CONNECTIONS OF CORTEX WITH LOWER CENTRES. 779 



fillet, and some perhaps also of the superior cerebellar peduncle, pass directly 

 to the same area, chiefly to the ascending parietal gyrus, but this direct 

 connection of fibres of the fillet with the cortex could not be found by Mott, 

 and is also denied by Dejerine. 1 The Rolandic areas are further commissurally 

 connected with one another through the middle part of the corpus callosum, 

 and by stimulation of these commissural fibres the grey matter of both sides 

 can be simultaneously set in activity. 2 If one hemisphere be cut away and 

 electrodes applied to the cut surface of the corpus callosum, the motor centres 

 in the remaining hemisphere may be excited through the fibres of this commis- 

 sure as readily as if the electrodes were applied directly to the grey matter, and 

 the effects produced are similar, but not so easily isolated. That the move- 

 ments which are produced by excitation of the callosal fibres are brought 

 about through the cortex, and not by fibres which have been described as 

 passing from the cortex of the one side across the corpus callosum directly to 

 the internal capsule of the opposite side, 3 is shown by the fact that they are 

 not obtained if the Rolandic cortex is destroyed, although the connection 

 between corpus callosum and internal capsule may not thereby be interfered 

 with. Nor is degeneration set up in the opposite internal capsule by removal 

 of portions of the cortex of one hemisphere, although localised degenerations 

 are thereby produced in the corpus callosum, and can be traced to the 

 corresponding part of the Rolandic area of the opposite cortex. 4 The fibres 

 connecting the different motor centres of the two hemispheres through 

 the corpus callosum, although massed together at certain places so as to give 

 specific results on exciting these places, nevertheless show a certain amount of 

 scattering, as can be ascertained by following the degenerations which result 

 from localised cortical lesions (Sherrington). The corpus callosum also 

 contains other fibres serving to connect the parts of the brain in front of and 

 behind the Rolandic region. It is constituted, in fact, by a great mass of fibres, 

 which must serve to effect an association of action between the two hemi- 

 spheres. But the exact purpose of such association is by no means clear. As- 

 sociation of movement between the two sides can, and does, occur in the lower 

 level centres, and although it is very probable that there is also association 

 through the corpus callosum, it is nevertheless the case that this commissure 

 can be completely divided in animals without any distinct symptoms being 

 manifested. Many cases have been recorded of congenital absence of the 

 corpus callosum in man. These are generally associated with a condition of 

 greater or less idiocy, but this may be the result of a general lack of cerebral 

 development, which usually accompanies absence of the commissure. In one 

 or two cases of its complete absence, which have been published, no symptoms 

 were noticed during life. 5 The Rolandic area further receives numerous 

 association fibres from other regions of the cortex. 



The prefrontal (non-excitable) region of the cortex sends corticifugal 

 fibres down in the anterior part of the internal capsule and in the mesial 

 bundles of the crusta, to end in the grey matter of the nuclei pontis. 



1 Compt. rend. Soc. de biol., Paris, 1895, p. 285. 



2 Brown-Sequard, Compt. rend. Soc. de biol., Paris, 1879 ; Mott and Schafer, Brain, 

 London, 1890, vol. xiii. p. 174. 



3 Hamilton, Proc. Hoy. Soc. Loudon, 1884, p. 349. The fibres described by Hamilton 

 appear to be really cortieipetal fibres passing from the optic thalamus of one side across the 

 corpus callosum, to the cortex of the opposite side (Ferrier and Turner, Phil. Trans., 

 London, 1898, B, p. 31). 



4 Mnratoff, Arch. f. Anat. u. Entwcklngsgesch., Leipzig, 1893, S. 97. Tschermak, 

 however, found it to obtain in the cat, ibid., 1898, S. 291. But Melius {Proc. Roy. Soc. 

 London, 1895, vol. lviii. p. 206) found in some cases, in the monkey, after lesions of the 

 cortex of the one side, degenerated fibres in considerable numbers, running (presumably 

 after having crossed by the corpus callosum) to the opposite internal capsule. 



5 For abstracts of a large number of cases recorded by others, and an account of a case 

 observed by himself, see a paper by Alexander Bruce in Brain, London, 1889, vol. xii. p. 

 171 ; see also Dunn, Gity's Hasp. Pep., London, 1889, vol. xlvi. p. 117. 



