The peculiarity in the course of the inferior laryngeal from which it derives its name of TC.• current, depends upon the changes in the rela tive position of the branchial arteries to the larynx in the embryo, after they have assumed the form presented in the adult by the arch of the aorta and the large vessels which spring from it. In those cases where the right subclavian artery, instead of arising along with the right carotid by a common trunk (arteria innominata), comes off from the arch of the aorta beyond the origin of the left subclavian, or, in other words, is the last in order of the large arteries which supply the head and thoracic extremities, and then proceeds across the spine behind the (esophagus to reach its usual position behind the scalenus anticus muscle on the right side, the recurrent does not arch round the right subclavian, but is given off from the trunk of the vagus as it is passing the larynx.* Course of the vagus through the thorax.— After the right vagus has given off the recurrent, it passes behind the ascending portion of the arch of the aorta, and proceeding downwards, inwards, and backwards behind the right bronchus, right pulmonary artery and veins, reaches the cesophagus as it lies in the posterior mediastinum. The left vagus, after passing from the anterior surface of the arch of the aorta, also proceeds downwards, inwards, and backwards behind the left bronchus, left pul monary artery and veins, and also reaches the cesophagus in the posterior mediastinum at the same part where the right vagus joins it. Both nerves closely accompany the cesophagus down the posterior mediastinum, and pass from the thorax into the upper part of the abdomen through the same opening (cesophageal open ing) in the diaphragm. At the upper part of the chest, the vagi become flattened from before backwards, and are consequently broader and thinner than in the neck.
Imrnediately after the vagus has given off the recurrent it sends numerous twigs inwards. Some of these pass upwards and inwards to assist in forming the cardiac plexus ; some pro ceed transversely inwards upon the anterior surface of the lower part of the trachea, and anastomose with other branches from the vagus arising higher up, and also with branches from the recurrent and sympathetic to form the ante rior and inferior tracheal plexus (plexus trache alis anterior et inferior); while others pass upon the posterior surface of the lower part of the tmchea, anastomose with other branches from the recurrent and sympathetic, and thus form the posterior and inferior tracheal plexus. The vagus at this part also sends a few twigs upon the upper part of the thoracic portion of the (esophagus, forming a free anastomosis on its surface with other twigs from the recurrent and the posterior bronchial plexus (plexus cesopha gei thoracici superior). It likewise sends some branches inwards and downwards to throw themselves into the lateral portion of the lower part of the cardiac plexus ; while a few others pass still more downwards to reach the anterior surface of the pulmonary veins and branches of the trunk of the pulmonary artery, and anasto mose with branches from the inferior part of the cardiac plexus prolonged upon these ves sels. Other branches proceed downwards and inwards upon the antenor surface of the bron chii, and anastomose with the descending bmnches of the anterior and inferior tracheal plexus, with the nervous filaments accompany ing- the pulmonary bloodvessels, and with some branches direct from the sympathetic to form the anterior pulmonary plexus (plexus pulmo nalis anterior). A few twigs also proceed from this portion of the vagus into the anterior me diastinum, and are chiefly distributed in the thymus gland. As the trunk of the vagus passes behind the bronchus it sends off several pretty large branches upon the posterior surface of that tube, and also a few smaller ones upon the posterior surface of the pulmonary bloodvessels. These branches form a great part of the posterior pulmonary plexus (plexus pulmonalis poste rior), and anastomose with twigs from the posterior and inferior bronchial, with some fila ments from the superior thoracic cesophageal and the anterior pulmonary plexuses. The branches of the pulmonary plexuses, after send ing off some nervous filaments which run for some distance below the pleura, (vide Reissei sen De Fabrica Pulmonum, 1822, tab. vi.
plate 2,) accompany the bronchial tubes and bloodvessels into the interior of the lungs, and follow the divisions and subdivisions of the bronchial tubes. The two trunks of the vagi, after leaving the lower edge of the bronchi, soon reach the cesophagus, where each nerve divides into three or four chords upon the sur face of the (esophagus; those formed by the subdivisions of the left vagus lying on its an terior and left side, those by the right vagus on its posterior and right side. The chords of the same nerve anastomose freely by large branches, and also by smaller and less nume rous branches on both sides of the cesophagus, with those of the opposite nerve, and thus form an extensive and open network upon the sur face of the (esophagus, called the inferior ceso phageal plexus (plexus cesophageus thoracis inferior).* From these chords nervous fila ments pass into the walls of the (esophagus, and they also exchange some communicating filaments with the sympathetic. Immediately before the vagi pass through the cesophageal opening of the diaphragm, the chords into which each nerve has divided again reunite ; those of the left nerve collecting into one trunk, while those of the right frequently form two branches which run close to each other4 As they pass through the cesophageal opening, the right nerve, or the larger, is placed on the posterior surface of the (esophagus, and the left, or the smaller, on its anterior surface.* , Distribution of the vagus in the abdomen. Left vagus.—As it enters the abdomen it sends some small branches upon the anterior surface of the lower part of the cesophagus, some of which enter the walls of that tube, others anas tomose with (esophageal twigs from the right vagus, and others are prolonged downwards upou the cardiac end of the stomach. As it proceeds downwards over the cardiac surface of the stomach it also passes towards the right side and forms a curve, the convexity of which looks to the left. From the convexity of this curve several small branches run across the anterior surface of the cardiac orifice and the upper part of the large cul de sac of the sto mach, and some of these anastomose with filaments from the left portion of the solar plexus, and from the phrenic. nerve.t From the concavity several small branches run up wards and to the right between the layers of the smaller omentum to join the left hepatic plexus.I The left yaps now divides itself into several branches, which pass towards the pyloric surface of the stomach, along the upper edge of the anterior surface of the stomach, very close to the smaller curvature of that organ, and along the lower edge of the coronary artery of the stomach, sending numerous filaments into the nervous plexuses of the sympathetic surrounding the coronary and superior pyloric arteries, and also branches downwards over the anteriqr surface of the stomach. These latter brdnches, after running a greater or less distance below the peritoneal covering- of the stomach, penetmte the muscular coat where some of their filaments terminate, while others pass through it to reach the mucous coat. The few branches of the left vagus which reach the pyloric orifice are partly distributed upon the walls of that portion of the organ, and partly throw themselves into the cceliac plexus. Some of the filaments of the latter portion may be traced into the numerous plexuses surrounding the gastro-duodenalis branch of the hepatic artery, into the right hepatic plexus, and may sometimes be followed as far as the artery of the gall-bladder. The branches which leave the left vagus as it lies on the anterior surface of the lower part of the cesophagus, and cross the anterior surface of the cardiac orifice of the stomach, divide and subdivide below the peri toneum in a forked manner, and also anas tomose freely with each other, forming a kind of plexus which has been termed the anterior cardiac plexus of the stomach. As the branches of the left vagus pass along, the smaller curva ture of the stomach, they not only anastomose freely with the plexuses of the superior coronary and superior pyloric arteries, but with each other, forming a plexus along the upper edge of the anterior surface of the stomach, stretch ing from the cardiac to the pyloric orifice, which Valentin* has termed plexus gastricus anterior et superior.