THE TRIGEMINAL NERVE. 



1231 



spinal root gives oflf collaterals and fibres that bend medially, enter the adjacent substantia gel- 

 atinosa and end in arborizations around the reception cells of that nucleus. Since the number oi 

 fibres is thus progressively reduced during the descent of the spinal root, the tract is tapering, 

 becoming smaller and smaller as it approaches the spinal cord until within the upper part of the 

 latter, at about the level of the second cervical nerve, it finally disappears. In its descent through 

 the brain-stem the spinal tract becomes more and more superficially placed, in the lower part of 

 the pons lying to the inner side of the restiform body, separated from it by the vestibular division 

 of the auditory nerve, and lower, in the lateral area of the medulla, occupying a position close 

 to the surface as it rests upon the expanded gelatinous substance of the tuberculum Rolandi. 



The central connections of the sensory part of the trigeminus (Fig. 1054), by way either of 

 the collaterals of the fibres of the spinal root or of the axones and collaterals of the axones of 

 the reception neurones, are undoubtedly very extensive, since the impulses collected by this 

 important nerve are widely dispersed. The most important paths for such distributions are : 



1. By axones that pass, as arcuate fibres, from the cells of the reception-nucleus across the 

 raphe to join the opposite mesial fillet 



and ascend to the optic thalamus and Fig. 1054. 



thence, after interruption in the cells of 

 the latter, by axones of thalamic neu- 

 rones to the cerebral cortex. It is prob- 

 able that some of the arcuate fibres do 

 not cross the mid-line, but ascend within 

 the mesial fillet of the same side. It is 

 also probable that collaterals of the 

 arcuate fibres pass to the trigeminal, 

 facial and glosso-pharyngeo-vagal motor 

 nuclei. 



2. By axones from the cells of the 

 reception nucleus that enter the infe- 

 rior cerebellar peduncle of the same side 

 and pass to the cerebellar cortex as con- 

 stituents of the nuclco-cerebellar tract. 



3. By collaterals that are distrib- 

 uted to the nuclei of origin of the hypo- 

 glossal and of the motor part of the tri- 

 geminus and facial nerves, whereby 

 these important motor nerves are 

 brought directly under the influence of 

 the sensory part of the fifth. 



The Motor Part. — In con- 

 trast to the median position of the 

 nuclei of origin of the oculomotor, 

 trochlear, abducent and hypoglos- 

 sal nerves, the deep origin of the 

 motor part of the trigeminus in- 

 cludes groups of cells that lie at 

 some distance from the raphe and 

 fall into series with the laterally 

 placed nuclei of the motor parts of 

 the other mixed cranial nerves — the facial, the glosso-pharyngeal and the vagus. 



Diagram showing relations of trigeminal root-fibres to nuclei 

 within brain-stem; GG, Gasserian ganglion with divisions (/, 

 //, ///) of sensory part of nerve ; SR, AIR, sensory and motor 

 roots ; 5', sensory nucleus ; SG, substantia gelatinosa ; Sp.R, spi- 

 nal or descending root ; i^, mesial fillet ; Cfi. nucleo-cerebellar 

 fibre : M, motor nucleus ; MsR, mesencephalic root ; Sf, sub- 

 stantia ferruginea; CB, cortico-bulbar fibres. 



1. The largest contingent of the motor fibres of the trifacial nerve arise as axones from the 

 neurones within the chief motor nucleus (nucleus masiicatorius) (Fig. 935). This nucleus con- 

 sists of a short columnar collection of gray matter, oval on cross-section, which lies in the upper 

 part of the pons, close to the median side of the sensory nucleus. It is composed of large stel- 

 late cells from which, as their axones, the motor fibres proceed outward through the tegmentum 

 to their superficial origin on the pons. A small number of fibres, from the more medially situ- 

 ated cells of the nucleus, pursue a dorsally convex course toward the raphe, which they cross 

 close beneath the floor of the fourth ventricle to join the motor nucleus of the opposite side and 

 become incorporated in the opposite trigeminal motor root. 



2. A second and smaller constituent of the motor root, the descending mesencephalic root 

 (radix descendens n. trigemini) includes fibres that arise from cells lying within the lateral part 

 of the gray matter surrounding the Sylvian aqueduct. In cross-sections (Fig. 936) this root 

 appears as a delicate crescentic bundle that descends from the mid-brain to join the larger tract 



