The Lower Portion of the Human Brain-Stern. 31 



aspect of the mesially projecting, cephalic knob and then curves laterally 

 and caudally. The surface shows a slight depression in its middle and a 

 very large and rather deep depression just above and connecting with the 

 sulcus which delimits the dorso-lateral surface. Below the sulcus the lateral 

 surface is large and smooth, pentagonal in outline. The angles of the 

 pentagon are formed by the junction of the sulcus with the dorsal margin, 

 by the dorsal angle of the dorsal margin, bj^ the ventral l)ulging of the ventral 

 surface and the two caudal angles. In its cephalic portion, the lateral face 

 looks laterally and somewhat ventrally, but in the caudal one-third of the 

 nucleus the surface is directed almost wholly laterally. This change in 

 direction of the surface is due to the narrowing of the dorsal, thickened 

 portion of the nucleus, showing on the lateral face as a caudal and mesial 

 sloping of the lower portion of the surface from the widened and thickened 

 middle portion. The lateral surface shows a curving of its whole face with 

 the convexity mesially. This curving is accounted for by the more lateral 

 position of the upper portion of the nucleus in its cephalic two-thirds. 



The greatest antero-posterior diameter of the nucleus occurs at the level 

 of the dorsal angle and middle ventral bulging (figure 3). Both above 

 and below this, the diameter is maintained with liut gradual diminution. 

 The nucleus exhibits its greatest transverse diameter, or is thickest, at the 

 same level. Caudal to this, the nucleus rapidly becomes a thin sheet of 

 cells, while in its superior portion it maintains a considerable transverse 

 diameter. 



NUCLEUS AMBIGUUS. 



The adult nucleus ambigvuis, as modeled on the left side of this recon- 

 struction, consists of two main masses separated by a considerable inter- 

 mission (figure 4). The lower cell-mass oval with its longer axis parallel 

 to that of the substantia gelatinosa is found on the level of the middle of 

 the true hypoglossal nucleus in a plane slightly cephalic to the caudal end 

 of the dorsal accessory olivary nucleus. It is very small and the large motor 

 cells can be identified only through a few sections in this region. The main 

 column of cells in the nucleus ambiguus begins on a level with the middle of 

 Roller's "small cell hypoglossal nucleus," caudal to the cephalic ends of the 

 nucleus nervi hypoglossi, the nucleus alse cinerea?, and the nucleus olivaris 

 accessorius dorsalis. It is surrounded by the formatio reticularis, in which 

 it lies; lateral to it is the substantia gelatinosa, and ventral are the dorsal 

 accessory olive and the dorsal leaf of the main olive. Dorsall}^ it is in rela- 

 tion to the nucleus tractus solitarii, the nucleus nervi hypoglossi, the nucleus 

 alse cinereae, and in the upper part the nucleus intercalatus and the nucleus 

 vestibularis medialis. Cephalad the nucleus extends to a level just below 

 that of the caudal pole of the nucleus nervi facialis. The whole nucleus is 

 shown in figure 4, a mesial view of the model. Between the level of the 

 smaller nuclear mass and the upper nuclear column an occasional large motor 



