THE DEEP CONNEXIONS OF THE CEKEBEAL NERVES. 601 



tractus spinalis. This tract descends on the lateral side of the column of gray matter 

 formed by the substantia gelatinosa, which constitutes its terminal nucleus, nucleus 

 tractus spinalis nervi trigemini. Fibres constantly leave it to enter the nucleus, 

 so that the lower it gets the smaller does the spinal tract become until, in the 

 upper part of the spinal medulla, about the level of the first or second spinal nerve, 

 it disappears altogether. 



The large spinal tract of the trigeminal nerve is a conspicuous object in sections 

 > through the pons and medulla oblongata. In the pons it traverses the dorsal or 

 tegmental part, first, between the emergent part of the facial nerve and the 

 vestibular nerve; and then lower down, between the restiform body and the nucleus of 

 the facial nerve (Fig. 498, p. 565). In cross sections it presents a well-defined semi- 

 lunar or curved piriform outline. In the superior part of the medulla oblongata it lies 

 on the ventral aspect of the restiform body, and therefore nearer the surface than in 



Brachium conjunctivum 



Anterior medullary velum 



Mesencephalic root of the trigeminal nerve 



Motor nucleus of the trigeminal nerve 



Motor root of the trigeminal nerve 



Sensory nucleus of the 

 trigeminal nerve 



Superior olive 



Sensory root of 

 trigeminal nerve 



Brachium ponti 



FIG. 532. SECTION THROUGH THE PONS OF THE ORANG, AT THE LEVEL OF THE NUCLEI 

 OF THE TRIGEMINAL NERVE. 



the pons (Fig. 495, p. 561). Here it is traversed and broken up into separate bundles 

 , by the olivo-cerebellar fibres and the roots of the glossopharyngeal and vagus nerves. 



Finally, it comes to the surface and its fibres are spread over the area on the side of 

 t the medulla oblongata known as the tuberculum cinereum of Rolando (Fig. 494, p. 561). 



The small motor part of the trigeminal nerve is distributed chiefly to the muscles 

 of mastication, and derives its fibres from the motor nucleus. 



The motor nucleus (Fig. 532) lies in the lateral part of the tegmental portion of 

 : the pons, close to the medial side of the main sensory terminal nucleus, but some- 

 what nearer the floor of the fourth ventricle. It is serially homologous with the 

 : motor nuclei of the lateral somatic group, namely, the facial and nucleus ambiguus. 

 It does not become displaced so far forwards as these nuclei, because its chief source 

 of sensory impulses the terminal nucleus of the trigeminal afferent fibres is 

 placed alongside it, and there is no need for any definite migration (Fig. 532). 

 The mesencephalic root or radix descendens nervi trigemini takes origin from 



