CHAP. XI.] FUNCTIONS OF THE COMMISSURES. 373 



and the hippocampi, the pons Varolii and the cerebellar hemi- 

 spheres. 



The anatomy of the corpus callosum favours the hypothesis that 

 it is the bond of union to the convoluted surface of the hemispheres, 

 and that it is the medium by which the double organic change is 

 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 it must be re- 

 marked that all these cases are accompanied with lesion or defect 

 of other parts, which weaken the inferences to be drawn respecting 

 the corpus callosum. Direct experiments upon this commissure yield 

 only negative results. Longet and others found that 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 dis- 

 turb voluntary movements ; and that, when he incised this body in 

 its whole length in rabbits standing, they have continued to main- 

 tain that position; or, when urged on, ran; and that no convulsive 

 movement whatever, nor any sign of pain was manifested. Such 

 statements are certainly favourable to the supposition that these 

 fibres are destined to connect centres whose appropriate stimulus is 

 mental. 



The fibres of the fornix manifest the same insensibility to me- 

 chanical irritants ; and their obvious anatomical connexion with 

 particular convolutions warrants but one conclusion, that they 

 associate the actions of those parts. Lallemand relates a case in 

 which the symptoms were altogether limited to mental disturbance, 

 without any affection of the sensitive or motor powers, and the 

 fornix and corpus callosum were found in a state of complete soft- 

 ening without discoloration. 



The fibres of the pons Varolii bring the cerebellar hemispheres 

 into connexion with each other, and with the vesicular matter of the 

 mesocephale. Direct experiments on these fibres can yield no satis- 

 factory result, because they are so intimately associated with the 

 deeper 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 likewise irritating these other parts. And it 

 is sufficiently evident, that these fibres have no necessary connexion 

 with sensation and volition, from their non-existence in birds; nor 

 rvru with the cerebellum when that organ is single. It will be borne 

 in mind, that at a previous page we have referred to the connexion 

 of these fibres with the mesocephale as explaining the crossed in- 

 fluence of lesion of one hemisphere of the cerebellum. 



