THE STRUCTURES AT THE BASE OF THE BRAIN. 101 



cortex, too, which covers the whole lobe is atrophic in the 

 olfactory tract. The fibres of the actual olfactory nerves arise 

 from the olfactory bulb. 



The olfactory-lobe cortex has numerous connections with 

 regions of the brain which lie farther back. Some of these are 

 plainly visible on the base of the brain. They pass over the 

 substantia perforata, and in part penetrate it. Formerly these 

 were thought to be real nerve-roots, but, according to our present 

 views, it is much more probable that they are tracts which serve 

 to connect the olfactory lobe with more central portions of the 

 brain. The most external of these bundles can be traced into 

 the tip of the temporal lobe. The results of physiological ex- 

 periments make : it probable that it terminates in the cornu 

 ammonis. The investigations of Zuckerkandl in comparative 

 anatomy show that these olfactory nerve-roots have their termi- 

 nation not only in the cornu ammonis, but probably in the 

 cortex of the whole gyrus hippocampi and the lobus lingualis, 

 which is associated with it. 



In the olfactory nerves of the lower animals there has been 

 demonstrated an extensive system of commissures and a chiasm 

 between them. We only know that fibres from the anterior 

 commissure pass into the region of the olfactory lobe. These 

 fibres, which can be plainly seen in Fig. 41, form, as it appears, 

 a commissure between the olfactory lobes. The other fibres of 

 the anterior commissure connect the two lobi linguales (which 

 are also points of origin for the olfactory nerve) with each 

 other. 



The gray lamina between the two olfactory lobes is directly 

 continuous in front with the knee of the corpus callosum. It 

 is called the lamina terminalis. It is the remnant of the em- 

 bryonic terminal lamina, that wall which closed the primitive 

 fore-brain, and from which the massive hemispheres have been 

 developed. At present it is only a small, unimportant gray 

 area, which lies at the most anterior point of the base of the 

 brain. 



