362 NERVOUS SYSTEM. 
supracallosal or callosal sulcus (e). Between the supracal- 
losal (¢) and splenial (@) sulci is the gyrus fornicatus (2). 
On the caudal surface of the hemisphere (that facing the 
cerebellum) appear the ends of the sulcus splenialis (a) and the 
sulcus rhinalis posterior (/). Hidden in the natural condition 
by the thalami and optic tract is the broad hippocampal sulcus 
(g), marking externally the course of the hippocampus. 
A further extension of the surface of the cerebrum has 
taken place in connection with the sense of smell. A mass of 
gray matter, the olfactory bulb (Fig. 144, a), reckoned as a 
part of the cerebrum, is separated from the latter and lies 
against the cribriform plate of the ethmoid bone. From it the 
olfactory fibres pass through the perforations of the plate to the 
olfactory mucous membrane. The olfactory bulb contains a 
cavity, a part of the lateral ventricle. 
' The bulb lies against the ventral surface of the frontal lobe 
and projects craniad of it. It is connected to the cerebrum by 
a tract of fibres, the olfactory tract (Fig. 138, @), which is 
divisible into two roots, medial and lateral. The medial root 
comes from the medial surface of the frontal lobe, where it is 
continuous with a tract extending to the cranial end of the 
corpus callosum. The lateral root is traceable from an elevated 
gyrus-like portion of the cerebrum which lies at the side of the 
infundibulum and is known as the lobus pyriformis or tractus 
postrhinalis (Fig. 138, 7). The lateral root is divisible into a 
medial white strand and a lateral gray strand. 
That part of the brain comprising the olfactory bulb and 
the parts intimately related to it are frequently included under 
the term rhinencephalon. 
In the triangular area between the two olfactory tracts and 
craniad of the optic chiasma appears a mass of gray matter, 
subdivided by a longitudinal fissure. This possesses numerous 
openings through which blood-vessels pass to the brain sub- 
stance, and is thence known as the anterior perforated sub- 
stance (substantia perforata anterior) (Fig. 138, 4). 
Internal Structures of the Cerebrum.—The cavity of each 
of the cerebral hemispheres is known as a lateral ventricle. 
The two lateral ventricles constitute the firs¢ and second of the 
