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, _a_), 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, _f_). 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 substance, and is thence known as the =anterior perforated substance= (=substantia perforata anterior=) (Fig. 138, _b_). =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 _first_ and _second_ of the ventricles of the brain, whence the application of the names _third_ and _fourth_ ventricles to the cavities of the ’tween-brain and hindbrain. The lateral ventricles do not grow at the same rate as the walls of the hemispheres, so that they remain comparatively small. The cavity of each ventricle is further reduced in size by the development on its floor of a large ridge-like thickening, the =corpus striatum= (Fig. 148, _f_).