CEEEBEAL COMMISSUEES TX THE YEETEBRATA. 481 



obvious to ucecl formal presentation ; but a glaucc at tlic past history of tliis discussion woukl appear to 

 show that the inference does not naturally suggest itself to most writers ; and that in deciding the 

 question of homology, the great majority of investigators have made use of criteria which may readily be 

 shown to be either utterly false or of limited applicability. 



In the foregoing descriptions it has been demonstrated that a large strand of fibres coming from the 

 hippocampus crosses to the corresponding region of the other hemisphere in the lamina terminalis and 

 forms the dorsal commissure. The fact that the morphological relations of this commissure are identical 

 with those of the dorsal commissure in the INIonotreme, which is known to be of purely hippocampal 

 origin, undoubtedly lends support to the contention that the dorsal commissure in the Reptilian 

 brain is also a commissure of the hippocampi, i.e. t\\c psa/terium. But such an argument can only be of 

 subsidiary value. The only essential reason for regarding the dorsal commissure as being hippocampal, 

 and not a true " corpus callosum," must be the demonstration of its origin from the hippocampus. So 

 far as 1 am awaie, no investigator, with the solitary exception of Ramon y Cajal, has ever seriously 

 attempted to claim an origin of any fibres of the dorsal commissure from the ])allial area beyond the 

 hippocampus. Edinger, it is true, appeared to do so in his first contribution to this discussion (op. cit. 

 1888), but, as I have already pointed out, his statements are not supported by his figures. And the 

 internal evidence of his memoir seems to point to the statement merely as an attempt to harmonize his 

 conclusions with the plausible and sweeping generalization of Osborn. 



When Osborn himself argued in favour of the existence of a " corpus callosum " in the Reptilia, he did 

 not trace any fibres of the dorsal commissure beyond the medio-dorsal cortical jjlate. He was led to the 

 erroneous belief in the presence of the corpus callosum, because he did not recognize in the medio- 

 dorsal plate the true hippocampus (H. F. Osborn, "The Origin of the Corpus Callosum," Morpholog. 

 Jahrblich, Bd. xii.). The same remarks apply to the earlier observations of Stieda (" Studien iiber das 

 Centralnerveusystem der Reptilieu," Zcitschr. f. wiss. Zoologie, Bd. xxv. 1875). 



Rabl-Riickhard, however, would not homologize the dorsal commissure in the Alligator with the 

 corpus callosum, and gave it the neutral name " coiumissui-a pal/ii anterior"*. His reason for this 

 hesitancy was that he regarded it as corresponding neither in position with the corjjus callosum, nor 

 with what one should regard as the primitive stage of the great Mammalian commissure. He somewhat 

 doubtfully suggested its homology with the commissure of the fornix, but gave no valid reasons for his 

 belief. 



In his numerous contributions, to which reference has already been made, Herrick regai'ded the dorsal 

 commissiu'c as a true " corpus callosum," for the reason that he refused to admit the homology of the 

 medio-dorsal plate of cortex with the hippocampus. 



Then, again, it was the relation of the dorsal commissure to the lamina terminalis, rather than its 

 origin from the region which he compared to the " outer Randbogen," which led Adolf Meyer to refuse 

 the title "corpus callosum" to the dorsal commissure in 1893 {(Jp. cit.). In order to justify his 

 ])osition ^leyer, in 1895, ])rought forward some rather arbiti'ary criteria of a " corpus callosum " (which 

 1 shall discuss later), but they added little to the force of his original argument. 



It is somewhat surprising to find that in 1896 Edinger {loc. cit.) regarded the dorsal commissure 

 (which he now calls the " commissura pallii anterior" after Rabl-RUckhard) as a, psalter ium, although 

 he regarded the origin of this commissure as being homologous not ouly with the hippocampus but also 

 with the gyrus fornicatns of Mammals. He attributed his change of belief to the work of Adolf Meyer 

 {op. cit.) and the writer (" The Cerebral Commissures in the Mammalia/' Proceedings of the Liunean 

 Soe. of N.S.W. 2nd series, vol. ix.), and very properly argued that, as the commissural link between the 

 hippocampi, it must represent the psalterium (Edinger, op. cit. 1896, p, 371). But in this ])assage he 

 seemed to forget that elsewhere iu the same memoir (p. 38 1 inter alia) he refused to admit that the 



* " Ueber d;is Centralnervcmsystem der Alligators," Zeitschrift fiir wiss. Zoologie, BJ. xxs. 1S7S. 



70* 



