40 A Reconstruction of the Nuclear Masses in 



On account of the thinness of this dorsal leaf, the mesial edge is fairly- 

 sharp as one passes upon its ventro-mesial surface. The leaf, however, 

 rapidly increases in thickness as it goes toward the main cell-mass, reaching 

 its greatest transverse width at the irregular ridge underlying the lateral 

 angle of the ventricle. 



NUCLEUS NERVI ABDUCENTIS. 



With its inferior termination just caudal to the ccjihalic end of the 

 nucleus intercalatus, the nucleus nervi abducentis extends almost to the 

 cephalic limit of the nucleus vestibularis (figures 3 and 4). It measures in 

 the long cephalo-caudal diameter 3.8 millimeters; in its dorso-ventral, 1.7 

 millimeters; and in the greatest transverse, 2.3 millimeters. 



Mesial to the nucleus are the genu of the seventh nerve and the fasci- 

 culus longitudinalis medialis (figure 13). Dorsal to it in its middle is the 

 loop of the seventh nerve, while above it is covered by the nucleus inter- 

 calatus (figures 3 and 13). The nucleus lies fairly superficial beneath the 

 eminentia abducentis of the floor of the fourth ventricle. Lateral to the 

 nucleus lie the nucleus vestibularis medialis and the emergent root of the 

 seventh nerve. The abrupt notch made in the vestibular cell-mass to 

 accommodate the nucleus is shown in figure 3. Ventral to the nucleus is 

 the formatio reticularis. Just caudal to its inferior pole is found the nucleus 

 vestibularis medialis, while slightly mesial to this is the cephalic pole of the 

 nucleus intercalatus. 



Miss Sabin describes and pictures the sixth nucleus in the new-born 

 as an "almost round body." The general shape of the nucleus in the adult, 

 as seen in figure 4, is that of an oval body with a dei)ressed mesial edge and 

 a bulging lateral border. The portion of the dorsal surface of the nucleus 

 not obscured by the nucleus incertus is shown in figure 3. The most dorsally 

 prominent portion of the nucleus is just superior to the caudal limit; it 

 shows as a projecting ridge on the dorsal and mesial surfaces (figure 4). 

 Cephalic to this margin, the mesial and dorsal surfaces are depressed, to 

 give room for the looping fibers of the ncu'vus facialis. The dejiression on 

 the dorsal surface persists, as is shown by the gradual ventral slope of this 

 section on mesial view. This slope forces the cephalic pole to a plane more 

 ventral than that of the inferior i)ole (figure 4). While the mesial and dorsal 

 surfaces show depressions, the lateral surface from dorsal view (figure 3) 

 shows a marked lateral bulging which gradually rounds into the superior 

 i:ole. Ventral to this lateral projection, seen from dorsal view, the nuclear 

 material is hollowed out in a mesial depression on the lateral surface. The 

 ventral surface, mesial to this de])ression, is well rounded into a mass which 

 shows a marked inferior shoulder from which tlie surface slopes concavely 

 into the cephalic i)ole. This may be S(>en in figure 4. This concavity 

 necessarily pushes the sujx'rior i)ole dorsally, so that it lies dorsal to the 

 mid-axis of the nucleus. The superior ])()le is a gi'adual convexity when 



