150 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
In the lamina terminalis, a little below the interventricular foramen, 
an anterior commissure, connecting the two hemispheres; a poster- 
ior commissure in the roof at the junction of di- and mesencephalon; 
and a superior or habenular commissure associated with the habenular 
ganglia and lying between the epiphysial structures and the velum 
transversum. In the amphibia, with the differentiation of the hip- 
pocampal region, a dorsal or hippocampal commissure appears in 
the lamina terminalis, just dorsal to the anterior commissure, connecting 
the hippocampi of the two sides. This persists, with slight modifica- 
tions, through the sauropsida and monotremes, but in the higher 
mammals it is subdivided into the hippocampal commissure proper and 
a more anterior portion, the corpus callosum. This corpus callosum 
is only in part the result of the division, but is more largely formed by 
new fibres, anterior to the hippocampal portion, connecting the neopal- 
lium of the two sides. The result is a broad band (the largest com- 
missure in the brain of man) which invades the intercerebral fissure 
from behind. In the lower vertebrates a few fibres pass downward from 
either side of the cerebellum beneath the fibre tracts of the medullary 
region and so to the other side of the cerebellum. In the mammals 
these are greatly increased in number, forming-a marked projection on 
the lower surface, the pons (Varolii), the prominence of which is in- 
creased by the great development of ‘nuclei’ in the medullary floor. 
The longitudinal tracts are more numerous and more complex. 
As will be recalled, there are dorsal, lateral and ventral columns in 
the spinal cord. These extend into the medulla oblongata and there 
pursue different courses. 
Some of the fibres of the dorsal columns end in connection with the nuclei of 
the medulla (p. 144), while others unite with fibres from the lateral column and with 
some from the oliva to form an enlargement, the corpus restiforme, and then 
bend upward (posterior peduncle) to enter the cerebellum. Other fibres from 
the lateral column, together with some from the dentate nucleus, enter the cere- 
bellum farther in front as the anterior peduncle, those from the dentate nucleus 
pass forward to the roof of the mid-brain, some terminating in the optic lobes, 
others continuing to the cerebrum. In this forward course, after leaving the cere- 
bellum, the fibres cross (decussate), those from the right side passing to the left 
side of the brain farther forward and vice versa. In the dorsal region of the medulla 
there is a short tractus solitarius (fasciculus communis) derived from fibres 
from the seventh to tenth nerves and extending-no farther forward than the seventh. 
In the higher vertebrates there are the crossed and the direct pyramidal tracts 
on the ventral side of the medulla, the direct being continuations of part of the ven- 
tral columns, the crossed of the deeper lateral columns. In the medulla these en- 
