Johnston, The Brain of Petromyzoti. 13 



in the lower VIII roots while a central process becomes a MCl- 

 LERiAN fiber. There is a very conspicuous decussation or chi- 

 asma of these fibers a short distance caudad from VIII. These 

 spindle cells and the coarse central processes coming from 

 them exist in the acusticum and will be described below in 

 connection with that center. These structures, however, have 

 nothing to do with the Mi"llerian fibers. The proof of this 

 is that the cells in question are much more numerous than the 

 MuLLERiAN fibers, that other cells of exactly the same char- 

 acter exist in the cephalic part of the acusticum and send their 

 fibers cephalad to cross in the ansulate commissure, and that 

 the MuLLERiAN fibers do not decussate. 



The fibers can be traced with ease to certain large cells 

 situated among the motor cells. The relative size of these and 

 the motor cells is shown with approximate accuracy in Fig. 

 1 1 a. I have counted these cells in a single haematoxylin 

 series and find them distributed as follows : in the caudal half 

 of the ventral motor column, 10; in the cephalic half, on the 

 left side 7, on the right side 6 ; in the immediate vicinity of the 

 V root, I pair ; in the vicinity of the IV root, I pair ; in the 

 vicinity of the III root I pair ; a little farther forward and dor- 

 sad, I pair; a little farther forward and ventrally, I pair. These 

 large cells are like motor cells in lying near the cavity and send- 

 ing large dendrites among the fiber tracts, and they closely 

 resemble the large cells in Acipenser from which the Mauth- 

 Ner's fibers arise. A pair of cells much larger than the others, 

 which lie in the lateral motor column at the level of VII are 

 possibly directly homologous with Mauthner's cells in Aci- 

 penser (Fig. 1 1 a). At the caudal end of the medulla there 

 'are about forty fibers large enough to be conspicuous and of 

 these from twenty to twenty-four are very much larger than the 

 rest. These probably arise from the very large cells, while the 

 smaller fibers arise from cells not easily distinguishable from the 

 motor cells. 



Ahlborn thinks, indeed, that his median uncrossed fibers 

 arise from the large cells here described, but he is in doubt 

 whether the fibers which he has seen are in reality MCllerian 



