952 THE NERVOUS SYSTEM 



(2) A muscular branch is distributed to the stylo-pharyngeus muscle. This branch re- 

 ceives a communication from the facial nerve (fig. 743). 



(3) The tonsillar branches are a number of small twigs which arise under cover of the hyo- 

 glossus muscle; they proceed to the tonsil, around which they form a plexus, the circulus 

 tonsillaris. From this plexus fine twigs proceed to the glosso-palatine arches (pillars of the 

 fauces) and to the soft palate. 



(4) The lingual branches are the terminal branches of the nerve and supply the mucous 

 membrane of the posterior half of the dorsum of the tongue, where, chiefly as taste-fibres, they 

 are distributed to the vallate papillse. Some small twigs pass backward to the follicular glands of 

 the tongue, and to the anterior surface of the epiglottis. Other twigs are distributed around the 

 foramen caecum, where they communicate with the corresponding twigs of the opposite side. 



The sensory fibres. — The sensory fibres of the glosso-pharyngeal nerve spring from the supe- 

 rior and petrosal gangha and pass peripherally and centrally. The peripheral processes of the 

 ganglion cells are those which are distributed to the mucous membrane (taste-buds) of the tongue 

 and pharynx, and the central processes pass medialward to the medulla. In the medulla they 

 pass dorsalward and median ward through the reticular formation and, bifurcating into ascend- 

 ing and descending branches, they end in the nucleus of termination of the glosso-pharyngeal 

 nerve, that is, in the superior part of the nucleus alae cinerese and in the nucleus of the tractus 

 soUtarius. 



The motor fibres arise from the nucleus ambiguus in the lateral funiculus of the medulla, 

 in line with the nucleus of origin of the facial nerve. From this nucleus they pass at first 

 dorsalward and then, turning lateralward, they emerge and join the sensory fibres and run with 

 them in the trunk of the nerve (fig. 646). 



Van Gehuchten's observations point to the conclusion that one motor nucleus of the glosso- 

 pharyngeal nerve is separate from and lies above and to the medial side of the nucleus ambiguus, 

 and that a portion of the nucleus of the ala cinerea is also a motor nucleus common to the 

 glosso-pharyngeal and vagus nerves. It is quite probable that the former motor nucleus is 

 that now considered as the dorsal motor nucleus of the vagus. An unknown proportion of 

 the motor fibres are visceral motor and course in the various communications of the glosso- 

 pharyngeal nerve with cephalic plexus. 



Central connections. — The nuclei of termination of the glosso-pharyngeal nerve are asso- 

 ciated with the motor nuclei of other cranial nerves by the medial longitudinal fasciculus, and 

 with the somsesthetic area of the cortex cerebri of the opposite side by the medial lemniscus 

 (fillet). The motor nucleus of the nerve is associated with the somsesthetic area by the pyra- 

 midal fibres. 



THE HYPOGLOSSAL NERVE 



The hypoglossal nerves are exclusively motor; they supply the genio-hyoidei 

 and the extrinsic and intrinsic muscles of the tongue except the glosso-palatini. 

 They are usually designated as the twelfth pair of cranial nerves. The fibres of 

 each nerve issue from the cells of an elongated nucleus which lies in the floor 

 of the central canal in the lower half of the medulla and in the floor of the fourth 

 ventricle in the upper half beneath the trigonum hypoglossi. This nucleus is the 

 upward continuation of the ventro-medial group of cells of the ventral horn of the 

 spinal cord. From their origin the fibres run ventralward and somewhat lateral- 

 ward, probabty joined in the medulla by a few fibres from the nucleus ambiguus 

 which is a segment of the upward prolongation of the lateral group of cells of the 

 ventral horn. The conjoined fibres issue from the medulla in the sulcus between 

 the pyramid and the olivary body, in a series of from ten to sixteen root filaments, 

 which pierce the pia mater and unite with each other to form two bundles (fig. 

 731). These bundles pass forward and lateralward to the hypoglossal (anterior 

 condyloid) foramen, where they pierce the arachnoid and dura mater. In the 

 outer part of the foramen the two bundles unite to form the trunk of the nerve. 

 At its commencement, at the base of the skull, the trunk of the hypoglossus lies on 

 the medial side of the vagus, but as it descends in the neck it turns gradually 

 around the dorsal and the lateral side of the latter nerve, lying between it and the 

 internal jugular vein, and a little above the level of the hyoid bone it bends for- 

 ward, and crosses lateral to the internal carotid artery, the root of origin of the 

 occipital artery, the external carotid, and the loop formed by the first part of the 

 lingual artery (fig. 743). After crossing the lingual artery it proceeds forward 

 on the lateral surface of the hyo-glossus, crossing to the medial side of the posterior 

 belly of the digastric, and the stylo-hyoid muscles. It disappears in the anterior 

 part of the submaxillary region between the mylo-hyoid and the hj^o-glossus, and 

 divides into its terminal branches between the latter muscle and the genio-glossus. 



As it descends in the neck the trunk lies deeply between the internal jugular vein and I 

 the internal carotid artery under cover of the parotid gland, the styloid muscles, and the pos- « 

 terior belly of the digastric, and it is crossed superficially by the posterior auricular and the occip- 

 ital arteries. As it turns forward around the root of the occipital artery the sterno-mastoid 

 branch of that vessel hooks downward across the nerve, and as it turns forward on the hyo- 



