The olfactory bulb is cut off. _T.ol._, Olfactory peduncle; _A.p._,
area parolfactoria; _Tr.O._, trigonum olfactorium; _T.op._, optic
tract; _C.o._, chiasma opticum; _G.s._, subcallosal gyrus; _C.c._,
corpus callosum; _S.p._, septum pellucidum; _F_, fornix; _G.c._,
callosal gyrus; _T_, cut surface of thalamus; _F′_, fimbria; _G.d._,
gyrus dentatus; _L.p._, pyriform lobe; _G.h._, hippocampal gyrus.
]
The =olfactory tracts= or =striæ= (Striæ olfactorii) are three in
number. The =external tract= (Stria lateralis) is much the largest and
most distinct. It passes backward, upward, and outward, widens out and
joins the pyriform lobe. It is clearly defined dorsally by the sulcus
rhinalis and is marked off from the trigonum olfactorium by the sulcus
arcuatus. The =intermediate tract= (Stria intermedia) is short, ill
defined, and flat; it contains the canal which opens into the anterior
horn of the lateral ventricle. The =internal tract= (Stria medialis) is
small, short, and not well defined; it bends over to the parolfactory
area (of Broca) on the inner face of the hemisphere below the genu of
the corpus callosum.
The =trigonum olfactorium= is the prominent gray area situated in the
angle of divergence of the internal and external olfactory striæ. It is
bounded externally by the external olfactory stria, from which it is
defined by the arcuate sulcus. It is continuous with the =area
parolfactoria= on the medial surface; behind the latter a band descends
from the rostrum of the corpus callosum and is continuous below with the
anterior perforated space; it is termed the =subcallosal gyrus= or
peduncle of the corpus callosum.
The =pyriform lobe= (Lobus piriformis) is the well-marked prominence on
the base lateral to the optic tract and cerebral peduncle, from which it
is separated by a deep fissure. Its nipple-like apex lies behind the
fossa transversa and covers the optic tract. The external surface is
marked by one or two sulci (lobi piriformis). The lobe contains a
cavity, the ventral horn of the lateral ventricle.
The fibers of the olfactory tracts go to the pyriform lobe and
hippocampus, the trigonum olfactorium, the area parolfactoria, the
subcallosal gyrus, and part of the gyrus fornicatus. The central
connections of the olfactory apparatus are complex and are not yet
fully understood. The anterior cerebral commissure contains fibers
which pass from the olfactory bulb of one side by way of the inner
tracts to the bulb of the opposite side; also fibers which cross in it
from the inner tract of one side to the pyriform lobe of the opposite
side. Many fibers pass by way of the septum pellucidum, fornix, and
fimbria to the hippocampus. Other fibers pass in the anterior pillar
of the fornix to the mammillary body and thence to the thalamus by the
thalamo-mammillary bundle.
[Illustration: