1865.] and Homologicul Interpretations. 131 



Part No. 72 of the Proceedings of the Royal Society. In this ' Abstract ' 

 I find that, to previous knowledge, and especially my own as represented 

 by the citation of a cerebral character from my Zoological Essay, given at 

 p. 71, Mr. Flower proposes to add, as discoveries of his own, and by way 

 of correction of alleged errors of mine, that the corpus callosum does 

 exist in the Marsupialia and Monotremata ; that the transverse fibres con- 

 necting the hippocampi are not parts of the fornix, " which is essentially a 

 longitudinal commissure " ; that the " two halves of the cerebrum are not 

 by any means disconnected, as the term ' Lyencephala ' would imply, but 

 that they are united in a remarkable manner by the immense size of the 

 anterior commissure " (p. 73). 



After these pretensions, put forward by a Fellow of the Royal Society 

 in opposition and assumed superiority to a previous labourer in the field, it 

 would naturally be taken for granted that the statements and opinions of 

 the author assailed had been fairly and fully quoted. It might be long be- 

 fore any one would deem it necessary to test the grounds of dispute by re- 

 ference to Prof. Owen's anatomical writings on the subject. I therefore 

 beg to leave to quote from those writings the following passages. 



With regard to the Marsupialia, I state, " This commissure [viz. the 

 commissure of the hippocampi] may nevertheless be regarded as repre- 

 senting, besides the fornix, the rudimental commencement of the corpus 

 callosum." (Phil. Trans. 1837, p. 91.) 



In a subsequent anatomical Monograph I wrote, in 1840, "The es- 

 sential function of the fornix, as a longitudinal commissure, uniting the 

 hippocampus major with the olfactory lobe of the same hemisphere, is more 

 exclusively maintained in the Ornithorhynchus, in consequence of the 

 smaller size of the transverse band of fibres uniting the opposite hippo- 

 campi, and representing the first rudiment of the corpus callosum, as it ap- 

 pears in the development of the placental embryo." Art. Monotremata, 

 Cycl. of Anat. and Physiol., vol. iii. p. 383, 1841.) 



In reference to other connexions of the opposite hemispheres I state, 

 " The anterior commissure is very large in the Monotremes, as in the Mar- 

 supials" (i6. p. 383) ; and that "it is the principal commissure of the 

 hemispheres, is subcylindrical, and measured, in the brain of a Platypus, 

 two lines thick vertically, and one and a half lines horizontally" (z'i.p. 385). 



With regard to the inner wall of the cerebral hemisphere, I describe 

 " the fissure upon which the hippocampus is folded " in the Marsupial 

 brain. (Phil. Trans. 1837, p. 90.) 



I regret to be compelled to show by the foregoing quotations the sources 

 whence Mr. Flower has derived, or might have derived, the ideas that 

 the essential nature of the fornix, as contradistinguished from its anthro- 

 potomical definition, is that " of a longitudinal commissure," that the 

 transverse fibres which connect together the two hippocampi, and form 

 part of the wall of the lateral ventricle, may be regarded as the homologue 

 of part, at least, of the corpus callosum, and that the absence of the main 



