132 Prof. Owen on Zoological Names [Mar. 23, 



part of the great commissure of Placentalia is compensated in the Impla- 

 centalia by the presence of a large "anterior commissure." 



Having no one's shortcomings to exaggerate, I did not, indeed, in the 

 above cited works attribute to this commissure an " immense " size *, but 

 preferred, finding it measurable, to give its dimensions, in the Ornitho- 

 rhynchus, e.g., and to show, as in fig. 1, g, Plate VII. Phil. Trans. 1857, 

 its large proportional size in the Opossum's brain. 



I nowhere assert that the mesial wall of the lateral ventricle (' septum 

 ventriculorum ' of Mr. Flower) is disconnected with what I affirm to be the 

 beginning of the corpus callosum ; on the contrary, both ia my original 

 paper in Phil. Trans. 1837, and in the art. Marsupialia, I describe that 

 wall or ' septum ' to be in part composed by or continued from the superior 

 and internal border of the hippocampal fibres, " forming, in the Wombat, 

 a thin lamina analogous f to the septum lucidum," and, " in the Kangaroo, 

 a stronger and thicker one." 



So far as I can comprehend Mr. Flower's account of " the upper and 

 anterior part of the transverse band which passes between the hemispheres 

 of the marsupial brain and radiates out in a delicate lamina above the an- 

 terior part of the lateral ventricle," he and I are recording observations of 

 similar facts. Only, inasmuch as the fibres which radiate from the hippo- 

 campal commissure to form a delicate lamina above the anterior part of the 

 lateral ventricle, contribute, according to my observations, to constitute part 

 of the wall of such ventricle, and, indeed, a greater proportion thereof than 

 its mere anterior part, I should not describe them as being, or as passing, 

 " between the hemispheres." 



The question that remains is the one of interpretation, whether, viz., in 

 reference to the placental condition of the great transverse commissure, and 

 to its relation to the " septum ventriculorum," any portion of the fibres from 

 the hippocampal commissure cross from hemisphere to hemisphere above 

 that septum, after the manner of a ' corpus callosum ' ? 



It is of course open to any anatomist to limit the definition of the fornix, 

 as suggested in my description of the brain of the Ornithorhynchus, to the 

 longitudinal commissural fibres of the hemispheres, and to expand the de- 

 finition of the ' corpus callosum ' to the transverse commissural fibres of the 

 same hemispheres. 



Accordingly, when Mr. Flower asks, " granted that only the psalterial 

 fibres are represented in the upper commissure of the Marsupial brain, why 

 should the name of 'corpus callosum ' be refused to it?" (p. 73), having 

 shown that no such refusal can be imputed to me, I reply by another 

 question " Granted that the chief inter-hemispheral connecting fibres in 



* " The two halves of the cerebrum are by no means ' disconnected,' the want of the 

 upper fibres is compensated for in a remarkable manner by the immense size of the an- 

 terior commissure." (Flower, loc. cit. p. 73.) 



t At that date (1836) the terms ' analogous ' and 'homologous ' had not settled sig- 

 nifications. 



