THE BASE OF THE FORE-BRAIN. 77 



bounds it on two sides and separates it from the posterior orbital 

 gyrus, in front; and from the temporal lobe, behind. Laterally 

 it is separated from the frontal and the parietal parts of the oper- 

 culum by an antero-posterior cleft continuous with the lateral 

 cerebral fissure. 



The insula is continuous with the anterior perforated substance, 

 and the area of transition from one to the other is called the 

 threshold, or limen insula (Fig. 26). 



Rhinencephalon. The smelling brain belongs to the basal 

 surface. It is rudimentary in man. Many connected parts 

 make it up. It is divided into two parts by the sulcus paroljac- 

 torius posterior. These are designated as the pars anterior rhin- 

 encephali and the pars posterior rhinencephali. The pars 

 anterior of the rhinencephalon embraces, first, the olfactory lobe, 

 which is made up of the olfactory bulb, tract, triangle and the 

 medial and intermediate striae; and second, the area parolfactoria. 

 In the pars posterior rhinencephali are included the anterior 

 perforated substance, the gyrus subcallosus, the lateral olfactory 

 stria, and the limen ihsulae. 



Olfactory Lobe. (Lobus olfactorius}. There is one lobe that 

 is studied only on the basal surface of the fore-brain. That is the 

 olfactory lobe (Fig. 26). Belonging to the pars anterior rhinen- 

 cephali, it comprises many connected parts; and the reason for 

 calling them the olfactory lobe is found in the lower animals 

 and in the human embryo, where it exists as a prominent hollow 

 process of the cerebral hemisphere (Figs. 17 and 18). 



Olfactory Lobe 



Bulbus olfactorius 

 Tractus olfactorius 

 Trigonum olfactorium 

 Stria medialis 

 Stria- intermedia. 



The olfactory bulb (bulbus olfactorius) is an ovoid mass of 

 brain matter about half an inch long, one- sixth of an inch wide 

 and a quarter of an inch in vertical diameter (Fig. 26). It is 

 lodged in the olfactory sulcus of the frontal lobe and rests upon 

 the cribriform plate of the ethmoid bone through which it receives 

 the twenty or thirty olfactory nerves. The center of the bulb 



