1865.] On the Cerebral Commissures of the Marsupialia, c. 71 



III. " On the Commissures of the Cerebral Hemispheres of the 

 Marsupialia and Monotremata, as compared with those of the 

 Placental Mammals." By W. H, FLOWER, F.R.S. Received 

 January 24, 1865. 



(Abstract.) 



As it is most convenient to pass from the best to the least known, and 

 especially as the terms used in describing the anatomy of the vertebrated 

 animals have iu most cases been originally bestowed upon parts of the 

 human body, the Paper commences by a short description of the septum 

 ventriculorum and commissures of the human brain. This is done with 

 a view to establish clearly, both by their structure and development, the 

 mutual relations of the great transverse commissure or corpus callosum 

 and the fornix. The latter is denned as essentially a longitudinal com- 

 missure, consisting of two lateral halves closely applied for a short space 

 in the middle line, but each half belonging to its own hemisphere, and 

 formed out of the longitudinal fibres bordering the superior margin of the 

 ventricular aperture. There are no transverse fibres in the fornix proper, 

 the so-called " psalterial fibres" connecting together the two hippocampi 

 majores being a portion of the same system of fibres as the corpus 

 callosum. 



The relations of these parts are shown in a series of longitudinal and 

 vertical sections of the brains of the Sheep, Rabbit, Two-toed Sloth, and 

 Hedgehog among Placental Mammals, and in the same way in the Kan- 

 garoo, Wombat, and Thylacine among Marsupials, and the Echidna among 

 Monotremes. 



After reference to the literature of the subject, more especially to the 

 writings of Professor Owen, whose statement (Phil. Trans. 1837) of the 

 absence in the marsupials of the "corpus callosum," or "great transverse 

 commissure which unites the supra-ventricular masses of the hemispheres," 

 in all placentally developed mammals * has been almost universally 

 adopted, the author proceeds to sum up the result of the present investi- 

 gation as follows. 



At the outset a confirmation is afforded of the important fact, first 

 observed by Professor Owen, that the brains of animals of the orders 

 Marsupialia and Monotremata present certain special and peculiar cha- 

 racters, by which they may be at once distinguished from those of other 

 mammals. The appearance of either a transverse or longitudinal section 

 would leave no doubt whatever as to which group the brain belonged. In 



* In the paper by the same author " On the Characters, Principles of Division, 

 and Primary Groups of the Class Mammalia " (Proc. Linn. Soc. 1858), the Subclass 

 Lyencephala ("loose" or "disconnected" brain), equivalent to the Marsupialia and 

 Monotremata, are characterized as having " the cerebral hemispheres but feebly and 

 partially connected together by the ' fornix ' and ' anterior commissure,' while hi the 

 rest of the class a part called ' corpus callosum ' is added, which completes the connect- 

 ing or commissural apparatus." 



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