482 



PHYSIOLOGY 



III. THE COMMISSURAL FIBRES 



These are arranged in three groups : 



(a) The corpus callosum forms a great mass of white fibres passing 

 transversely in both directions between the two hemispheres. Its 

 fibres are derived from every part of the cerebral cortex with the 

 exception of the olfactory bulb and the hind and fore parts of the 

 temporal lobe. As the fibres cross the middle line they become 

 gradually scattered, so that they tend to connect wholly dissimilar 

 parts of the cortex of opposite hemispheres. Each callosal fibre 

 arises in one hemisphere and ends by fine arborisations in the opposite 

 hemisphere. It may represent either the axon of one of the cortical 



Fia. 217. Schematic section through cerebral hemispheres, to show chief 



classes of nerve tracts. (After RAMON Y CAJAL.) 



A, corpus callosum ; B, anterior commissure ; c, pyramidal tract ; a, cell 

 giving off projection fibre ; &, cell giving off commissural fibre ; c, cell with 

 axon forming association fibres. 



cells or a collateral from a fibre of association or a collateral from a 

 projection fibre (Fig. 217). 



(b) The anterior commissure is situated in the anterior wall of the 

 third ventricle between the two pillars of the fornix. It connects 

 together the two olfactory lobes and portions of the opposite temporal 

 lobes. In lower vertebrates it is almost entirely olfactory in function, 

 but in man the olfactory fibres only form a small proportion of the 

 total number making up the bundle. 



(c) The psalterium or hippocampal commissure is a thin lamina 

 formed of transverse fibres filling up the small triangular space on the 

 under surface of the hinder part of the corpus callosum formed by 

 the divergence of the posterior pillars of the fornix. Like the anterior 

 commissure, the hippocampal commissure is closely associated with 

 the sense of smell. Its fibres arise from the pyramidal cells in the 



