530 MANUAL OF PHYSIOLOGY. 



Stimulation of the proximal cut end of the trunk of the vagus 

 causes inspiratory effort and cessation of the breathing move- 

 ments in the position of inspiration. Stimulation of the central 

 cut end of the superior laryngeal branch, causes reflex spasm of 

 the muscles of the larynx and a fixation of the expiratory mus- 

 cles in the position of expiration. The fibres which regulate the 

 respiratory rhythm consist of two sets, probably passing from the 

 lungs to the inspiratory and expiratory centres, and causing each 

 to act before its ordinary automatism would transmit any dis- 

 charge of impulse to the thoracic muscles. 



In the laryngeal branches are fibres which bear centrifugal 

 impulses to the vaso-motor centres in the medulla, and excite the 

 centres to action. These, which may be grouped with the excito- 

 motor channels, are spoken of as " pressor " fibres, from the influ- 

 ence they exert upon the pressure of the blood in the arteries. 



3. Excito-inhibitory fibres pass from the heart to the vaso-motor 

 centre. Stimulation of these fibres, which take somewhat different 

 courses in different animals, checks the tonic action of the vaso- 

 motor centre, and greatly reduces the blood pressure. Hence 

 these fibres form the depressor nerve. Its terminals in the heart 

 are stimulated by distension of that organ ; and the vaso-motor 

 centre is thereby inhibited, the arteries dilate and the blood pres- 

 sure falls so that the over-filled heart can empty itself. 



4. Exeito-secretory fibres. Stimulation of the gastric endings 

 of the vagus causes not only gastric, but also the salivary secre- 

 tion, which occurs as a precursor of gastric vomiting. 



Section of both vagi in the neck causes the death of the animal 

 within a day or two after the operation, and the following changes 

 may be observed while it lives : 1. The heart-beat is much quicker 

 as shown by the increased pulse-frequency. 2. The rate of breath- 

 ing is very much slower. 3. Deglutition is difficult, the food 

 easily passing into the air-passages through the insensitive larynx. 



Section of the superior laryngeal nerves is followed by slight 

 slowness of breathing, loss of sensibility in the larynx, entrance 

 of food into the' air-passages, chronic broncho-pneumonia, and 

 death. 



Section of the inferior laryngeal nerves give rise to the same 



