568 



THE NERVOUS SYSTEM. 



Upon the medial and dorsal aspect of the superior olive there is a dense groin; 

 of longitudinal fibres. These constitute the fasciculus thalamo-olivaris or centra! 

 tegmental tract, to which we have already referred in discussing the inferior olivary 

 nucleus (Fig. 498). It is uncertain whether this tract arises in the thalamus. 



(4) The medial longitudinal bundle and the lemniscus medialis come next. At 

 they proceed upwards through the tegmental part of the pons, these longitudina 

 tracts occupy the same relative position as in the medulla oblongata. They ar< 

 placed close to the median raphe; but they have drawn further apart from each 

 other, and their fibres are more distinctly concentrated into separate strands, wit! \ 

 an interval of some little width between them, which is occupied by the tecto-spina 

 tract. The medial longitudinal bundle lies immediately under cover of the gra- 



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 pont 



FIG. 500. TRANSVERSE SECTION THROUGH THE PONS AT THE LEVEL OF THK NUCLEI OF THE 



TRIGEMINAL NERVE (Orang). 



matter of the floor of the fourth ventricle. The lemniscus medialis is placed close 

 the trapezial fibres, many of which traverse it as they pass towards the median pla . 



(5) The nucleus of the abducens nerve also forms a conspicuous object in secti<J 

 through the lower part of the pons. It is a round mass of gray matter, which $ 

 situated close to the lateral side of the medial longitudinal bundle, and immediat ' 

 under cover of the gray matter of the floor of the fourth ventricle. From its mec 1 

 side numerous root-bundles of the abducens nerve pass out and proceed forwas 

 between the lemniscus medialis and the superior olivary nucleus. They occi' 

 in the pons, therefore, a position similar to that occupied by the hypoglossal re - 

 fibres in the medulla oblongata. 



Up to the present only the inferior part of the dorsal portion of the p s 

 has been described, i.e. the portion immediately adjoining the medulla oblong, i- 

 As we proceed upwards and gain a point above the level of the trapezial fib', , 

 many] of the structures which have attracted attention lower down gradu;/ 

 disappear from the formatio reticularis. The lemniscus medialis becomes marke J 



