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 ed, or which the mind can develope, have no nerves implanted in them. Such are the con volutions, 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 implanted in the olfactory lobe ; the latter in the chiasma, which is formed by the junction of the optic tracts, and these ought no more to be regarded as portions of the optic nerves, than the olfactory lobes should be considered as nerves.
Functions of the commissures.—The anatomy of the parts which we call commissures indi cates that the name by which they have long been known is not misapplied, inasmuch as they seem to unite particular portions of the nervous centres with each other. The most obvious object of such an union would be to ensure 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 developement to that of the parts they unite, and that, when these parts are absent or defec tive, the commissures are deficient or wholly wanting. Thus the corpus callosum and the hemispheres are developed together; the fornix and the hippocampi, the pons Varolii and the cerebellar hemispheres.
In Stilling's experiments on the spinal cord it was fotmd that when division of that organ vvas made along the median plane, a stimulus applied to one leg caused only reflex actions of that leg, and not at all of the other side of the body. The power of transmitting org-,anic change from one side of the cord to the other was destroyed by the section of the cominissure.
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 OS made to correspond with the working of a 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 likevvise 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.f Direct experiments upon the corpus callo sum yield only negative results. Longet and
others found that mechanical 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 as regards the posterior pillars of the fornix, a direct ratio to each other.
Lallemand relates a case in which the symp toms were altogether limited to mental distur bance, without any affection of the sensitive or motor powers, and the fornix and corpus callo sum were found in a state of complete softening without discolouration.
The fibres of the pons Varolii bring the cerebellar hemispheres into connection with each other, and with the vesicular matter of the mesocephale. Direct experiments on these fibres can yield no satisfactory result, because they are so intimately associated with the deep er seated parts of the mesocephale, and with the nerves of the fifth pair and others, that it is impossible to irritate them in the living animal without affectin,,c, these parts likewise. The anatomy of the fibres, however, sufficiently in dicates that they belong properly to a double cerebellum : for when the cerebellum becomes single, as in birds, reptiles, and fishes, no such fibres are found in the encephalon. Morbid lesion of the pons is productive of very serious results from the number and importance of the parts in its neighbourhood, the pyramids, the medulla obloncrata, the quadrigeminal tuber cles; so that the symptoms it produces cannot be referred solely to the injury to the commis sural fibres. It is very probable, however, that the crossed effect of deep-seated disease of either hemisphere of the cerebellum may be accounted for by the influence of these com missural fibres upon the adjacent anterior py ramids, which again would influence the oppo site side of the spinal cord.