive 

 and negative ventilation are repeated. The effects previously seen are 

 absent. It is possible, however, by excessive ventilation of the lungs 

 to reduce the pressure of carbon dioxide in the blood and produce 

 apnoea. 



The rabbit is now killed, and the tracheal cannula is connected with 

 a water manometer. Excitation of the peripheral end of the vagus with 

 a faradic current will cause a small rise of pressure in the manometer, 

 due to the contraction of the bronchial muscles. 



The rhythm of respiration is maintained by the changes in the 

 composition of the blood, not by reflex action due to alterations in the 

 distension of the lungs. The vagus, however, influences the respiratory 

 centre, thereby co-ordinating the respiratory movements, and, in 

 addition, it is probable that it influences at the same time the cardiac 

 centre and harmonises the working of the heart and lungs. 



After section of both vagi the animal dies in a day or two from 

 septic broncho-pneumonia, due to the passage of food or saliva into the 

 trachea and bronchi. The larynx is paralysed by section of the vagi 



