954 THE NERVOUS SYSTEM 



nerve within the thorax. The nerve to the posterior belly of the omo-hyoid runs in a loop of 

 the cervical fascia below the central tendon of the muscle. 



(b) The nerve to the thyreo-hyoid leaves the hypoglossus near the tip of the great cornu of 

 the hyoid bone, and runs obliquely dowTiward and medialward to reach the muscle. All the 

 fibres in (a) and (b) are derived from the first, second and third cervical nerves. 



(c) The nerve to the genio-hyoid arises under cover of the mylo-hyoid, where loops are 

 formed with the lingual nerve from which loops branches pass into the muscle. It probably 

 contains some true hypoglossal fibres. 



(3) The branches of the hypoglossus proper, the rami linguales, supply the stylo-glossus, 

 hyo-glossus, genio-glossus, and the intrinsic muscular fibres of the tongue. 



The nerve to the stylo-glossus is given off near the posterior border of the hyo-glossus. 

 It pierces the stylo-glossus, and its fibres pursue a more or less recurrent course within the muscle. 



The nerves to the hyo-glossus are several twigs which are supplied to the muscle as the 

 hypoglossal nerve crosses it. 



The nerve to the genio-glossus arises under cover of the mylo-hyoid in common with the ter- 

 minal branches to the intrinsic muscles of the tongue. It communicates freely with branches of 

 the lingual, forming long loops which lie on the genio-glossus. From these loops twigs pass into 

 the genio-glossus and into the muscular substance of the tongue. 



Central connections. — The nucleus of origin of the hypoglossus is associated with the som- 

 sesthetic area (operculum) of the cortex cerebri of the opposite side by the pyramidal fibres, 

 and it is connected with the sensory nuclei (nuclei of termination) of other cranial nerves by 

 way of the reticular formation and the medial longitudinal fasciculus. 



THE VAGUS OR PNEUMOGASTRIC NERVE 



The vagus or pneumogastric nerves are the longest of the cranial nerves, and 

 they are remarkable for their almost vertical course, their asymmetry, and their 

 extensive distribution, for, in addition to supplying the lung and stomach, as the 

 name 'pneumo-gastric' indicates, each nerve gives branches to the external ear, 

 the phar3^nx, the larynx, the trachea, the oesophagus, the heart, and the abdominal 

 viscera. They are commonly referred to as the tenth pair of cranial nerves. 



Each nerve is attached to the side of the medulla, in the postero-lateral sulcus, 

 dorsal to the olivary body, by from twelve to fifteen root filaments which are in 

 linear series with the filaments of the glosso-pharyngeal nerve. The filaments 

 contain both sensory and motor fibres. They pierce the pia mater, from which 

 they receive sheaths, and, traced outward, they pass into the posterior fossa of the 

 cranium toward the jugular foramen and unite to form the trunk of the nerve, 

 which passes through openings in the arachnoid and the dura mater which are 

 common to it and to the spinal accessory nerve. In the jugular foramen a small 

 spherical ganglion, the jiigular ganglion (ganglion of the root), is interposed in the 

 trunk which here turns at right angles to its former course and descends through 

 the neck. As it leaves the jugular foramen it is joined by the internal or accessory 

 portion of the spinal accessory nerve, and immediately below this junction it 

 enters a large ovoid ganglion, the ganglion nodosum or ganglion of the trunk (fig. 

 743). As it descends through the neck the nerve passes ventral and somewhat 

 lateral to the superior cervical sympathetic ganglion, and in front of the longus 

 capitis and longus colli, from which it is separated by the prevertebral fascia. In 

 the upper part of the neck it is placed between the internal carotid artery and the 

 internal jugular vein, and on a plane dorsal to them, the artery being ventral and 

 mesial, and the vein ventral and lateral. In the lower part of the neck it occupies 

 a similar position in regard to the common carotid artery and the internal jugular 

 vein, and the three structures are enclosed in a common sheath derived from the 

 deep cervical fascia, but within the sheath each structure occupies a separate 

 compartment (fig. 743). In the root of the neck and in the thorax the relations of 

 the nerves of the two sides of the body differ somewhat, and they must, therefore, 

 be considered separately. 



The right vagus passes iu front of the first part of the right subclavian artery in the root of 

 the neck and then descends in tiio liiorax, passing obliquely downward and backward on the right 

 of the trachea, and beliirid tli(! right innoiiiinatc vein and the superior vena cava, to the back of 

 the root of tlu; right lung. .lust befon; it nsaches the right bronchus it lies close to the medial 

 flido of the vena azygos as th(! latter hooks forward over the root of the lung. At the back of the 

 right bronchus the right vagus breaks up into a number of brandies wliich join with the branches 

 of the sympathetic to form the right posterior pulmonary plexus, and from this plexus it issues 

 in'the form of one or more cords, combined sensory, visceral motor and sympathetic, which de- 

 scend on the (esophagus anrl l)r(;ak uj) into branches whicli join with branches of the left vagus, 

 forming the posterior oesophageal plexus. At the lower part of the thorax fibres of this plexus 

 become again assf)(;iii(c(i in one trunk which passes through the diaphragm on the posterior 



