142 J. B. JOHNSTON, 
histological character which surround the ventral part of the cavity 
of the fore brain throughout nearly its whole extent (Phot. 73, 
Fig. N). These cells are characterized by having a central process 
which runs among the ependymal cells to the central cavity. Cells 
of two kinds are to be recognized. The greater number are very 
similar in form to those in the torus longitudinalis (page 103) and 
in the corpus mammillare (page 118). These cells measure 7—16 
by 11—24 u, have stellate bodies with two or several dendrites 
directed outwards and a central process which varies in length from 
almost nil to several times the diameter of the cell body. Cells 
of this form are shown in Phot. 73, which is a frontal section at 
the cephalic edge of the commissura anterior. The dendrites are 
slender and smooth, so much so, in fact, that it is sometimes difficult 
to distinguish the neurite of a cell from its dendrites. The neurites 
arise from one of the dendrites. In the cephalic part of the nucleus 
they join the tractus strio-thalamicus. 
The most important part of this nucleus is situated in the walls 
of the recessus praeopticus. I have traced the neurites of part of 
the cells of this region caudad over the chiasma into the corpus 
geniculatum. Whether they end there or in some part of the lobi 
inferiores, I do not know. Some of the neurites cross to the opposite 
side beneath the recessus praeopticus before entering the hypo- 
thalamus. Other cells send their neurites at first dorsad. I have 
been unable to trace these for any great distance from the cell but 
think it probable that they join the tracts to the ganglia habenulae, 
since a large part of these tracts come from this region. 
The cells of the second form are chiefly or wholly confined to 
that part of the nucleus situated caudal to the anterior commissure. 
These cells are fusiform with one long, thick dendrite from each 
end of the cell body. The cell bodies measure 12—16 by 16—32 u. 
One process is shorter than the other and, although usually having 
the appearance of a dendrite which gives off several small branches, 
it is sometimes reduced to a mere central process. The other 
dendrite is always long and straight and is disposed longitudinally, 
lying among and parallel with the fibres of the tractus strio-thalamicus, 
which probably determine its position mechanically. This dendrite 
bears numerous small branches at right angles and gives rise to 
the neurite, always at a long distance from the cell and often as a 
continuation of the dendrite itself. The neurites run in the tractus 
strio-thalamicus ventralis to the hypothalamus. 
