Nuclei Tuberis Laterales and the Ganglion Opticum Basale. 33 



tioD types, whose significance will be discussed later. The nuclei 

 tuberis laterales must be sliarply distinguished from certain regions of 

 the substantia grisea in whicli the cells are more densely packed 

 together than in other portions of the substantia grisea. Differences 

 in the density of different portions of the substantia grisea may be seen 

 in many sections, and very definite areas of closely packed cells are 

 shown in Figs. 17, 23 and 34. While in such dense masses the cells 

 are possibly somewhat larger than the remaining cells of the substantia 

 grisea, I do not consider the difference in cell type clear enough to 

 justify one in setting aside such dense cell masses as distinct nuclei. 

 Whatever the significance of these denser cell masses of the substantia 

 grisea, one fact is certain : they diifer both in location and in cell 

 type from the nuclei tuberis laterales; the radical difference between 

 these two kinds of cell groups should be clearly recognized. Of course 

 the structure of the cells of the nuclei tuberis laterales as well as that 

 of the cells of the substantia grisea is radically different from that of 

 motor cells, while the small size of both types of cells would indicate 

 that the connections of each cell were very limited; this correlation 

 between cell size and the extent of the connections of each cell has 

 already been referred to. 



Eelationship Of The Various Cell Groups To Oxe Another. 



From the previous description of the various cell groups together 

 with a reference to the figures showing their cell types, it will be evident 

 that the cell groups herein considered fall naturally into two classes: 



1. Those composed of small cells, including the substantia grisea, 

 and the nuclei tuberis laterales, and 



3. Those composed of large cells, including the basal optic gang- 

 lion, the nucleus paraventricularis hypothalami, the nucleus tubero- 

 mammillaris, and perhaps also the nucleus ansae peduncularis (gang- 

 lion basale). 



Taking up the first class of nuclei a most interesting relation will 

 be found to exist between the substantia grisea and the nuclei tuberis 

 laterales. The cells of the substantia grisea are the most primitive 

 of the hypothalamus. The constancy of cell type in all four animal 

 forms would suggest this, but the strongest evidence is derived from the 

 nature of the cell tyjie. The cell type of the substantia grisea, ^Wth its 

 scanty cytoplasm, resembles that of embryonic cells; it occurs often in 



