PHYSIOLOGY OF THE NERVOUS SYSTEM. * 723D 
directed very strongly, and for some time, to 
any part, it may become the seat of pain, for 
which the most effective remedy is to engage 
the thoughts as much as possible on some other 
object. In many instances, where pain has 
been excited by a physical cause, there can be 
no doubt it has been continued long after the 
cessation of its exciting cause, by the attention 
of the patient having been directed to it. It is 
probable, that in such cases the perceiving parts 
of the brain (so to speak) become habituated to 
_ acertain condition of the centre of sensation, 
_ produced by the original exciting cause of the 
_ pain. And, on the other hand, pain, at first 
excited by the mind, may be rendered perma- 
nent by habit; a certain physical alteration in 
some part of the centre of sensation being in- 
“duced by the frequent repetition of the mental 
_ act in reference to a particular part of the body. 
_ Those parts of the brain which are capable 
only of mental nervous actions, that is, of ac- 
tions by which the mind is immediately affect- 
1, or which the mind can develope, have no 
rves implanted in them. Such are the con- 
‘lutions, the corpora striata, the optic thala- 
“mi, and the cerebellum. The only apparent 
exceptions to this statement are the olfactory 
and optic nerves: these nerves, however, have 
in truth no immediate connection with any of 
the parts above mentioned. The former are 
mplanted in the olfactory lobe; the latter in 
le chiasma, which is formed by the junction 
of the optic tracts, and these ought no more to 
garded as portions of the optic nerves, than 
2 olfactory lobes should be considered as 
unctions of the commissures.—The anatomy 
ae parts which we call commissures indi- 
that the name by which they have long 
known is not misapplied, inasmuch as 
seem to unite particular portions of the 
ous centres with each other. The most 
ous object of such an union would be to 
ure the harmonious cooperation of the parts 
thus united. And this view of their function 
is strengthened by the fact that the principal 
commissures bear a direct ratio in point of 
velopement to that of the parts they unite, 
i that, when these parts are absent or defec- 
_ the commissures are deficient or wholly 
iting. Thus the corpus callosum and the 
0 *P s are developed together; the fornix 
i the hippocampi, the pons Varolii and the 
erebellar hemispheres. 
Tn Stilling’s experiments on the spinal cord 
was found that when division of that organ 
was made along the median plane, a stimulus 
applied to one leg caused only reflex actions of 
hat leg, and not at all of the other side of the 
;. The power of transmitting organic 
2 from one side of the cord to the other 
was destroyed by the section of the commissure. 
_ The anatomy of the corpus callosum is fa- 
\vourable to the hypothesis that it is the bond 
of union to the convoluted surface of the hemi- 
Spheres, and that it is in all probability the 
medium by which the double organic change 
is made to correspond with the working of a 
VOL, III. 
oe 
single mind.* There is nothing in the recorded 
observations of morbid change or congenital 
defect of this part to militate against this idea; 
but as all these cases are accompanied with 
lesion or defect of some other parts, and of the 
convolutions themselves, it is impossible to 
gather from them what is the precise conse- 
quence of the defect of the corpus callosum. 
This commissure is defective in the marsupiate 
class, as was shown by Professor Owen, and 
likewise in birds; but we have yet to learn 
whether there is any psychological character in 
either of these groups of animals, which would 
give us material assistance in our search into the 
nature of its function. \ 
Direct experiments upon the corpus callo- ~ 
sum yield only negative results. Longet and 
others found that mecnanical irritation of it did 
not cause convulsions ; and Longet states that 
injury to the corpus callosum in young rabbits 
and dogs did not appear to disturb voluntary 
movements; and that when he incised this 
body in its whole length in rabbits standing, 
they continued to maintain that position, or, 
when urged on, ran; and that no convulsive 
movement whatever, nor any sign of pain, 
was manifested. Such effects are not unfa- 
vourable to the view above taken, as the con- 
nection of the centres of intellectual action is 
probably in no degree necessary to locomotion, 
which function would no doubt be as well per- 
formed without a corpus callosum as with one. 
The fibres of the fornix manifest the same 
insensibility to mechanical irritants, and their 
obvious anatomical connection with particular 
convolutions warrants but one conclusion, that 
they associate the actions of those parts. The 
connection of this commissure with the optic 
thalami and the corpora mamillaria indicates 
that it also associates these gangliform bodies 
with :the convolutions at the posterior part of 
the brain, and with the hippocampi. A marked 
relation exists between these latter convolutions 
and the fornix; they bear, indeed, especially 
* Mr. Solly and Mr. Grainger think that they 
can trace the fibres of the corpus callosum distinct] 
to the convoluted surface of the hemispheres. Wit 
the greatest respect for these able anatomists, 
I must express my doubts that all the fibres which 
they have represented can be regarded as fibres of 
the corpus callosum. See fig. 99 in Mr. Solly’s 
work on the Brain, p. 251, ed. 1847. Although 
the anatomical views of these writers correspond 
with and confirm the physiology of the organ advo- 
cated in the text, I feel that great caution should 
be used in drawing conclusions from tracing the © 
fibres of brains hardened in alcohol. By these 
means any speculative.anatomist may make prepa- 
vations to illustrate his views, as is, indeed, abun- 
dantly shown by what I must call the fanciful ana- 
tomy of the brain put forward by Foville. 
+ An excellent account is given by Mr. Paget of © 
a case in which the corpus callosum and fornix 
were imperfect, in the xxixth vol. of the Med. 
Chir. Trans., accompanied by some very judicious 
remarks upon the office of those commissures, and an 
analysis of other similar cases. Mr. Paget refers 
to some oblique fibres as existing in the corpns cal- 
losum, and serving to connect the anterior convo- 
lutions of one hemisphere with the posterior ones 
of the other. 
Q z eRERERE 
