NUCLEI OF Till- CRAMAL .YA'ATA'X MC5 



The central connections of the cranial nerves arc most easily homoln 

 with spinal-cord structures. Functionally the cranial nerves are of three varieties: 

 (1) the motor or efferent nerves, comprising the oculomotor, the trochlear, the 

 abducens, (lie spinal accessory, and the hypoglossus; (2) the sensory or afferent, 

 comprising the olfactory, the optic, and the acoustic: and (3) the mixed, motor and 

 ,-cnsory nerves, comprising the triiremimis. the facial, the glosso-pharvngeal, and the 

 vagus. The nuclei of origin of the motor or efferent cranial nerves and the efferent 

 portions of the mixed nerves are directly continuous with the cell columns of the 

 ventral horns of the spinal cord, while the emerging root filaments and roots of these 

 nerves correspond to the ventral roots of the spinal nerves. The nuclei of termina- 

 tion of the afferent or sensory cranial nerves and of t lie sensory portions of the mixed 

 nerves correspond directly to the nuclei of the funiculus gracilis and funiculus cunea- 

 1 u and, functionally, are merely anterior continuations of these nuclei. 



The nuclei of the efferent or motor cranial nerves lie in two parallel lines, one near 

 the mid-line and the other more laterally placed. The nuclei giving origin to the 

 oculomotor, the trochlear, the abducens, and the hypoglossus are near the mid-line, 

 and correspond to the ventro-medial and dorso-medial cell groups of the ventral horns 

 of the spinal cord: the nuclei of origin of the motor portion (portio minor) of the 

 trigeminus, of the facial, and the nucleus ambiguus giving origin to the motor por- 

 tions of the glosso-pharyngeal and vagus nerves, together with the nucleus of the 

 spinal accessory, correspond to the ventro-lateral and dorso-lateral cell-groups of 

 t he ventral horns of the spinal cord. The nerve-roots having medial nuclei of origin 

 are those which make their exit from the brain-stem along the more medial super- 

 ficial line, while those having the more lateral nuclei comprise the more lateral line 

 of roots apparent on the surface of the stem. The first two pairs of cranial nerves, 

 the olfactory and optic, are attached to the prosencephalon. These are purely 

 sensory, ami make their entrance near the mid-line of the brain, both having super- 

 ficially placed nuclei of termination. Of the other nerves, all having sensory or 

 afferent functions enter the brain along the lateral or more nearly dorsal line, and the 

 ganglia giving origin to their afferent axones correspond directly to the 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 structures 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. 



Of the twelve pairs of cranial nerves, eight pairs are attached to the medulla 

 oblongata and pons, viz., the trigeminus, abducens, facial, acoustic, glosso-pharyn- 

 geal. vagus, spinal accessory, and hypoglossus. 



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

 ginning in the lower portion of the floor of the fourth ventricle at the level of the 

 stria 1 acustica;. 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 entire thickness of the medulla (fig. 595) , 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 cerebnrn 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 (associa- 

 tional). None of its axones are supposed to decussate, though numerous com- 

 missural 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 continuous with the nucleus giving motor fibres to the vagus and glosso- 

 pharyngeus (nucleus ainbiguiis). Below, it consists of the dorso-lateral group 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 upwards parallel with the medulla to turn outwards in company with 

 the vagus. (See fig. ."SO.) The upper rootlets arise from that part of the nucleus 

 contiguous to the nucleus ambiguus. and are described as comprising the medullary 



