362 NERVOUS SYSTEM. 



supracallosal or callosal sulcus (e). Between the supracal- 

 losal (e) and splenial (a) 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 

 (,), 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, 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, _/"). 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, ). 



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 



