820 THE NERVOUS SYSTEM 



prosencephalon. These are purely sensory, and make their entrance near the 

 mid-line of the brain, both having superficially placed nuclei of termination. Of 

 the other nerves, all having sensory or afferent functions enter the brain along the 

 lateral or more dorsal line, and the ganglia giving origin to their afferent axones 

 correspond directly to the spinal ganglia of the dorsal or afferent roots of the 

 spinal nerves. 



Commissural and associational neurones are much more numerous in the 

 brain-stem than in the spinal cord. Their axones serve to connect the struc- 

 tures on the two sides of the mid-line and to associate the different levels of the 

 same side. Just as in the spinal cord, those of longer course correspond to the 

 fasciculi proprii. Many of their axones descend into the spinal cord. 



Of the fifteen pairs of cranial nerves, eleven pairs are attached to the medulla 

 oblongata and pons, viz., the trigeminus, the masticator, abducens, facial, 

 glosso-palatine, vestibular, cochlear, glosso-pharyngeal, vagus,, spinal accessory, 

 and hypoglossus. 



The hypoglossus, the motor nerve of the tongue, has its nucleus of origin beginning 

 in the lower portion of the floor of the fourth ventricle at the level of the acustic striae. It 

 is a long nucleus, lying close to the mid-line and just under the floor of the ventricle (hypoglossal 

 eminence) and extending down to the region of the funiculus separans. Here it curves ventrally 

 to a slight degree, and below the obex assumes a position ventro-lateral to the central canal, 

 and thus extends a short distance below the level of the inferior tip of the olive. The nerve 

 arises as a series of rootlets which traverse the entne thickness of the medulla (fig. 643), to 

 emerge in line in the furrow between the olive and the pyramid and fuse to form the trunk of 

 the nerve. The lowermost of the rootlets usually emerge below the olive. The nucleus 

 receives impulses — (1) from the cerebrum by way of divergent fibres from the pyramid of the 

 opposite side (voluntary); (2) impulses brought in by the sensory fibres of the cranial nerves 

 (reflex); and (3) by axones from other levels of the medulla (associational). None of its axones 

 are supposed to decussate, though numerous commissural fibres are known to pass between 

 the nuclei of the two sides. 



The spinal accessory is likewise a purely motor nerve, and has a laterally placed, long, and 

 much attenuated nucleus of origin. Above, its nucleus is in line with and practically continu- 

 ous wuth the nucleus giving motor fibres to the vagus and glosso-pharyngeus (nucleus ambiguus). 

 Below, it consists of the lateral and dorso-lateral groups of cells of the ventral horn of the first 

 five or six segments of the spinal cord. The nerve arises as a series of rootlets which emerge 

 laterally and join a common trunk, which passes upward between the dorsal and ventral roots 

 of the upper cervical nerves and parallel with the medulla to turn lateralward in company 

 with the vagus. (See fig. 629). The upper rootlets arise from that part of the nucleus con- 

 tiguous to the inferior end of the nucleus ambiguus, and are described as comprising the medullary 

 or accessory part of the nerve; those which arise from the ventral horn cells below are described 

 as the spinal part. The trunk of the spinal accessory fuses with the vagus in the region be- 

 tween its two ganglia, and, before separation, contributes fibres (the accessory part) to the 

 trunk of the vagus. Some of the accessory fibres are distributed as motor fibres to the muscles 

 of the larynx and some of them are visceral efferent fibres. The latter probably terminate 

 chiefly in sympathetic ganglia which send axones to the heart. The spinal part is distributed to 

 the sterno-mastoid and trapezius muscles. The nucleus of the spinal accessory receives termi- 

 nal twigs of pyramidal fibres from the opposite side and is otherwise subjected to influences 

 similar to those affecting the cells giving origin to the motor roots of the spinal nerves. 



The vagus or pneumogastric and the glosso-pharyngeus, though they have widely different 

 peripheral distributions, are so similar in origin and central connections that they may be 

 described together. Both contain efferent fibres, though both are in greater part sensory. 

 They are similar as to the origin of both their efferent and afferent components. The afferent 

 fibres of the vagus arise in its jugular ganglion and its nodosal ganglion (ganglion of the trunk); 

 the afferent fibres of the glosso-pharyngeus arise in its superior ganglion and its petrosal ganglion. 

 In both nerves these fibres enter the lateral aspect of tlae medulla and bifurcate into ascending 

 and descending Ijranches, similar to those of the dorsal root-fibres in the spinal cord. Some 

 of these branclics terminate in practically the same level of the medulla about cell-bodies 

 situated on the same and the opposite sides. Such branches end chiefly in the nuclei of the 

 hypoglossal and spinal accessory, and about the cells giving origin to the efferent components 

 of the vagus and glosso-jiharyngeus themselves — short reflex arcs. However, most of the 

 afferent fibres tenniiiaio in the nucleus of termination of the vagus and glosso-pharyngeus: — (1) 

 the nucleus of the ala cinerea, the middle portion of which is indicated in the floor of the fourth 

 vontricU; \>y the ala cinerea; (2) in the dosed portion of the medulla, the lower end of the 

 nucleus of tlie ala cinerea comes to lie in tlie dorso-lateral proximity of the central canal, and 

 this portion is known as tlie commissural nucleus of the ala cinera (figs. 642 and 645) from the 

 fact that fibres may be seen which pa.ss directly from it across the mid-line; (3) the longer of 

 the descending branch(\s of the bifurcated fibres collect to form the solitary tract, a compact 

 bundle situated dorsally just ventro-lateral to the nucleus of the ala cincuea and quite con- 

 spicuous in sections of the medulla. The fibres of this l)undle terminate in the nucleus of the 

 solitary tract, which is but a ventro-lateral and downward continuation of the nucleus of the 

 ala cinerea enclosing the bundles forming the tract. It is most probable that the fibres of the 

 solitary tract are chicfiy from the vngus (pneumogastric), though Bruce has found evidence 

 that the glosso-pharyngeal contributes to it appreciably. It decreases rapidly in descending 

 the medulla, owing to the rapid termination of its fibres about the cells of its nucleus. It, 



