GENERAL SURVEY OF THE STRUCTURE OF THE BRAIN. 373 



link of the projection system, the system of the crus cerebri (Pa), 

 the peripheric extremity of which is found in the grey substance 

 of the third category, i.e. the grey substance around the central 

 cavities. The third link of the projection system is formed by 

 the nerves which arise from the above-named grey substance of 

 the central cavities, extending from the origin of the third pair 

 of cerebral nerves in the grey matter around the aquseductus 

 Sylvii to the nucleus of origin of the lowermost coccygeal 

 nerves of the spinal cord. These, perhaps without exception, 

 end peripherically in definite microscopic terminal organs that 

 have already been described in several chapters of this work. 



The region through which the first link of the projection 

 system pursues its course is that formed by the cerebral hemi- 

 spheres, within which it is apparently accompanied by two 

 medullary formations, the trabecular fibres (fibres of the corpus 

 callosum) and the arcuate system. While the projection system 

 forms the medium of communication for the cells of the cortex 

 with the outer world, the communication of the cortical cells of 

 the cerebral hemispheres amongst themselves is accomplished 

 in a threefold manner, so that that portion of the protoplasm of 

 the primordial cells from which the innumerable cells of the 

 cortex are developed, again unites to form a morphological 

 unity ; a process that can only be regarded as an instance of 

 reunion. This view is most obviously based on the union of the 

 transverse commissural fibres in the middle line during foetal 

 life, after they have perforated the median wall of the vesicles 

 of the hemispheres. The commissural trabeculse (callosal fibres, 

 figs. 230 and 231, T) unite the corresponding and identical 

 regions of the cortex of the two opposite halves of the cerebral 

 hemispheres. 



The different regions of the cortex of the same hemisphere 

 are, on the other hand, continuously connected together by the 

 grey fibre plexus formed by the anastomoses of the cell pro- 

 cesses. It forms, however, the most satisfactory proof of the 

 law of isolated conduction, which is even here in operation, that 

 in addition the regions of the cortex are also connected together 

 by medullated fibres (figs. 230 and 232, aa), the fasciculi of the 

 fibrse proprise, fibrse arcuatse, which form a continuous layer, 

 lining the inner surface of the cortex, composed of fasciculi of 



