218 ANATOMY OP THE CENTRAL NERVOUS SYSTEII. 



the biilbus olfactorius. It has been learned in earlier chapters that both of 

 these brain-parts are everywhere present, from the fishes on upward. 



The lobus pyriformis and the cornu Ammonis are, indeed, parts which 

 are connected with the olfactory apparatus; nevertheless, tliey are character- 

 ized by their structure as immense separate territories which are able of 

 themselves to bring about the most manifold associations, etc. 



They are most probably the cortical areas of smell; their very large 

 development also argues in favor of this. 



Let us now examine more closely what tracts they receive and what 

 others they gi^'e origin to. 



l^he cortical area of smell receives its tracts, on the one hand, through 

 the olfactory bundle; on the other, through the fibers running superficially 

 in the layer of tangential fibers. But what tracts does it send out? These 

 collect at its mesial edge as the fimbria (Fig. 143), and then pass toward the 

 anterior. It is soon recognized that they belong to at least two difEerent sys- 

 tems. A large part separates at the anterior end of the gyrus hippocampi, 

 and even somewhat farther posteriorly also, and passes over to the cornu 

 Ammonis of the other side. These connecting fibers, in their entirety, are 

 called the psalterium. These are the mesially-lying fibers. From the more 

 lateral fibers, however, another bundle is collected. Tracts, directed for 

 the most part longitudinally, here pass forward and run for a short distance 

 next to the olfactory bundle, entering at this point, but soon leave it again 

 to pass backward in a downward-directed course. These are the fornix. 

 It ends near the base of the brain in the corpus mamillare and also in the 

 opposite thalamus. The fornix is consequently that part of the medulla of the 

 gyrus hippocampi ivhich, not used for commissures, connects tMs medulla with 

 the interirain. With the "descending fornix," as that part is called in con- 

 tradistinction to the bundle rimning longitudinally forward from the cornu 

 Ammonis — ^the "ascending fornix" — there is associated, however, another 

 tract of fibers which arises from that part of the marginal gyrus that is not 

 inrolled to form the cornu Ammonis, the gyrus limbicus. In order to reach 

 down to the fornix its tracts must pass through the corpus callosum, which 

 always covers over the ventricle in mammals. The bundle is called the 

 fornix longus. Its fibers always lie just under the corpus callosum and an- 

 teriorly, forming the most mesial bundles of the pillar of the fornix; they 

 pass down with this to the bottom of the interbrain. 



It is very probable that the descending fornix, as well as the fornix 

 longus, receives in its course fibers from the opposite olfactory cortex by 

 way of the psalterium. 



In the smaller mammals tlic relations of the fimbria and the psalterium, as well 

 as of the fornix, are better known than in man. This is because, on the one hand, 

 they are relatively much larger structures in the osmatic animals investigated than 



