The Lower Portion of the Huvmn Brain-Stem. 76 



but its ventral border is placed more laterally from the mid-line than its 

 dorsal margin. The mesial surface is smooth and slightly curved, with a 

 mesial convexity extending cephalad for half of the extent of the nucleus, 

 where it forms a marked notch. The surface then extends mesially in a 

 small shoulder. Just posterior to this notch is a deep caudal incision, dorsal 

 to which appears a similar, somewhat thicker sheet of cells (figure 4), which 

 merges with the mesial cell-cohmm, the two ascending as a widened, thickened 

 cell-sheet in the characteristic direction of the columns in the nucleus. Just 

 caudal to the cephalic termination, the ventral border turns abruptly dorsally 

 and then cephalad, decreasing the width of the cell-sheet by almost one-half. 

 This is quickh' followed by the abrupt cephalic ending of the nucleus, the 

 cell-sheet disappearing very quickly. The ventral border of this mesial 

 cell-column shows a slight ventral projection somewhat cephalad to its 

 caudal origin. Slightly above this, the dorsal border exhibits a dorsal pro- 

 jection (figure 4). The mesial surface is smooth, but in the caudal half is 

 marked by a slight eminence and in the ventral portion of the cephalic half 

 a similar eminence occurs. Cephalic to this superior elevation, the mesial 

 surface shows a slight concavity. The lateral surface of the mesial cell- 

 column is quite irregular and rough. At a point corresjionding to the notch 

 in the mesial surface, a marked irregular lateral projection occurs on this 

 surface. At the superior pole there is considerable thickening of the nucleus 

 in the transverse diameter. 



The lateral of the two cell-columns is really double throughout the middle 

 portion of its extent, although it arises singly from the mesial surface of the 

 dorsal union of the three primary radiate columns. Arising from this union, 

 the column extends as a triangular cell-column, placed dorsally and some- 

 what mesially to the dorsal border of the mesial column. It approximates 

 in part the ventro-mesial border of the nucleus nervi facialis (figure 4). It 

 continues cephalad somewhat irregularly, in the characteristic dorsal deflec- 

 tion, with slight lateral deviation. At the level of the superior pole of the 

 seventh nucleus a marked and smooth dorso-mesial spur is given off. At 

 this point the cell-column bends laterally and dorsally across the superior 

 pole of the olive and then pursues a cephalo-lateral course to fuse quickly 

 with its second portion. This second portion of the lateral column arises 

 just caudally to the superior pole of the seventh nucleus as an elongated oval, 

 with the long axis in the dorso-ventral plane. It ascends cephalad and fuses 

 with the lateral cell-mass after its abrupt lateral deflection. Here the com- 

 bined cell-column, oval in shape, lies lateral to the mesial collection. It 

 passes cephalad in the characteristic direction of these cell-columns, flatten- 

 ing out as a dorso-ventral sheet of cells. As such, it abruptly terminates 

 just caudal to the lateral jirojection on the lateral surface of the mesial cell- 

 column. Hence the lateral column possesses only half the longitudinal extent 

 of the mesial cell-mass. 



