370 
BRAIN OF MAN. 
pass from each, hemisphere to its fellow through the corpus 
callosum, whilst others again bring the different convolutions 
of the same hemisphere into mutual connexion. The hemi¬ 
spheres are (so to speak) wrapped round the collection of 
Sensory Ganglia in which the spinal cord may be said to ter¬ 
minate at its upper end, in such a manner as to leave two 
cavities, one on either side, which are called the lateral ven¬ 
tricles. 1 The Sensory Ganglia are so small relatively to the 
Cerebrum, that they would scarcely attract notice as inde¬ 
pendent centres, if they were not carefully compared with the 
ganglionic centres corresponding to them among the lower 
animals. The olfactory ganglia are mere bulbous enlarge¬ 
ments upon the cords (1), which, though commonly termed the 
olfactory nerves, are really (as in the Shark, § 453) footstalks 
connecting these ganglia with the rest of the series ; it being 
from these ganglia that the true olfactive nerves are given off 
(§ 506). The optic ganglia, g , only in part represent the 
optic lobes of Fishes ; the function of the latter being shared 
by two large masses termed the thalami optici, which form 
the hinder part of the floor of the lateral ventricles, and 
which also seem to participate in the sense of touch, as the 
sensory columns of the spinal cord may be traced up to them. 
This close connexion of the sensorial centres of Sight and 
Touch is just what we might anticipate from the continual 
co-operation of these two senses (§§ 556 , 557). In front of 
the optic thalami is another pair of large ganglionic masses, 
termed the corpora striata , which is in the like close relation 
with the motor columns of the spinal cord ; and it is chiefly 
from them and from the thalami optici, that the fibres pro¬ 
ceeding to the surface of the Cerebral hemispheres radiate. 
The Cerebellum , which has no direct communication with the 
Cerebrum, but possesses independent connexions of its own 
with the upper part of the spinal cord, has its grey or vesicular 
and its white or fibrous substance so peculiarly disposed, as 
to present in section the appearance delineated at d , which 
is termed the arbor vitae, , or tree of life. 
459. Of the nerves given off within the skull (figs. 196,197), 
1 There are other ventricles, which are merely spaces left on the 
middle plane by the imperfect coalescence of the two lateral columns 
of the nervous axis, like the openings formed by the divergence of the 
two halves of the nervous cord in Insects (fig. 188). 
