532 MANUAL OF PHYSIOLOGY. 



In the laryngeal branches are fibres which bear centrifugal 

 impulses to the vasomotor 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 in- 

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



3. Excito-inhibitory fibres pass from the heart to the vasomotor 

 centre. Stimulation of these fibres, which take somewhat differ- 

 ent courses in different animals, checks the tonic action of the 

 vasomotor centre, and greatly reduces the blood pressure. Hence 

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

 are stimulated by distention of that organ ; and the vasomotor 

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

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



4. Excito -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 

 final result, because the muscles of the larynx are paralyzed, 

 and closure of the glottis is impossible. A change in voice fol- 

 lows the section or injury of even one inferior laryngeal, as may 

 often be seen in man from the effect of the pressure of an aneurism. 



IX. HYPOGLOSSAL NERVE. 



This nerve appears in the furrow between the olivary body 

 and the anterior pyramid, on a line with the anterior roots of 

 the spinal nerves. It corresponds with the anterior roots in 

 function, being a purely motor nerve. It bears impulses to the 



