OLFACTORY CENTERS IN TELEOSTS 233 
preoptici and also secondary olfactory fibers from the tractus 
olfactorius medialis, pars lateralis, before its decussation (fig. 136). 
It thus receives both secondary and tertiary olfactory fibers. 
Very similar to the precommissural, and of great morphological 
significance, are the descending connections of the primordium 
hippocampi. The latter receives secondary olfactory fibers from 
the tractus olfactorius medialis, pars medialis, and gives rise to 
fibers for the tractus olfacto-thalamicus, pars dorsalis, for the 
diencephalon. 
The important descending pathway from the nucleus preopti- 
cus is the tractus praethalamo-cinereus from the nucleus magno- 
cellularis to the hypophysis, together with the nuclei lateralis and 
ventralis tuberis. Besides this there is the tractus preoptico- 
tuberis from the same nucleus to the region of the nucleus pos- 
terior tuberis and the nucleus posthabenularis. Both of these 
are probably neurones of the fourth order. 
Important neurones, chiefly of the third order, connect the 
nucleus preopticus with the habenulae, originating from all parts 
of the nucleus (figs. 141, 142). 
Neurones of the fourth order originate in the habenular ganglia 
and pass caudo-ventrad, the fasciculus retroflexus for the corpus 
interpedunculare, and the tractus habenulo-diencephalicus for the 
formatio reticularis in the region of the nucleus posterior tuberis _ 
(figs. 141, 142). 
It will be noted, then, that the olfactory neurones of the first 
order, or olfactory nerve, carries impulses to all parts of the lateral, 
rostral and mesal aspects of the bulb. From the lateral part of the 
bulb, chiefly, but also from the mesal, impulses are carried by 
neurones of the second order to the lateral area of the basal lobes. 
Thence neurones of the third order carry the impulse either to the 
habenula, or else to the nucleus posterior thalami, or the diffuse 
cellular area of the caudal part of the inferior lobes. From the 
mesal portion of the bulb impulses are carried to all parts of the 
mesal olfactory area, or corpus precommissurale and primordium 
hippocampi, by neurones of the second order, which also reach the 
nucleus preopticus, further caudally. From the mesal area im- 
pulses may travel by neurones of the third order to the palaeostria- 
