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\b[The two <pneumogastric nerves> are the right and left <vagus nerves>, the tenth <cranial nerves>: mixed sensory, branchial motor and <parasympathetic> pathways from the <medulla oblongata> to the neck, <thorax> and abdomen.] A useful description separates their apparent origin at the surface of the <brain> from the origins and terminations of their component fibres.
Rootlets emerge in the groove behind the <inferior olive>, between the <glossopharyngeal nerve> above and the accessory rootlets below. Branchial motor fibres arise chiefly in the <nucleus ambiguus>; preganglionic <parasympathetic> fibres arise in the <dorsal motor nucleus of the vagus> and, particularly for cardiac control, the <nucleus ambiguus>. Visceral sensory fibres have their cell bodies chiefly in the <inferior vagal ganglion> and terminate centrally in the <nucleus of the solitary tract>; somatic sensory fibres associated with the <superior vagal ganglion> project to the <spinal trigeminal nucleus>. Consequently a sensory fibre is not accurately described as originating in a motor nucleus simply because its <peripheral nerve> emerges nearby.<Neuroanatomy, Nucleus Ambiguus (StatPearls)>[Introduction and Structure and Function: branchial motor output and cardiac vagal control]{ref}
Each <vagus nerve> leaves the <skull> through the <jugular foramen>, with the <glossopharyngeal nerve> and <accessory nerve>. It bears the small <superior vagal ganglion> at the foramen and the larger <inferior vagal ganglion> below it. Connections include communications with adjacent <cranial nerves> and the cervical <sympathetic nervous system>. Traditional descriptions assign fibres joining the <vagus nerve> from the cranial part of the <accessory nerve> to the latter <peripheral nerve>; their modern classification is more nuanced and does not change the peripheral branches being described. In the neck the <vagus nerve> descends in the <carotid sheath>, behind and between the <internal jugular vein> and the internal, then common, <carotid artery>.
Its meningeal and auricular branches supply small sensory territories of the cranial covering and external ear. Pharyngeal branches contribute to the <pharyngeal plexus>, supplying much of the <soft palate> and <pharynx>; the <stylopharyngeus muscle> belongs to the <glossopharyngeal nerve> and the <tensor veli palatini muscle> to the mandibular division of the <trigeminal nerve>. The internal branch of the <superior laryngeal nerve> supplies sensation above the <vocal folds>; its external branch supplies the <cricothyroid muscle>. The <recurrent laryngeal nerve> supplies the other intrinsic <laryngeal muscles> and sensation below the <vocal folds>. The right <recurrent laryngeal nerve> loops beneath the <right subclavian artery>; the left loops under the <aortic arch> beside the <ligamentum arteriosum>. Both return towards the <larynx> near the groove between <trachea> and <oesophagus>.<Neuroanatomy, Cranial Nerve 10 (Vagus Nerve) (StatPearls)>[Structure and Function; Vagus Nerve Branches; Muscles]{ref}
In the <thorax>, both <vagus nerves> pass behind the roots of the <lungs>, contrasting with the anterior course of the <phrenic nerves>. Cardiac branches join the <cardiac plexus>; pulmonary branches join the <pulmonary plexus>. The trunks break up into the <oesophageal plexus>, then collect principally as an anterior trunk derived from the left <vagus nerve> and a posterior trunk derived from the right. Through the <oesophageal hiatus> they reach the <stomach>; hepatic and coeliac branches distribute to abdominal viscera. Their <parasympathetic> territory includes foregut and midgut derivatives, extending into the proximal <colon>, rather than every abdominal and pelvic organ.<Neuroanatomy, Cranial Nerve 10 (Vagus Nerve) (StatPearls)>[Structure and Function: thoracic course and oesophageal plexus]{ref} This extensive distribution explains why the level of experimental section is indispensable information.
The main consequences of section can be organized by the interrupted functions:
* <Heart>: cutting both cervical <vagus nerves> removes tonic inhibition and usually increases <heart rate>. The <heart> continues beating because its rhythm is intrinsic. Electrical stimulation of the peripheral cut end can slow the <heart>, whereas stimulation of the central end tests reflex effects through the <brainstem>. Cutting a pathway and stimulating its intact output are different experiments.
* <Breathing>: bilateral cervical section commonly produces slower, deeper breaths in experimental mammals. Sensory information from the <lungs> has been interrupted; the <respiratory diaphragm> is not directly paralysed, because its motor <peripheral nerves> are the <phrenic nerves>. Persistence of <breathing> establishes that the <vagus nerves> modify the respiratory rhythm rather than being its sole generator. The subsequently named <Hering-Breuer inflation reflex> provides a later explanation of one component of this feedback; its formal description belongs to 1868, after this examination.<Reflexes from the lungs and airways: historical perspective>[Discussion of the 1868 lung inflation and deflation experiments]{ref}
* <Larynx> and <lungs>: section proximal to the <recurrent laryngeal nerves> can impair opening and closure of the <vocal folds>, causing altered voice, obstruction and impaired airway protection. Classical bilateral cervical experiments sometimes ended in death with pulmonary congestion, <oedema> or <aspiration> changes. Such outcomes must be distinguished from an immediate necessary arrest of <breathing>: species, age, lesion level and airway management affect them.
* <Stomach>: bilateral section can reduce vagally driven <gastric acid secretion> and disturb gastric motor coordination and emptying. Nevertheless <digestion> and <nutrient absorption> are not abolished as chemical possibilities: <enteric nervous system> activity, local stimuli and <hormones> remain. Likewise, interruption of gastric sensory fibres does not establish that the entire experience of hunger originates in them.
A discriminating historical experiment would record respiratory movements and <heart rate> before and after section, distinguish unilateral from bilateral section, and compare an intact airway with an airway bypass. A <tracheal cannula> can separate obstruction at the <glottis> from changes in the respiratory rhythm, while the central-versus-peripheral stimulation comparison separates incoming reflex traffic from outgoing motor effects. Later separate-lung recordings confirmed that a change following unilateral section could affect both <lungs>, supporting a central reflex interpretation rather than simple local paralysis.<A Study of the Hering-Breuer Reflex>[Summary and Conclusions, points 1–4; separate recordings of the two lungs, 1927]{ref} The gastric distinction is also borne out by measurements of persistent acidity after surgical <vagotomy>, despite major reductions in stimulated <gastric acid secretion>.<Twenty-four hour gastric acidity after vagotomy>[Abstract: hourly intragastric sampling, seven patients, four years after operation]{ref}
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