CENTRAL TRACT.
A. Connection of the bulbus olfactorius with the primary centres. Within the glomeruli, the impulse is transferred to the olfactory brush of the mitral and brush cells ; it then reaches the mitral or brush cells and thence, by means of their axones, is conducted centrally to the primary centres (Fig. 137). The bulbus olfactorius is, as it were, an intercalated ganglion, being the end-station of the peripheral tract and the starting point of the central tract. The primary centres include the gray substance of the tractus olfactorius and of the trigonum olfactorium, the substantia perforata anterior and the adjoining part of the septum lucidum.
B. Connection of the primary centres with the secondary or cortical centres. The secondary or cortical centres are : the gyrus hippocampi, the hippocampus and the gyrus dentatus. The connection is established by : a. The stria olfactoria laterals. The fibres pass from the trigonum olfactorium through the gyrus olfactorius lateralis to the anterior end of the gyrus hippocampi and terminate within the cortex of the same.
b. The olfactory bundle of the, hippocampus (Zuckerkandl). The fibres arise within the trigonum olfactorium and the substantia perforata anterior, extend to the septum lucidum, are augmented by fibres from the septum and then pass backward through the fornix as far as the hippocampus.
c. The stria Lancisii. The fibres pass from the trigonum olfactorium as the stria olfactoria medialis towards the gyrus subcallosus, thence over the corpus callosum and through the gyrus dentatus to the hippocampus formation.
According to Dejerine, the nucleus amygdalae is also a cortical centre. With this nucleus a fibre bundle, the taenia semicircularis, stands in close relation. The fibres arise within the substantia perforata anterior and the septum lucidum, are augmented by fibres coming from the anterior commissure, then extend convergingly toward the sulcus intermedius, where they run backward between the nucleus caudatus and the thalamus and end within the nucleus amygdalae. During the ascending anterior course of the bundle, fibres are given off at right angles and enter the thalamus (Fig. 138).
The fornix has already been referred to as the corona radiata bundle of the hippocampal formation. The fornix fibres arise from the pyramidal cells of the hippo campus and the polymorphic cells of the gyrus dentatus. They extend, first as the fimbria and then as the posterior limb of the fornix, toward the splenium corporis callosi. In this locality fibres pass across to the opposite fornix limb, thus forming the fornix transversus or the commissura hippocamfii. During its course beneath the corpus callosum, the fornix receives accessions from the striae Lancisii in the form of fibres which pierce the corpus callosum. These are known as the fibrae perforantes and constitute the fornix longus of Forel. In addition to those from the striae Lancisii, other fibrae perforantes from the gyms fornicatus penetrate the cal losum. The fornix fibres continue downward, as the columnae fornicis, behind the anterior commissure. The majority of the fornix fibres terminate within the corpus mamillare—tractus ; another part of the fibres, however, passes to the stria medullaris thalami and with these to the ganglion habenulae, as the tradus Some fibres of the fornix reach their end-station by another route. Such aberrant fibres branch off either above the foramen Monroi, passing in front of the anterior commissure, or at the level of the tuber cinereum, and course to the corpus mamillare as the stria alba tuberis of Lenhossek (page 57).
The paths proceeding from the corpus mamillare, as well as those related to the ganglion habenulae, may here be considered.
The corpus mamillare consists of two nuclei or ganglia, a medial and a lateral one. The medial ganglion forms the chief portion, while the lateral ganglion is small and arches around the medial. From the medial ganglion arises the fasciculus mamillaris princeps, which extends obliquely upward and outward. The fibres of this bundle divide into two branches, one of which becomes the fasciculus or tractus mamillo-thalamicus, the other the fasciculus tegnzento-mamillaris or the tractus mamillo legmentalis (Fig. 139).