OKIGIN OF THE CORPUS CALLO.SUM, 65 



hippocampus as I have represented in fig. 2. This anterior portion of indented hippo- 

 campus is the "gyms grnicuW' of Zuckerkandl. 



It is ai:)parent from this that the dorsal comiiiissure, which in. the Ilarsupial liesroithin 

 a well-defined arc formed by the hippocampus (fig. 1), never breaks through this hippo- 

 campal arc even in its much more extended and reconstructed form as a corpus callosum, 

 but pushes the dorsal part of the arc before it as it elongates in. both directions (fig. 2). 



We have seen that in tlae Marsupial a large mass of grey substance, containing both 

 commissures and formed by the "corpus prfecommissurale " and "commissure-bed," is 

 situated i;pon the ventral aspect of this hippocampal arc. Since the hijipocampal 

 formation may be recognized surrounding the corpus callosum even in the brains of the 

 ox, the monkey, and man, it is clear tliat the matrix of this commissure can be formed 

 of no other structure than the " precommissural body " and the " commissure-bed." 

 This introduces us to the problem of the nature of the " septum pellucidum." 



We have already seen that the first " callosal " fibres rej)lace the hippocampal fibres of 

 the dorsal limb of the " commissura dorsalis," and in every respect joresent the same 

 relations as the fibres, the places of which they usurp. The first " callosal " fibres, 

 therefore, must lie in the structure which is exactly homologous to what we have 

 called, in the Marsupial, the " commissure-bed." 



In discussing the natiu"e of the " commissure-bed " we came to the conclusion that 

 with the increasing dimensions of the commissures the dorsal part of the lamina 

 terminalis became thickened or invaded by the posterior parts of the " corjjora prte- 

 commissuralia," so that a bridge of grey matter — the " commissure-bed " — was formed, 

 connecting the two hemispheres, and forming a matrix for the two cerebral 

 commissures. This secondaiy thickening appears to take place gi-adually in the 

 ontogeny of each individual, and cannot be regai'ded as a mere mechanical fusion of the 

 precommissural areoe of the two hemispheres. We may therefore safely say that the 

 dorsal commissure of the Metatherian and (for the reasons stated aljove) of the early 

 Eiitherian cerebrum is contained in a matrix which is formed by a thickening of the 

 dorsal part of the lamina terminalis at the expense of the " corpora praecommissuralia." 

 As new commissural fibres crowd in aud the commissure increases ia length and depth, 

 this matrix becomes extended, but I believe it always continues to enclose the dorsal 

 commissure, even in its most extended form. 



Paul Martin says tha in the cat the cephalic extremity of the corpus callosum or 

 genu is formed by the apposition of fibres in front of those which are situated in the 

 thickened lamina terminalis. It must be evident (if we glance at fig. 1) that if these 

 apposed fibres grow in a " secondary fusion of the walls of the hemisphere," such a 

 fusion must consist of a " soldering " of the two "precommissural hippocampi" {hip"). 

 But such a " soldering " does not take place, because in the gyrus genicuU we have seen 

 the representative of the " precommissural " hippocampus pushed before the advancing 

 corpus callosirm. There seems to be no other .alternative which could be for a moment 

 maintained but that the corpus callosum is contained ui its original matrix or " com- 

 missure-bed," which has become enormously but gradually elongated. This conclusion 

 is strongly forced upon us by comparison, in the order mentioned, of such a series of 



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