234 RALPH EDWARD SHELDON 
-tum, and thence by quaternary fibers of the tractus strio-thalami- 
cus to practically all the nuclei of the thalamus and hypothalamus. 
Or impulses will more usually take a tract of the third order, 
the median forebrain bundle, for the region of the nuclei rotundus, 
subrotundus, posterior thalami. Other impulses may continue 
into the nucleus preopticus with fibers of the third order, the 
tractus mediano-preoptici, or may reach the more rostral parts of 
the nucleus by means of fibers of the second order. Neurones of 
the third order, largely, carry impulses from all parts of the 
nucleus preopticus to the habenular ganglia. It is, therefore, 
probable that the nucleus preopticus stands in much the same rela- 
tion to the habenulae as does the nucleus pyriformis. From the 
nucleus preopticus fibers of the fourth order reach the nucleus pos- 
terior tuberis and hypophysis, while from the habenulae such fibers 
pass to the corpus interpedunculare and the medial thalamus. 
Motor correlation probably takes place through two connections; 
one of these is by.means of the corpus interpedunculare, which sends 
fibers, according to Ramon y Cajal and Edinger, to the nucleus 
dorsalis tegmenti in higher forms, from which fibers undoubtedly 
pass into the great bulbar and spinal descending tracts for the 
transmission of somatic motor impulses. Other connections may 
also develop when this nucleus and its relations are more thor- 
oughly worked out. Another connection is by way of the tractus 
thalamo-bulbares et spinales from the thalamus to the medulla 
and cord (Johnston, ’06). In teleosts the more usual motor 
pathway for the simple direct olfactory impulses is probably by 
way of the corpus interpedunculare. This pathway is the more 
definitely laid down and involves the more direct connections. 
An impulse may pass to any part of the bulb, practically, from 
the olfactory mucous membrane, thence to the lateral olfactory 
area, thence by the definite, medullated tractus teniae to the 
habenular ganglia, thence by the powerful fasciculus retroflexus 
to the corpus interpedunculare and thence to the tegmental region 
of the mesencephalon, whence it may come into relation with the 
motor areas of the midbrain, medulla and spinal cord. 
The olfactory connection with the thalamus is not so simple 
and direct. An impulse must pass from the corpus precommis- 
