350 



On the Human Corpus Callosum. 



[Feb. 28, 



from this source is the anterior peduncle of the optic thalamus. 

 (Vorderer Stiel, Meynert.) 



5. The callosal fibres derived from other regions of the cortex 

 pursue a similar course : — They sweep downwards into the corpus 

 callosum, slant somewhat backwards or forwards when they enter 

 it, pass over to the opposite side, and then instead of becoming 

 attached to a corresponding part of the cortex in the opposite hemi- 

 sphere, turn upwards, outwards, and downwards, in a dense well- 

 marked arch, and enter the inner and outer capsules. This mass 

 of arched fibres I propose to name the crossed callosal tract. 



6. The fibres of the opposite hemisphere, having thus become united 

 to the inner and outer capsules, terminate in the following manner : — 

 A few of them enter the head of the caudate nucleus, but by far the 

 greater number become attached to the thalamus opticus opposite 

 the bend or knee formed by the junction of the anterior and posterior 

 divisions of the inner capsule. Those entering the outer capsule become 

 attached respectively, from before backwards, with the following 

 parts : the inner capsule, the olfactory tract, the temporo-sphenoidal 

 lobe, and the optic tract. The olfactory and optic nerves are thus, 

 I believe, connected directly with the cerebral cortex. 



7. The corpus callosum, therefore, is not an interhemispheric com- 

 missure at all, but is the decussation of the cortical fibres in their 

 progress downwards to become connected with the basal ganglia. 



In the human brain I have never seen any fibres which pass 

 directly from one hemisphere to another. 



8. The "corona radiata," as usually described, does not exist. The 

 only fibres which pass from the cerebral peduncle directly up to the 

 cortex of the same side are contained in the posterior division of 

 the inner capsule. They correspond to those fibres which have 

 decussated already in the anterior pyramids, and are a mere fraction of 

 the whole " inner capsule " fibres. All the other fibres of the " inner 

 capsule " pass into the crossed callosal tract, and, instead of running 

 directly up to the cortex of the same side, cross in the corpus callosum 

 to the cortex of the opposite side. 



9. The same arrangement prevails in the callosal fibres from the 

 tip of the frontal to the tip of the occipital lobes. 



10. The crossed callosal tract comes so near to the cortex of the 

 opposite side in arching downwards that injuries or stimulations of 

 one side of the brain would be liable to affect the fibres coming from 

 the opposite, and hence to produce a misleading result. 



