(c) 
(e) 
THE CENTRAL NERVOUS SYSTEM. 272 
cut end represents the point of origin of the first cranial 
or olfactory nerve, the various divisions of which may be 
found on the skull, where they may be traced into the per- 
forations of the cribriform plate. 
The olfactory bulb is the anterior portion of the olfactory 
lobe, or olfactory brain. When traced backward on the 
ventral surface of the brain it is seen to be replaced by a 
white band of fibres, the latter forming the olfactory tract 
(tractus olfactorius). The tract ends posteriorly in an 
expanded portion of the brain, which, from its shape, is 
described as the lobus piriformis. The olfactory brain as 
thus defined is separated superficially from the remaining 
portions of the cerebral hemisphere by a longitudinal furrow, 
the limbic fissure (fissura limbica), which passes along its 
lateral margin. The anterior portion of the furrow, known 
as the anterior rhinal fissure, separates the olfactory tract 
laterally from the narrow anterior portion of the cerebral 
hemisphere. The corresponding posterior portion of the fur- 
row, the posterior rhinal fissure, separates the lobus 
piriformis from the posterior enlarged portion of the hemi- 
sphere. The slight angle formed at the junction of the 
anterior and posterior rhinal fissures is the point of origin 
of a faint depression extending upward on the cerebral 
hemisphere. It represents a rudimentary, lateral cerebral 
(Sylvian) fissure. 
The corpus callosum is a broad white commissural band 
passing transversely from one hemisphere to the other. Its 
median portion may be exposed by pressing apart the 
medial margins of the hemispheres at the longitudinal 
cerebral fissure. 
The pineal body (corpus pineale) is a small, somewhat 
conical structure lying between the dorsal posterior tips of 
the cerebral hemispheres, and connected by a hollow stalk 
with the unpaired portion of the brain (the thalamence- 
phalon) lying below it. The connection is concealed by a 
mass of pigmented vascular tissue, the beginning of the 
chorioid plexus of the third ventricle, and usually also by a 
small portion of the dura mater containing part of the 
sagittal venous sinus. The latter may be carefully detached. 
