^82 PROF. (I. ELLIOT SMITH OX THE MORPHOLOGY OF THE 



cortical area in question was hippocampus only, and concluded by regarding it as also representing the 

 gyrus t'ornicatus. 



His lack of consistency is almost as pronounced as it was eight years earlier, when he called the 

 medio-dorsal cortex " hijipocampus " and its commissure " coi'pus callosum." In 1896 he almost 

 reversed tlie process by calling the commissure " psalterium," but declining to admit that its area of 

 origin is simply "hippocampus." 



I have dwelt at great length and with, 1 tear, much repetition upon the views of Edinger, because his 

 opinion has been widely accepted from time to time by writers of all nationalities as authoritative, and 

 hence, in all recent memoirs and text-books which treat of the comparative anatomy of the brain, the 

 doi'sal commissure in Reptiles and Amphibians is called " psalterium." It only needs a writer to come 

 forward (as Flower did in 1865, in the controversy regarding the commissures in the Monotremes and 

 Marsupials) and indicate the weakness of the basis for the general belief, to cause a general revulsion to 

 the conviction of the presence of a corpus callosum in all Vertebrates. For so history might repeat 

 itself. But although Edinger's argument laclvs cogency and consistency, the co)iclusion at which he 

 arrived regarding the dorsal commissure is undoubtedly correct. For the fibres of the dorsal commissure 

 must be hippoeampal, i. e. psalterium, because they spring from the hippocampus. 



In this discussion I have not yet referred to the possibility of the fibres of the dorsal commissure 

 arising h'oni a wider cortical area than the hippocampus. The only author who, so far as I am aware, 

 directly admitted this possibility is Pedro Ramon y Cajal (' Investigacioncs niicrograficas en el Encephalo 

 de los Batraceos y Reptiles,' Zaragoza, 1894). I am only acquainted with this work at second hand 

 from the excellent figure which Edinger reproduces [op. cit. 1896, p. 319). In this figure a group of 

 fibres is represented passing upward from the dorsal commissure (which he calls " corpus callosum ") 

 through the paraterminal body and hippocampus to end in the pallia! area beyond the latter. These 

 fibres are labelled " Mehr lateral aufgezweigte Balkeufasern fiir die laterale Rindenplatte.'' In no 

 specimens which I have examined have I been able to satisfy myself that any such fibres came from the 

 dorsal commissure and extend beyond the hippocampus. It may be that they merely consist of fibres 

 which associate the hippocampus and the lateral pallial area, and have no connection with the com- 

 missural fibres. Sucii fibres are found in the Mammalian brain. But Ramon y Cajal distinctly 

 represented the fibres in question as coming from the dorsal commissure. 



If these fibres really exist as he represents them, and if the suggestion which I have already made to 

 the effect that the area beyond the hippocampus represents the neopallium of the mammalian brain is 

 correct, it must of necessity follow that the fibres in question are strictly homologous with the mammalian 

 corpus callosum. That such is the case, however, I am very loth to admit, not merely for the reason 

 that I have never been able to detect such commissural fibres passing to the lateral pallial ai-ea, but for 

 wider morphological reasons. There can be no doubt that no such corpus callosum exists in eitlur the 

 Monotremata or Marsupialia, and that therefore, presumably, the ancestors of the Mammalia had no 

 true corpus callosum, /. e. they possessed a dorsal commissure which was pttrehj luppocampul. ^Nlore- 

 over, a study of the conditions prevailing in the Mammalian lirain when the corpus callosum makes its 

 first appearance, seems to point clearly to the conclusion that the true corpus callosum develops in 

 response to the demand of the rapidly growing neopallium for a shorter route for its commissural fibres. 

 Such a demand can never exist in the Reptilia, in which the neopallium is of such insignificant area ; and, 

 « priori, one woiild be extremely surprised to find a representative of the true corpus callosum in the 

 Reptilia. Yet all these strung a priori objections would not be of sufficient iceight to counterbalance the 

 positive evidence for a true corpus callosum which the substantiation of Ramon y Cajal's evidence would 

 afford. The arguments of Meyer concerning its relation to the lamina terminalis, or of other writers 

 concerning its relation to the recessus superior and roof of the third ventricle, would be of no avail if this 

 crucial fact were established. In the meantime the question must remain in abeyance, for Ramon v 

 Cajal's conclusions need confirmation. His observations were made and recorded without anv due 



