52 A Recondnailon. of the Nuclear Masses in 



NUCLEUS INCERTUS. 



Following Streeter, the indefinite nuclear mass, placed directly beneath 

 the floor of the fourth ventricle in the prominence which runs toward the 

 aqueduct of Sylvius from the eminent ia abducentis, has been modeled as 

 the nucleus incertus. It extends from its caudal limit at the middle of the 

 nucleus nervi abducentis (figure 13) cephalad beyond the superior limits of 

 this model (figure 14). Streeter diagrams it as beginning dorsally in the 

 region of the knee of the facial nerve and extending cephalad with a lateral 

 convexity. 



Mesial to the nucleus is the raphe and mid-line of the pons; ventral to 

 it occurs the formatio reticularis with also the nucleus reticularis tegmenti 

 pontis (figure 4). Ventral to its most caudal extremity is the cephalic half 

 of the nucleus nervi abducentis (figure 3). Lateral to the nucleus is the 

 cephalic portion of the nucleus vestibularis and the nucleus nervi trigemini. 

 Dorsal to the nucleus is the floor of the fourth ventricle. 



In general, the nucleus may be said to be long and flat with a somewhat 

 wider caudal extremity. Its limits are very poorly defined in cross-sections 

 and it is felt that the terminations given this nucleus are subject to more 

 individual variation than are those of any other nucleus modeled. The 

 nucleus lies beneath the ventricular floor and constitutes in whole or in part 

 the flattened nuclear material seen there. This gray matter is rich in a 

 fine fibrillar network and in its midst there is diff'erentiated, in part, the 

 nuclear material as modeled. The nucleus may be said to present for 

 examination only two surfaces, the dorsal and ventral, uniting with each 

 other at the lateral and mesial borders. 



As shown in figure 3, the nucleus begins abruptly in its caudal part but 

 soon widens and presents a convex smooth surface dorsally. The mesial 

 border is at first (juite close to the mid-line of the ventricle, but after a short 

 distance, in which a parallel relation is maintained, it curves laterally around 

 the fovea mediana. After this lateral deflection, the mesial surface continues 

 cephalad as an irregularly notched border which in general parallels the 

 mid-line, although it gradually l^ecomes more remote from it. This mesial 

 curve corresponds in general outline to that gi\'en by Streeter in his diagram 

 of the floor of the fourth ventricle. Th(> lateral border of the nucleus is shown 

 in part in figure 3. Beginning at the caudal extremity of the model, the 

 border i)ursues at first a straight, oblique direction ce]ihalad and laterally. 

 Then it i)arallels the mid-line for a distance, forming the narrowetl cephalic 

 portion of the nucleus. As the nucleus narrows, this lateral margin curves 

 toward the mid-line in a mesial convexity (o form a notch in the narrowest 

 part of the nucleus (figure 3). From this area cephalad, the nucleus widens 

 somewhat and then narrows as it ai)i)r()aches the sujxM-ior limit of the model. 

 In accord with this, the lateral margin shows a gradual lateral convexity 

 situated vcntrally to the mesencephalic portion of the fifth nucleus. 



