56 BULLETIN OF THE LABORATORIES 



XIV, Fig. 3.) The median nucleus {Fig. 5, b,) still continues, and 

 there is a gradual separation of the cells of the nucleus into a dorsal 

 and ventral group, as will be seen by reference to Fig. 6, d, and c, of 

 the same plate. The remaining portions of the gray matter are about 

 as in the previous section. At about the middle of the decussation 

 we find that the median nucleus has entirely differentiated into two 

 distinct nuclei, the ventral being the caudad extension of the nucleus 

 of the twelfth nerve, (Plate XIV, Fig. 4,) and the dorsal, the nucleus 

 of the eleventh nerve. The pyramidal tracts in decussating and pass- 

 ing dorsad are divided up into a great many fasciculi, and these are 

 thickly interwoven with each other in crossing and also with the fibres 

 of the gray matter. 



The surface of the fasciculi is very thickly covered with nutri- 

 tive corpuscles, even more so than the gray matter at this point. The 

 gray matter contains many longitudinal bundles which are seen here 

 in cross section and which mark the first appearance of the fonnatia 

 reticularis of the medulla. In this section also there appears for the 

 first time a small ventral cluster of cells, (Plate XIV, Fig. 6, a,) which 

 marks the beginning of the olivary body. These cells are rather smaller 

 than those in any nucleus previously discussed, measuring about. 02 

 mm. in diameter. They tend to be bipolar and fla.sk shaped in form 

 and the processes start more abruptly from the cell body. Plate XIV, 

 Fig. 2, represents this entire nucleus. 



It may be noted here that our study has led us to the opinion that 

 most of the so-called flask- shaped and unipolar cells are in reality 

 cells of the bipolar type which have been sectioned obliquely to their 

 axis. This will account for the fact that many of the flask-shaped 

 cells are non-nucleated. We do not mean to imply that there are no 

 unipolar cells, for those do in some cases undoubtedly exist, but that 

 in a nucleus the cells all tend to be of the same type and it is with 

 doubt that we accredit the presence of unipolar cells in a multipolar 

 nucleus. 



Passing cephalad from this level, the medulla, enlarges rapidly, 

 ih^ir r'alis centralis is flexed dorsad and rapidly expands into the fourth 

 veitricle. There is an aggregation of gray matter towards the median 

 portions and on the floor of the ventricle. The white matter is limited 

 to a narrow zone around the lateral and ventral surface and the reticu- 

 lar formation. 



\\\ a .section taken at the level of the twelfth nerve, (Plate XV, 



