The Central Nervous System. 273 



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. 



(c) 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 limbics), 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. 



(d) 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. 



(e) 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- 

 phalic) 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. 



