THE VAGUS NERVE. 531 



the forces of the circulation pass along the vagus to the ganglia 

 of the heart. As already explained in detail (see p. 282), these 

 fibres are always acting, as shown by the fact that section of the 

 vagi causes a considerable quickening of the heart beat. On the 

 other hand, if the distal end of the cut vagus be stimulated, the 

 heart beats more slowly, and in some animals may come to a 

 stand-still in a condition of relaxation. 



B. The AFFERENT FIBRES, still more widely spread, are im- 

 portant for the functions of the various viscera. They are : 



1. Sensory fibres carry impulses from the pharynx, oesophagus, 

 stomach, and intestine, and from the larynx, trachea, bronchi, 

 and the lungs generally. The pneumonia which follows section 

 of the vagi depends on (1) the removal of the sensibility, and 

 the ease with which foreign matters can enter the air passages ; 

 or (2) the violent breathing necessary when the motor nerves of 

 the larynx are cut ; or (3) the injury of trophic or vasomotor 

 fibres. 



2. Excito-motor nerves. There is no nerve that can be com- 

 pared with the vagus in the variety of the reflex phenomena in 

 which it participates. Afferent fibres in this nerve cause spasm 

 of the muscles of the thorax and govern the respiratory rhythm, 

 and preside over the inhalation of the air and excite the expira- 

 tory muscles. Thus irritation of the mucous membrane at the 

 root of the tongue, the folds of the epiglottis, larynx, trachea, or 

 bronchi, causes spasmodic fits of coughing. Irritation of the 

 pharyngeal or the gastric fibres gives rise, by reflex stimulation, 

 to the act of vomiting. 



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 muscles 

 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 

 discharge of impulse to the thoracic muscles. 



