The Lower Portion of (he Human Brain-Stem. 67 



lateral projection shows a similar relation to the superior of the notches. 

 The caudal end of this portion is rounded and shows a widened transverse 

 portion, cejihalic to which the nucleus preserves almost intact its transverse 

 diameter. The cephalic end of this first portion of the left arcuate nucleus 

 inclines somewhat ventrally, as shown in figures 2 and 4. At the superior end 

 of this first portion, two small masses of nuclear material occur, separated 

 from each other and from the superior cell-column of filler biuidles. These, 

 with their long axes transverse, arc shown in figures 1, 2, and 4. They lie 

 cephalic, mesial, and slightly ventral to the cephalic pole of the first portion. 

 The superior of these lies more cephalic, mesial, and ventral than the inferior. 

 Cephalic to these small nuclear masses, the arcuate nucleus on the left again 

 becomes a continuous mass which runs to the pontine nuclei. As shown in 

 figure 1, the whole column shows a marked convexity toward the mid-line 

 and also a curving, so that the superior end is drawn dorsally before it pro- 

 jects ventrally in the pontine enlargement. The mesial margin of this part 

 is irregularly curved toward the mid-line (figure 1); the lateral margin shows 

 a marked mesial notch, superior to which it i)rojects irregularly laterally to 

 fuse with the pontine nuclei. The nuclear material in the middle of this part 

 is split into two masses which soon unite (figure 1). Cephalic to the union 

 of these two masses, the ventral surface is smooth and widens gradually; 

 about its middle is a gradual ventral ridge shown in figures 1, 2, and 4. 



At the level of the caudal end of the separation of this left arcuate 

 nucleus into two columns, there occur cell-masses belonging to the arcuate 

 system, dorsal to the ventral mass, lying deeply along the side of the anterior 

 longitudinal fissure as it deepens into the foramen caecum. A small irregular 

 cuboid mass and a more ventral elongated mass appear in figure 4 at this 

 level, t^'omewhat superior to this, the lateral surface of the longitudinal 

 fissure is lined bj' a cell-column which joins the ventral arcuate mass in a 

 gentle curve (figure 4). This dorsally projecting cell-column is thickened 

 at its ventral origin, but soon becomes a slender spur. Cephalic to this, 

 the whole arcuate plate curves around to the mesial surface. This is con- 

 tinued into two marked dorsal spurs before it merges with the pontine nuclei 

 which surround the pyramids. The middle of these dorsal spurs is projected 

 toward the mid-line in a ridge-like (>minence. The whole arcuate complex, 

 curving around the pyramids and exhibiting the characteristics mentioned, 

 is shown in figure 4. The fusion of this convex plate with the ventral pontine 

 nuclei is well shown. The inner surface (lateral and dorsal) of this curved 

 comjilex is grooved ])y cephalo-caudal fissures and marketl by a single ridge 

 which lies in the same plane as the cuboid mass and runs in the same cephalo- 

 caudal direction. 



The right nucleus arciformis is considerably different from the left, which 

 has just been described. In general, it consists, like the left, of a slender 

 caudal mass of cells which widens laterally and curves about the mesial 

 side of the pyramids cejjhalad to fuse with the pons. The most caudal 



