PHYSIOLOGY OF THE NERVOUS SYSTEM. 



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 

 a certain 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- 

 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 found that when division of that organ 

 was 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 organic 

 change 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. 



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 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 



* Mr. Solly and Mr. Grainger think that they 

 can trace the fibres of the corpus callosum distinctly 

 to the convoluted surface of the hemispheres. With 

 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- 

 rations 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. 



t 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 corpus cal- 

 losum, and serving to connect the anterior convo- 

 lutions of one hemisphere with the posterior ones 



of the other. 



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