OEIGIN OF THE CORPUS CALLOSUM. 59 



the hippocampal fissure [f.h.), a less prominent hippocampal eminence in the lateral 

 ventricle, and a less broad and slightly less convoluted fascia dentata {f.d.). These 

 variations in the form of the hippocampus are of little moment, for, as I have before 

 remarked, great variations are found in any order. Moreover, it is noteworthy that in 

 very small Marsupials like Notoryctes one finds a simplicity of hippocampal formation 

 similar to that which the miniite Miniopterus presents (compare fig. 6 of my paper 

 on Notoryctes*). 



In the next section, whicli passes through the posterior extremity or splenimn of the 

 dorsal commissure (PL 16. fig. 13), we have a condition which recalls that presented 

 by the corresponding section through the brain of Perameles (Pi. 15. fig. 2). 



But if we compare the hippocampus in figs. 12 and 13, a distinct simplification will be 

 noticed in the foremost section (fig. 13). The fascia dentata (f.d.) is narrower, the 

 layer of pyramidal cells {pyr.) is shorter, and the hippocampus as a whole is distinctly 

 flatter. In fig. 12 there is a very sharp angle between the alveus and tlie corona 

 i-adiata, while in fig. 13 the angle has become rounded otF. It is possible that fibres 

 coming from a wider cortical field may round off the angle, but the carmine stain does 

 not permit us to definitely decide this. But in the next section (fig. 11) there is 

 no question, even with the carmine stain, of the much wider field of origin of the dorsal 

 commissiu'e. 



If we are undecided whether any fibres other than hippocampal are passing to the 

 dorsal commissure in fig. 13, we have in fig. 11 a very distinct demonstration of the 

 existence of a verv considerable bundle of fibres arisina: from the sjeneral cortex and 

 forming part of the superior commissure. In fiict, the insignificant flattened band of 

 grey matter which now represents the hippocampus can contribute but a very small 

 share towards the formation of the thick commissural tract (c.D.) upon which it lies. 

 The hippocampus is now much more insignificant and more flattened than it was in the 

 last figure. The regular row of nuclei immediately above the dorsal commissure belongs 

 to the layer of pyramidal cells of the hippocampus {pyr.), and the group of closely- 

 aggregated smaller nuclei which cap the mesial end of the layer of pyramids represents 

 the stratum granulosum of the fascia dentata. In fig. 15 the region of the fascia 

 dentata from fig. 11 has been rei)resented upon a larger scale, and in figs. 12, 13, 11, 

 and 15 the stratum moleculare of the fascia dentata has been shaded in order to render 

 its recognition more easy. 



It will be noticed, if we comj^ai-e figs. 13 and 11, that the commissural fibres of the 

 general cortex pursue their new course through the alveus of the hippocampus. In 

 other words, the alveus of the " dorsal hippocam^nis " of the bat consists of a mixture 

 of fibres derived from the hijijiocampus, which we might call " true alveus," and 

 commissural fibres derived from the general cortex — "false alveus." Our "false 

 alveus " is the corpus callosum proper. Prom the examination of this series, therefore, 

 we may infer that the fibres of the corpus callosum proper reach the mesial platie by 

 passing through the alveus of the dorsal part of the hippocampus. 



* " The Cerebrum of Notoryctes tupldops,"' Transactions of the Eoyal Society of South Australia, vol. xix. pi. viii. 

 (1895). 



