THE VAGUS NERVE. 529 



leaving the cord by the roots of the accessory nerve to the in- 

 trinsic muscles of the larynx ; these fibres lie in the inferior or 

 recurrent laryngeal nerve except that to the crico-thyroid, which 

 lies in the superior laryngeal branch. The tracheal muscle and 

 the smooth muscle of the bronchial walls are also under the con- 

 trol of the pulmonary branches of the vagus. 



2. Vctso-motor fibres are said to be supplied to the stomach and 

 small intestine. These fibres are probably derived from some of 

 the numerous connections with the sympathetic. 



3. Inhibitory impulses of great importance for the regulation of 

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

 of the heart. As already explained in detail (see p. 276), 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 

 standstill 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 vaso-motor 

 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. 



