Johnston, The Brain of Petivmyzon. 17 



3. TubercLilum acusticum (Figs. 10, 11). 



In GoLGi preparations it is difficult or impossible to dis- 

 tinguish between the caudal part of the acusticum and the 

 nucleus of the spinal V and the nucleus funiculi. For this rea- 

 son the following description of the acusticum is taken from 

 that part in front of the lateral line X root. In this part of the 

 acusticum there are three kinds of cells : large and small cells 

 resembling those of the nucleus funiculi, and a peculiar type of 

 spindle shaped cells. The first two are found in all three acus- 

 ticum nuclei, the third only in the lateral nucleus. The large 

 cells are found in part among the fiber bundles and in part ad- 

 joining the ventricle, with a central process among the epen- 

 dyma cells. They vary greatly in both size (8-16 X 14-48 /v.) 

 and form, many of them vividly recalling the large irregular 

 cells in the acusticum of Acipenser. The neurites of these 

 cells go as arcuate fibers across the ventral raphe. 



The small cells do not appear to be very numerous, but 

 this is probably due in part to their failure to become impreg- 

 nated. They measure 8—9 X 8—16 /^, and are always sharply 

 distinguished from the large cells by the facts that the cell 

 bodies are compact (rounded) and lie among the fiber bundles, 

 never adjoining the ventricle, and that the dendrites are short 

 and poorly branched. The neurites could not be traced far 

 from the cells. 



The spindle cells are seldom impregnated in Golgi prepar- 

 ations, but in iron haematoxylin sections they are clearly dif- 

 ferentiated by their simple form and by their sharp deep black 

 stain. Their position is indicated in Figs. 11, 1 1 a, 12; their 

 structure is shown in Fig. 23. As noted above, Ahlborn de- 

 scribed these cells as having central processes which become 

 Mullerian fibers and peripheral processes which go out in the 

 VIII nerve. It has been shown above that the Mullerian 

 fibers are not processes of these cells and it will be shown here 

 that the VIII fibers do not arise from them. In iron haema- 

 toxylin sections certain coarse fibers of VIII and the lateral 

 line VII have a direct course and take the same deep stain as 

 do the spindle cells and their processes. These sharply stained 

 fibers pass in part forward and in part backward in the lateral 



