408 J. B. JOHNSTON 
complex from which arises the great olfactory projection tract 
(fig. 17). The relations to the amygdaloid will be further de- 
scribed in the next section. The extreme rostral and upper end 
of this nucleus is the very dense collection of cells seen near the 
recessus superior in figures 19 and 39. There is no break in the 
continuity of the nucleus described but on the basal surface in 
front of the optic chiasma and lateral to the optic tract, the 
number of cells is much less than in the medial wall. Through- 
out the length of the nucleus its cells are small and are some- 
what elongated in the direction of the densely packed bundle of 
fine fibers among which they lie. 
The fibers in question enter into the precommissural bundles 
of the fornix system in the medial wall. They are the olfactory 
radiations of Zuckerkandl and constitute the chief pathway 
from the amygdaloid region to the hippocampus. Compare 
"13 b, p. 410. From the amygdaloid forward along the basal 
surface these fibers form a low ridge parallel with the optic tract 
(figs. 1, 8, 19) and in this position the bundle is clearly visible in 
- the entire brain (fig. 2). The bundle is clearly distinct from 
the medial forebrain bundle which lies ental to this. In front 
of the optic chiasma the bundle is seen to be composed of two 
parts, one of which is derived from the medial olfactory tract as 
above described, while the other bends up into the medial wall, 
spreads out in the area between the fissura prima and the com- 
missures, and enters the hippocampus (figs. 56, 57). The tri- 
angular area between the fissura prima and the commissures in 
which this bundle spreads out like a fan is the equivalent of the 
mammalian gyrus subcallosus. Lateral to the optic chiasma 
this bundle passes close to the supra-optic nucleus and there 
seems to be a connection between the two which has not yet 
been carefully studied. 
The homology of the area described in this section with the 
gyrus subcallosus and the diagonal band of the mammalian 
brain is obvious. Its chief morphological significance is that it 
represents the persistence of the preoptic and precommissural 
connection between the medial olfactory area and hippocampus 
on the one hand and the lateral olfactory area and amygdaloid 
