This is a most important nerve, not only on account of its wide distribution, but equally so in reference to the variety and complexity of its functions.

It is a mixed nerve, and arises from the side of the medulla oblongata immediately behind the ninth nerve. It then passes out of the cranium through the posterior part of the foramen lacerum basis cranii, where it joins the inner division of the eleventh nerve, with which it unites for about an inch of its course.

In the foramen the nerve presents an enlargement - the jugular ganglion. This gives off the auricular branch of the vagus, which enters the aqueduct of Fallopius to join the seventh nerve, and subsequently passes out with it to be distributed to the lining membrane of the external auditory canal. Below the occipital artery it becomes united with the cervical sympathetic cord, and joined in this way it passes down the neck with the carotid artery as far as the entrance to the chest, where it resumes its independence. From this point it passes onwards above the division of the trachea, and assists in forming the bronchial plexus. It now gives off the oesophageal nerve, which goes to the stomach and the solar plexus. In its course the pneumogastric nerve gives branches -

(1) To the superior cervical ganglion of the sympathetic.

(2) The pharyngeal branch unites with a branch of the ninth pair given off near the termination of the common carotid, and these, together with a branch of the sympathetic, form the pharyngeal plexus, which distributes fibres to the pharynx and commencement of the oesophagus.

(3) The superior laryngeal nerve enters an opening beneath the appendix of the superior border of the thyroid cartilage, and supplies the mucous membrane of the larynx with sensibility. It also gives branches to the mucous membrane of the root of the tongue, the pharynx, and oesophagus, likewise to the crico-thyroid, and crico-pharyngeus muscles.

The inferior laryngeal differs not only in its point of origin, but likewise in the course it subsequently takes.

The right inferior laryngeal or recurrent nerve arises from the parent trunk near to the dorso-cervical artery at the entrance to the chest. It passes round the root of this vessel and escapes from the chest between the carotid artery and the trachea, to which and the oesophagus it gives filaments. It then passes up the neck in company with the former as high as the larynx, and gives its fibres to the posterior crico-arytenoicleus, the lateral crico-arytenoideus, the arytenoideus, and the thyro-arytenoideus muscle. Before leaving the chest this nerve gives branches to the cardiac plexus, and communicates with the middle cervical ganglion of the sympathetic.

(4) The left inferior laryngeal or recurrent nerve is given off from the pneumogastric opposite the root of the aorta; it then courses its way round that vessel, as did the right round the cervico-dorsal artery. Passing out of the chest between the two first ribs, it follows the carotid artery up the neck to reach the larynx. Here it distributes its branches to the posterior and lateral crico-arytenoideus, the arytenoideus, and the thyro-arytenoideus muscles.

It is longer than the right nerve, having to pass round the aorta, and when paralysed gives rise to the disease known as " roaring and whistling".

A branch of the pneumogastric amalgamates with the middle or inferior cervical ganglion of the sympathetic, and a pulmonary plexus is formed at the bifurcation of the trachea; branches from it follow the divisions of the bronchi along their ramifications, and others enter into the cardiac plexus.