The Lower Portion of the Huiium Brain-Stem. 63 



NUCLEUS OLIVARIS ACCESSORIUS MEDIALIS. 



Placed between the stratum interolivare and the inferior oHvary nucleus, 

 but separated from the latter by the fibers of the nervus hypoglossus, is 

 the nucleus olivaris accessorius medialis. This medial accessory olive is a 

 flattened irregular cell-mass, arising caudally considerably nearer to the 

 decussatio pyramidum than the inferior olive, but extending ce[jhalad to a 

 point somewhat caudal to the cephalic pole of the inferior olivary body 

 (figure 4). In cephalo-caudal extent, it practically equals that of the olive, 

 beginning and ending caudally to the corresponding points on the main 

 nucleus. As a broad, thin sheet of cells it covers almost completely the 

 hilum of the olive on the mesial side, although the cell-masses are lacking 

 somewhat along the dorsal border. The whole nuclear mass of this medial 

 accessory olive is shown in figure 4. 



The nucleus olivaris accessorius medialis, on the left side of this adult 

 medulla, consists of a continuous flat and broad column of cells, presenting 

 two constrictions in the mass and also showing separated nuclear masses 

 at different portions of the whole extent. Beginning abruptly in the formatio 

 reticularis, the caudal portion of the main mass of the medial olive quickly 

 widens into the broadest ]iortion of the whole mass. The general direction 

 of this portion is obli(iucly lateral, making an angle of about 135 with the 

 raphe. With the broadest portion existing at the level of the inferior olivary 

 pole, this sheet of cells, convex medially, continues uj^ward to the level of 

 the third dorsal transverse fissure, where it exhiliits a marked constriction. 

 Just caudal to this constriction its dorsal border is continued posteriorly 

 into a well-marked projection lying entirely behind the general line of this 

 border. Undoubtedly this constriction corresponds to the division between 

 the caudal and middle cell-masses found by Sabin in the new-born, but 

 in the babe the degree of constriction is apparently far more marked. 

 Above this constriction the main cell-mass extends as a continuous, fairly 

 broad sheet to the level of the first dorsal transverse fissure. The ventral 

 border of this portion is slightly curved with the convexity dorsally. The 

 dorsal border, from the point of constriction, gradually and irregularly 

 projects more dorsally, until at the level of the second dorsal transverse 

 fissure it juts suddenly dorsally, to form the neck of an irregular cell-mass 

 (figure 4). This new cell-mass is almost square from mesial view, showing 

 a deep notch on its cephalic surface and separated from the main mass of 

 the mesial oli^'e by a still deeper notch. Its pedicle is rather broad, but is 

 perforated by a distinct band of fibers. The main nuclear mass, after an 

 abrupt ending of the principal broad column of cells at the level of the first 

 transverse fissure, is continued cephalad by a narrow mass of cells which 

 stretches dorsally and cephalad to widen above into an irregular cephalic 

 broadening. This broadened mass exhibits a dorsal i)rojection in its superior 

 half and a sharper, narrower ventral spur at its cephalic pole. 



Such, then, is the main sheet of cells comprising the m(\sial accessory 

 olive. In the main, it shows the same primary divisions as found by Miss 



