THE CEREBRUM OR PROSENCEPHALON. 



171 



development of the olfactory apparatus; so dependent that, in aquatic mam- 

 mals with atrophied olfactories, this cortical area is demonstrable only in 

 rudimentary form, while in the burrowing rodents it may attain an enor- 

 mous development. 



Through these investigations the demonstration seems complete that 

 the cortical portion Just described is the cortical center for the sense of 

 smell. 



Not only does a fiber-system end in the Cornu Ammonis, but a bundle 

 arises from it and commissu.res enter it. A large assortment of fibers come 

 into relation with this cortical area. Before they enter they all distribute 

 themselves along the ventral margin and form an important collection of 

 nerve-fibers designated the Fimh-ia. The fimbria always occupies the same 



Fig. 119. — Section through the posterior portion of the left hemisphere: A, of a 

 Lizard — Varaniis griseus; B, of a Mouse-embryo. 



position in all vertebrates; it accompanies the ventral margin of the cornu, 

 and, therefore, in reptiles lies dorsal to the Fissura arcuata septi. In the 

 posterior portion of the hemispheres where the median wall of the brain 

 merges into the Plexus chorioideus the fiber-system of the fimbria lies be- 

 tween these structures and the cortex. Let us now consider the fibers which 

 arise in the olfactory cortex, and also the commissural fibers. 



Eeptiles and mammals which have well-developed olfactory cortices, 

 probably also amphibians and birds, possess two bundles which characterize 

 this cortical region and always occur in the same place. The two bundles 

 are generally together designated the Fornix. But it is more advisable to 

 subdivide the bundle into two parts according to their terminations. Com- 

 ing from the posterior part of the olfactory cortex they together pass a 

 short distance ventrally to about the level of the commissura anterior, and 

 then they turn toward the posterior. Here the previotisly united bundle 



