62 DE. G. ELLIOT SMITH ON THE 



derived from the precommissural and supracommissural segments form the dorsal limb, 

 and those derived from the postcommissural segment form the splenium and ventral 

 limb of the " commissura dorsalis." 



Now, we have seen that in the brain of the bat commissural fibres arise from a small 

 area of the mesial cortex, which is distinctly not hippocampal, but which is placed above 

 the hippocampus, and that, after passing through the alveus of the svipracommissural 

 hippocampus, they cross the mesial plane intimately intermingled witli the hippocampal 

 fibres of the dorsal limb of the " commissura dorsalis." Coincidently with this invasion of 

 the alveiTS, and possibly as a direct result of it, retrogressive changes occur from before 

 backward in the " precommissural " and " supracommissural " segments of the hippo- 

 campus. So that soon these parts of the hipiiocampus {hip' and h'qf) are reduced to 

 mere vestiges, which, however, may still retain their characteristic histological elements. 

 These vestiges are the so-called strise Lancisii (mesial and lateral) and the associated 

 film of grey substance. It is natural to suppose that with the retrogressive changes 

 in the supracommissural hippocampus the liippocampal factor in the dorsal limb of the 

 dorsal commissure wanes, so that the usurping pallial fibres remain practically 

 if not actually unmixed with hippocampal fibres, to constitute the dorsal limb of the 

 dorsal commissure. 



This change is so gradual, and the pallial invasion so insidious, that at first the 

 appearance of the commissures is unchanged, as in Nyctophilus, from our Marsupial type, 

 and we are apt to overlook the fact that there has been thus introduced " the greatest 

 modification exhibited by the brain in the whole series of vertel)rated animals," as 

 Huxley has said. It is unnecessary to discuss the question of the homology of these 

 pallial fibres in the dorsal commissure ; for, in the ' Transactions of the Linncan Society 

 of New South Wales,' I have already (October 1894) called attention to the fact that 

 while in the Marsupial the whole pallium is bound to its fellow of the opposite hemi- 

 sphere by means of fibres of the ventral commissure, in the Eutherian hemisphere part 

 of the homologous pallium sends its commissural fibres by the much shorter route which 

 the commissura dorsalis provides. Therefore we cannot regard the corpus callosum as 

 an entirely new group of commissural fibres, but rather as fibres the undoubted homo- 

 logues of which arc found in the ventral commissure of the Metatherian cerebrum. The 

 passage of these pallial fibres through the dorsal commissure is an adaptive feature 

 which is obviously advantageous to the organism, since the new route provides a very 

 considerably shorter and more direct path for the commissural fibres of the rapidly- 

 increasing dorsal part of the palliiun. It is not at all sur2)rising, therefore, to find a great 

 and rapidly-growing demand upon this advantageous pathway oid the dorsal limb of the 

 dorsal commissure. Thus quite early this dorsal limb becomes completely transformed 

 from a practically purely "hippocampal" to a practically purely "paUial" structure. 

 (In fig. 1, p. 63, this reconstructed limb of the commissure has been darkly shaded.) But 

 the continued crowding in or intussusception of new commissural fibres soon expands the 

 bulk of the dorsal limb not only in thickness, but also in length. At first the increase in 

 length is most noticeable in the caudal direction, in correspondence with the rapid backward 

 growth of the hemisphere. This backward extension of the commissru^e takes place in the 



