The Loivcr Portion of the Human Brain-Stem. 47 



three-fifths. Above this the nucleus tapers grackially to its cephalic pole; 

 Avhile in the lower two-fifths the nucleus narrows to a very thin edge. The 

 caudal two-fifths of the nucleus present four surfaces for examination 

 dorsal, ventral, mesial, and lateral but the cephalic portion of the nucleus 

 is more or less rounded on transverse section, so that no distinct surfaces 

 are made out. 



From observation of serial sections of the brain-stem, inspected in order 

 from the spinal cord upward, the nucleus intercalatus is first noticed caudally 

 as a thin cap over the dorso-lateral surface of the nucleus nervi hypoglossi. 

 This cap of cells gradually broadens, taking the place of the decreasing 

 hypoglossal nucleus mesially and widening somewhat laterally, to reach its 

 greatest transverse and dorso-ventral diameters at the level of the cephalic 

 termination of the nucleus of the twelfth nerve (figures 3 and 4). The 

 mesial surface of this caudal two-fifths of the nucleus, hence, faces mesially 

 and somewhat ventrally as it conforms to the dorso-lateral face of the 

 hypoglossal nucleus. The surface is a smooth plane, narrowing in its upper 

 portion, although still showing dorsally to the motor nucleus but terminating 

 very quickly to pass into the cone-like upper three-fifths. The wide caudal 

 beginning of the nucleus intercalatus gives to this mesial caudal surface a 

 triangular aspect. 



The dorsal view (figure 3) of the nucleus in its caudal two-fifths shows 

 also this triangular appearance with the apex downward. The mesial dorsal 

 angle of the nucleus is about 110, sharply defined and fairly straight. The 

 lateral dorsal edge, however, shows considerable curving, with the convexity 

 lateralwards, as the nucleus widens with the ventral dipping of the nucleus 

 alae cinerefe. This convexity gradually slopes into the upper core of the 

 nucleus. The dorsal surface of the lower two-fifths of the nucleus is fairly 

 smooth, with a gentle dorsal convexity. Two poorly defined and shallow 

 ventral depressions occur on its surface. 



Ventrally, the surface of the nucleus intercalatus is also smooth, with a 

 slight ventral convexity. The mesial ventral border of this nucleus exhibits 

 a marked lateral convexity in its lower portion, followed above by a mesial 

 curvature. The lateral ventral border is fairly straight and runs almost 

 exactly in the cephalo-caudal axis. 



A rectangle is formed by the lateral surface of the nucleus. This face 

 is not so smooth as are the other three surfaces in the caudal two-fifths of 

 the nucleus; two mesial concavities mark this surface, between which occurs 

 a fairly prominent ridge. 



The arbitrary division for description of the nucleus intercalatus is 

 marked by a slight transverse ridge on the dorsal and mesial surfaces, and 

 by an elevation at the cephalic end of the ventro-lateral angle. Above 

 this the nucleus forms a gradually tapering truncated cone, as seen in 

 figures 3 and 12. The mesial surface shows a marked lateral bowing to 

 accommodate for the nucleus funiculi teretis (figure 12); the border turns 



