The Loiver Portion of the Human Brain-Stem. 71 



and worthless from a inoriihologieal standpoint. However, on the lateral 

 surfaces of the pons the {iber-l)undles do make fairly definite pathways in 

 the gray matter, permitting these to be modeled with some benefit. The 

 gray matter surrounding the longitudinal fibers of the pyramids has been 

 modeled, and all of the longitudinal fibers in the neighborhood of the main 

 mass of jjyramidal filK>rs have been cut away. This gives rise to the resulting 

 nuclear shell about the pyramids, not shown in any of the drawings of the 

 model, but seen in figure 14 on cross-section. 



The nuclei pontis in the caudal part may be considered as a huge mass 

 of cells surrounding the pyramids. The portion dorsal to the pyramids 

 takes its caudal origin just inferior to the superior pole of the nucleus olivaris 

 inferior (figure 2), while the ventral portion is extended caudall}- into the 

 nuclei arciformes (figure 1). The lateral portion is continuous caudall}^ with 

 the corpus ponto-bulbare (figure 1). The pyramids take a fairly direct 

 cephalo-caudal direction through the portion of the pontine nuclei modeled, 

 although at the superior end of the model they are divided into many separate 

 bundles by invading spurs of nuclear material (figure 14). With the dorsal 

 portion, arising caudally near the superior pole of the oliva, its cell-mass 

 quickly enlarges in all directions and joins with the ventral portion, at the 

 cephalic end of the arcuate nuclei, to surround the pyramids. This is shown 

 in figure 2, the pyramids of course not appearing in the drawing. As soon 

 as the pyramids are completely surrounded by gray matter, the pontine 

 nuclei enlarge cephalad in all directions (figures 1, 2, and 4). The dorsal 

 portion extends dorsally as one goes cephalad; the lateral i)ortion extends 

 laterally; and the ventral extends ventrally. Such, then, is the general 

 ground plan of the pontine nulcei in the caudal portion modeled. 



When viewed laterally (figure 2) the relations of the corpus ponto- 

 bulbare and the arcuate nuclei to the pontine nuclei are evident. The caudal 

 margin of the pontine nuclei extends in a dorso-ventral direction from the 

 mesial and ventral i)lates of the arcuate nuclei, surrounding the pyramids 

 and fusing with the small dorsal part of the pontine mass. From this point 

 of fusion the lateral border, caudally, takes a cephalic deflection and a general 

 dorsal course, so that it comes into relation with the nucleus nervi trigemini 

 and its sul:)stantia gclatinosa (figures 2 and 14). This lateral wall is, in its 

 caudal part, merely a thin sheet of cells projecting dorsally far beyond the 

 limits of the main nuclear mass. From lateral view, the ventral margin 

 shows the continuation of the ventral curve exhibited by the most cej^halic 

 lX)rtion of the arcuate nuclei. This convexity continues upward for some 

 distance with the assumption of almost an exact cephalo-caudal direction; 

 it is ended in an angle to be succeeded by a second ventral convexity. This 

 change in direction probably corresponds to the irregular surface-markings 

 of the ventral bulging of the pons. The whole lateral surface shows a 

 cephalo-lateral deviation from the longitudinal plane and also a gradual 

 convexity from its dorsal border around the pyramids to the mid-line. This 



