STRUCTURAL ARRANGEMENTS OF CEREBRUM 



425 



(6) The cingulum is closely associated with those parts of the cerebral cortex known 

 together as the limbic lobe. In front it originates in the neighbourhood of the anterior 

 perforated space, passes round the genu of the corpus callosum, and then is carried 

 backwards over the upper surface of this body to its hinder end, where it turns round 

 and is distributed to the hippocampal gyrus and to the temporal lobe. 



(c) The longitudinal superior fasciculus lies in the base of the frontal and parietal 

 lobes, and passing from before backwards connects the frontal occipital, and temporal 

 parts of the cerebral cortex. 



(d) The longitudinal inferior fasciculus runs along the whole length of the occipital 

 and temporal lobes, being situated behind on the outer aspect of the optic radiation. 



(e) The occipito-frontal fasciculus lies on the inner aspect of the corona radiata in 

 intimate relation to the caudate nucleus, and projects out over the upper and outer 

 aspect of the lateral ventricle immediately outside the ependyma. 



III. THE COMMISSURAL FIBRES 



These are arranged in three groups : 



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

 versely in both directions between the two hemispheres. Its fibres are 



FIG. 218. Schematic section through cerebral hemispheres, to show chief classes 



of nerve tracts. (After RAM6N Y CAJAL.) 



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

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

 forming association fibres. 



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

 projection fibre (Fig. 218). 



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

 ventricle in front of 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 



