Ascending fibres from the lower brain-segments, as strands from the vicinity of the end nucleus of the trigeminus, are also credited with terminating within the primary olfactory centres. Since terminal arborizations of the trigeminus are found within the regio olfactoria of the nasal mucous membrane and since this nerve, perhaps, also shares in the conduction of olfactory stimuli, it is not impossible that impulses may be carried from the olfactory region to the cortical olfactory centre by means of the ascending central trigeminal tract.
5. Connection of the cortical centres of the two is accom plished by the fibres of the fornix transversus, and, perhaps, by the pars interhemi sphaerica of the anterior commissure.
6. Further connections of the cortical fornix periphericus of Arnold, or the cingulum, is to be regarded as an association bundle of the rhinencephalon. It appears as an arcuate bundle, which surrounds the rostrum, knee, body and splenium of the corpus callosum ; at the isthmus it becomes narrow and expands toward the front end of the uncus. It consists of fibres which do not extend the entire length of the tract, but form larger or shorter strands, whose crooked ends radiate within the white substance of the neighboring convolutions. The cingulum appears to be, therefore, not properly an association bundle of the rhinencephalon, but an association strand of the different convolutions of the medial surface of the hemisphere (Fig. 126).
Reviewing the entire fibre-tracts of the rhinencephalon, we recognize, in the first place, that a centripetal' projection path conducts the impulse from the regio olfactoria to the primary centres and thence to the cortical centre proper ; secondly, that a centrifugal projection path transfers impulses from the cortical olfactory centre to subcortical centres (corpus mamillare, ganglion habenulae), from which latter then, by means of further paths, still other nuclei may be influenced. Thirdly, the tracts that arise within the primary centres and pass directly to the subcortical ganglia, constitute special reflex paths, and by means of these, in consequence of the transference of the impulse to the most diverse nuclei of the brain-stem, as the nuclei of the motor nerves, the most varied reflex movements may be induced. Finally, the peripheral and central districts of the rhinencephalon of both hemispheres are brought into relation with each other by means of certain systems of commissural fibres ; through the fornix periphericus, the central district is also connected with the adjoining regions of the pallium.