490 THE NEBVOUS SYSTEM 



respectively terminate. The central neurones, whose cell bodies 

 form these nuclei, send their neuraxes ventralward and toward 

 the anterior median raphe, which they cross to the opposite side, 

 and enter the tract of the mesial fillet (mesial lemniscus, laqueus). 

 Thus it is that the posterior columns of the spinal cord disappear 

 in the medulla, and the dorsal horns of grey matter are in this 

 way brought into direct relation with the layer of ependymal cells 

 which lines the floor of the fourth ventricle. 



The Inferior Olivary Body. Another and still larger isolated 

 grey nucleus, beginning at a slightly higher level than the above 

 and extending farther cephalad, is the inferior olivary body. This 

 nucleus is inserted between the ventrally situated pyramids on 

 the one hand, and the lateral columns, lateral nucleus, and forma- 

 tio reticularis on the other. The inferior olivary body is a large 

 ovoid mass with a central core of white fibres and a crinkled shell 

 of grey matter ; its convoluted surface presents a ribbon-like ap- 

 pearance when seen in transection. The olivary nucleus extends 

 from near the spinal border of the medulla oblongata upward to 

 the lower portion of the pons Varolii. 



The white matter of the medulla oblongata presents two 

 prominent decussations of long fibre tracts, which have already 

 been mentioned as cutting off the lateral from the medial cell 

 groups of the ventral horns. The more spinalward of these is 

 the motor decussation, the more cephalad is the great sensory 

 decussation of the mesial fillet. 



The motor decussation occurs at the junction of the first cervi- 

 cal segment and spinal border of the medulla oblongata; it is 

 found at the level of the lower portions of the nuclei of Goll and 

 "Burdach. The greater portion of the nerve fibres composing the 

 pyramids of the medulla decussate through the median raphe at 

 this level, and pass obliquely downward to the opposite lateral 

 column of the spinal cord. 



The sensory decussation occurs at a slightly higher level, its 

 fibres chiefly coming from the nuclei of Goll and Burdach, to pass 

 cephalad in an obliquely ventro-mesial direction, piercing the me- 

 dian raphe just dorsal to and a little above the motor decussation. 

 Having reached the opposite side they immediately turn cephal- 

 ward to form the mesial fillet, a large flattened bundle of longitu- 

 dinal white fibres lying next the median raphe" and just dorsal to 

 the pyramidal tracts. The interlacing of these fibres as they 

 approach the raphe forms a reticular mass of white matter which 



