570 



THE NEEVOUS SYSTEM. 



matter of the floor of the ventricle, is the collection of pigmented cells which con- 

 stitutes the substantia ferruginea. 



The medial longitudinal bundle, as it is traced upwards through the tegmental 

 part of the pons, maintains the same position throughout, and as it ascends it 

 becomes more clearly mapped out as a definite and distinct tract. It lies close 

 to the median raphe, and immediately subjacent to the gray matter of the floor 

 of the fourth ventricle. 



The lemniscus medialis, as it ascends through the tegmental part of the pons, 

 undergoes striking changes in shape. In the lower portion of the pons its fibres, 

 which in the medulla oblongata are spread out along the side of the median raphe, 

 are collected together in the form of a loose bundle, which occupies a wide field, 

 somewhat triangular in shape, on either side of the median raphe and immediately 

 behind the basilar portion of the pons. As it proceeds up, the fibres spread out 

 laterally until a compact ribbon-like layer is formed in the interval between the 

 tegmental and basilar portions of the pons (Figs. 501 and 502). 



Above the level of the trigeminal nuclei another flattened layer of fibres come: 



Upper end of fourth ventricle 



Trochlear nerve 



Mesencephalic root of trigeminal nerve 

 Fasciculus anterolateralis 

 superficialis 

 Medial longitudinal 

 bundle 



Brachium conjunctivum 



Lateral lemniscus 



Formatio reticularis 



Medial lemniscus 



A B 



FIG. 502. Two SECTIONS THROUGH THE DORSAL PORTION OF THE PONS AT ITS SUPERIOR PART, '. 



CLOSE TO THE MESENCEPHALON. 



A is at a slightly lower level than B. 



into view to the lateral side of the lemniscus medialis. To this the name of lemnisc 

 lateralis is given. These fibres spread laterally and backwards, and finally ta 

 up a position on the lateral surface of the brachium conjunctivum. In the an* 

 between the medial and^ lateral lemnisci a little knot of compact gray matt 

 termed the nucleus lemnisci lateralis, comes into view (Fig. 501). This appe- 

 to be in more or less direct continuity with the superior olivary nucleus. Ma 

 of the fibres of the lemniscus lateralis take origin in this nucleus. Bruce cal I 

 attention to the continuity between the superior olive and the nucleus of the late I 

 lemniscus in man, and Cunningham confirmed the observation in so far as 

 orang brain is concerned. In many other mammals the nuclei are quite distr 



THE CEREBELLUM. 



istinc 



ied fi J 



In the foregoing account it has been seen that the cerebellum is formed 

 two distinct rudiments, each derived from the posterior edge of the alar lair a 

 immediately above the pontine flexure and the insertion of the vestibular ne - 

 As development proceeds during the second month there is a rapid prolifera< 

 of cells in the mantle layer of the cerebellar rudiments, and they become consi' 

 ably thickened. But at first this thickening manifests itself not so mucl: 

 a swelling of the superficial aspect of the cerebellum but as a bulging inw* 

 into the cavity of the fourth ventricle (Fig. 503). 



