1865.] of the Marsiqnalia, $c. 73 



psaltcrial fibres or transverse commissure of the hippocampi ? Undoubtedly 

 a large proportion of its fibres do come under the latter category. But 

 even if they should nominally be all so included, it is important to bear in 

 mind that we have still a disposition in the marsupial brain very different 

 from that which would remain in the brain of any placental mammal after 

 the upper and main part of the corpus callosum had been cut away. In 

 the latter case the commissure of a very small part of the inner wall of the 

 hemisphere alone is left, that part folded into the hippocampus. In the 

 former there is a commissure, feeble it may be, but radiating over the 

 whole of the inner wall, from its most anterior to its posterior limits. 

 Granted that only the psalterial fibres are represented in the upper com- 

 missure of the marsupial brain, why should the name of " corpus callosum " 

 be refused to it ? These fibres are part of the great system of transverse 

 fibres bringing the two hemispheres into connexion with each other ; they 

 are inseparably mingled at the points of contact with the fibres of the 

 main body of the corpus callosum, and are only distinguished from it in 

 consequence of the peculiar form of the special portions of the hemisphere 

 they unite. Indeed they are scarcely more distinct than is the part called 

 " rostrum " in front. And although, like the fibres of the hinder end of 

 the corpus callosum, they blend at each extremity with the fibres of the 

 diverging posterior crnra of the fornix, they certainly cannot be con- 

 founded with that body, which, as shown before, is essentially a longi- 

 tudinal commissure. 



But is not the main part of the " corpus callosum" of the placental mam- 

 mals also represented by 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 anterior part of the lateral ventricle ? 

 The most important and indeed crucial test in determining this question, is 

 its position in regard to the septum ventriculorum, and especially the pre- 

 commissural fibres of the fornix. Without any doubt in all marsupial and 

 monotreme animals examined (sufficient to enable us to affirm without 

 much hesitation that the character is universal) it lies above them, as dis- 

 tinctly seen in the transverse sections. This is precisely the same relation- 

 ship which obtains in Man and all other mammalia, and this is one of the 

 chief points in which not only the interpretation of facts but the observation 

 of them recorded in the present paper differs from that of Professor Owen. 



The defective proportions of the part representing the great transverse 

 commissure of the placental mammals, which appear to result from or 

 to be related to the peculiar conformation of the wall of the hemisphere, 

 must not lead to the inference that the great medullary masses of the two 

 halves of the cerebrum are by any means " disconnected." The want of 

 the upper fibres is compensated for in a remarkable manner by the immense 

 size of the anterior commissure, the fibres of which are seen radiating into 

 all parts of the interior of the hemisphere. There can be little doubt 

 that the development of this commissure is, in a certain measure, comple- 



