The Lower Portion of the Human Brain-Stem. 27 



seems more sensible to consider the morphology of the nucleus in such a 

 reconstruction than the arbitrary division adopted by other authors without 

 reference to its actual morphology. The relation of this nucleus to the area 

 hypoglossi in the floor of the fourth ventricle has been considered under that 

 subdivision of this paper. 



NUCLEUS AL^ CINERE^t. 



The caudal extremity of the nucleus alse cinerese is situated just cephalad 

 to the caudal limit of the nucleus nervi hypoglossi (figures 3 and 4, ac), 

 at a point slightly more than 5.G millimeters above the cephalic limit of the 

 decussatio pyramidum. From this caudal pole, the nucleus extends dorsally 

 and cephalad, to become superficial under the floor of the fourth ventricle; 

 then continues ventrally and cephalad, with slight lateral deflection, to 

 terminate ventrally to the nucleus vestibularis medialis, just cephalad to the 

 sujiierior termination of the nucleus tractus solitarii, aliout on a level with 

 the middle of the nucleus intercalatus (figure 4). The extreme cephalo- 

 caudal diameter measures 11.7 millimeters and its greatest transverse meas- 

 urement is 2.0 miflimeters. Streeter gives 13.5X2 miUimeters as the dimen- 

 sions of the nucleus in his study. 



The nucleus alse cinereiB presents varying relations in the several 

 portions of its extent. Throughout the caudal three-fourths of the nucleus 

 of the hypoglossal nerve, the nucleus alae cinerete lies dorsal and lateral to 

 it, covering the dorso-lateral surface of the former (figures 3, 4, 10, and 11, 

 oc). The nucleus intercalatus, in the cephalic half of the nucleus under 

 consideration, lies ventral and mesial to it until the nucleus ahie cinerea; 

 dips ventrally when it assumes a mesial and finally a dorso-mesial relation 

 (figure 3). Lateral to the nucleus alse cinereae in its caudal portion is a 

 small nuclear mass ])laced on the ventro-mesial convexity of the nucleus 

 fasciculi gracilis, and also the nucleus fasciculi gracilis and the nucleus 

 vestibularis medialis (figure 10). In its most cephalic portion the nucleus 

 tractus solitarii (Melius) lies lateral (figure 11). In its caudal beginning, 

 the nucleus alse cinerese is ventral to the nucleus fasciculi gracilis (figures 

 3, 4, and 10), but as the central canal of the cord widens out the nucleus 

 vagi liecomes superficial lieneath the floor, to dip finally ventrall}' to the 

 nucleus vestibularis medialis. Ventral to the nucleus ala? cinerese is the 

 nucleus nervi hypoglossi and the nucleus intercalatus. The formatio reticu- 

 laris is in ventral relation in a small part of its coiu-se. 



Mention has been made of a small nuclear mass lying lateral to the 

 nucleus alse cinerese, between it and the ventro-mesial surface of the nucleus 

 fasciculi gracilis. This mass of nerve-cells and neuroglia-cells forms a thin 

 sheet which follows the convexity of the ventro-mesial wall of the gracile 

 nucleus. Its dorsal border is seen from dorsal view in figure 3 and also 

 on mesial inspection, in figure 4. In its longest cephalo-caudal diameter 

 it measures 3.7 miflimeters, while its dorso-ventral diameter (somewhat 



