PAR VAGUM NERVE. (HUMAN ANA TOMY.) - (1Vervus Vagus ; Pneumogastric ; part of the sixth pair of nerves of the older anatomists; one of the three divisions of the eighth pair in the classification of Willis ; the ninth pair of Andersch ; the tenth pair of Sommerring ; the nioyen sympathique of Win slovv.) The par vagum, like the other cerebro spinal nerves, consists of two nerves exactly similar at their origin, and placed on different sides of the mesial line of the body. It has a very long course,—passing down the neck, and through the thorax to the upper part of the abdornen,—is distributed upon numerous and dissimilar organs, and anastomoses very freely and extensively with the sympathetic and vari ous cerebro-spinal nerves. It is the chief' nerve of the lungs and stomach, and hence its appel lation of pneumo-gastric.
The nervus vagus arises by several filaments, generally from six to ten, from the restiform body of the medulla oblongata, pamllel to and a little posterior to the groove between the olivary and restiform bodies, and from a line to a line and a half distant from the posterior edge of the olivary body. Thearciform band of superficial filaments passing between the anterior pyramidal and res tiform bodies cross among,- the lower filaments of this nerve. The filaments of the vagus are attached to the restiform body in a vertical, straight, and thin band of from three to four lines in length, the upper end of which is sepa rated froin the lower edge of the glosso-pha ryngeal nerve by a few small bloodvessels only. The upper half of these filaments of the vagus are at their origin closely approximated, so that the lower edge of the one above is in contact with the upper edge of the one below, while the lower filaments, especially the two last, are considerably more distant from each other. 'The lowest filament is placed only a little above and in the same line with the uppermost filament of the spinal accessory, and it is fre quently difficult to determine where the fila ments of the accessory begin, and where those of the vagrns end. From this origin each vagus proceeds forwards and outwards between the lower surface of the lateral lobe of the cerebel lum and that portion of the dura mater cover ing the basilar process of the occipital bone, to reach the foramen lacer= posterius, through the anterior part of which opening it escapes from the interior of the cranium. In this part of its course it frequently anastomoses with the glosso-pharyngeal, and its filaments be come more subdivided, but at the same time more closely aggregated, so that it is thicker and narrower. On reaching the foramen lace
rum posterius it enters a sheath or canal in the dura mater, anterior and a little internal to the commencement of the internal jugular vein, immediately anterior to the spinal accessory nerve, and posterior to the glosso-pharyngeal. As these three nerves enter the foramen lace rum, they perforate the dura mater, the glosso pharyngeal by a separate and distinct opening, the nervus vagus and spinal accessory by an opening common to both. Sometimes there is a small bridle of dura mater, at other times only a fold of the arachnoid separating the vagus and accessory at this part. At the lower part of the foramen lacerum the spinal acces sory is closely applied to the posterior surEce of the vagus. The dum mater is prolonged downwards into the foramen lacerum upon these three nerves in the form of two sheaths, one sheath surrounding the glosso-pharyngeal, and the other the vagus and accessory. From the proximity of these three nerves, as they pass through the foramen lacerum posterius, and from their intimate connection in some parts of their course and subsequent distribu tion, they were long considered to form only a single nerye.
As the vagus lies in the foramen lacerum it presents a greyish oblong swelling, resem bling the ganglion on the posterior root of a spinal nerve (ganglion primum ncrvi vagi of Utilizer, ganglion radieis n. v. nj Bendz, ganglion superius n. v., ganglion jugulare n. v.) This ganglionic enlargement begins immedi ately after the nerve has entered the foramen lacerum, so that its upper edge may be sotne times seen from within-the cranium ; it is of an oval forrn, and it extends along the course of the nerve from a line. and a half to two lines.* Mr. James Spencet has pointed out that a small filament belonging to the lower part of the vagus passes over the posterior surface of this ganglion without entering it, and joins itself to the superior filaments of the spinal accessory./ This fact, as we shall afterwards find, has a direct bearing upon the physiology of the nerve. • A communicating filament passes between the ganglion superius of the vagus and the superior cervical ganglion of the sympathe tic, another between it and the ganglion petro sum of the glosso-pharyngeal, (vide article