The vagus arises from the lower part of the floor of the fourth ventricle, and is connected with many of the important groups of nerve cells in this neighborhood.
The functions of its widely-distributed fibres may be thus briefly stated: -
(A) The Efferent Fibres may be divided into -
1. Motor-nerve channels, going to a great portion of the alimentary tract and air passage; the following muscles getting their motor supply from the branches of the vagus - the pharyngeal constrictors, some muscles of the palate, oesophagus, stomach and greater part of the small intestine. Motor impulses also pass along the trunk of the vagus - though leaving the cord by the roots of the accessory nerve - to the intrinsic 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 control of the pulmonary branches of the vagus.
2. Vasomotor 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 hnpulses 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 page 280), 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 important for the functions of the various viscera. They are: -
Sensory Fibres carrying impulses from the pharynx, oesophagus, stomach and intestine, and from the larynx, trachea, bronchi and lungs generally. The pneumonia which follows section of the vagi depends on - (i) the removal of 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.
There is no nerve that can be compared with the vagus in the variety of reflex phenomena in which it participates. Afferent fibres in this nerve cause spasm of the muscles of the glottis and thorax, and govern the respiratory rhythm, preside over inhalation of air and excite the expiratory 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 breathing movements 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.
In the laryngeal branches are fibres which bear centrifugal impulses to the vasomotor centres in the medulla, and excite them to action. These, which may be grouped with the excito-motor channels, are spoken of as "pressor" fibres, from the influence they exert upon the pressure of the blood in the arteries.
Excito-Inhibitory Fibres pass from the heart to the vasomotor centre. Stimulation of these fibres, which take somewhat different 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 pressure falls so that the over-filled heart can empty itself.
Stimulation of the gastric endings of the vagus causes not only gastric, but also salivary secretion, 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 breathing 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 gives 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 follows 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.