Johnston, The Brain of Petrontyzon. 15 



more dense and much more deeply stained. It is the tuberculum 

 acusticum. Its deep stain is due to the density and complexity 

 of the fiber endings in it. It appears first as a small deeply 

 stainmg mass which soon grows larger a<nd occupies the outer 

 half of the extreme dorsal part of the medulla wall, the inner 

 half being occupied by the lobus vagi (fasciculus communis. Fig. 

 9 a). At about the level of IX the acusticum comes to overtop 

 the lobus vagi which continues forward as a fasciculus between 

 the acusticum and the lourth ventricle (Fig. 10). The acusti- 

 cum becomes the largest grey mass in the medulla at the level 

 of the VII— VIII root complex and contains three more or less 

 distinct nuclei (Fig. 1 1 a). A short distance cephalad from IX 

 there are recognized two groups of cells which occupy ventro- 

 lateral and dorso-median positions in the acusticum. The dorsal 

 group receives the root of the lateral line X nerve. The ven- 

 tral group contains somewhat farther forward the spindle cells 

 from which Ahlbokn derived the Mullekian fibers, and the 

 greater part of the VII-VIII roots enter through tnis group. 

 Dorsal to both these nuclei, and partially separated from them 

 by an area of fine fibers, is another group of cells which re- 

 ceives the larger part of the lateral line VII nerve. Farther 

 cephalad this most dorsal of the three nuclei wholly disappears. 

 The dorsal one of the other two gradually takes a more median 

 position while the ventral one lies lateral to it. The area of 

 fine fibers which separates these from the third now forms a 

 distinct fine fibered layer on the dorso-lateral surface of these 

 two nuclei. The median nuclei of the two sides now approach 

 one another and at the same time the median and lateral nuclei 

 become almost indistinguishable. In this manner they both 

 enter into the formation of the granular layer of the cerebellum, 

 while the fine fibered layer forms the molecular layer. 

 From this description it appears : 



1. That there is a special, although diffuse, nucleus funi- 

 culi at the junction of the cord and medulla. 



2. That this nucleus is in direct continuity with both the 

 nucleus of the spinal V and the tuberculum acusticum. 



3. 'That the acusticum is in direct continuity cephalad 



