BRAIN OF THE OPOSSUM. 153 



being shortened from behind forwards so that its hinder edge, instead of being over the 

 tubercula quadrigemina as in man, is just above the anterior commissure. Owen, however, 

 maintains that the corpus callosum in Implacentals has a niorphok)gical vakic only and not 

 a zoological, and is no more the corpus callosum of Placentals than a "bract" is a " leaf" 

 By a parity of reasoning a Batrachian might be described as destitute of a cerebellum. 



When the hemispheres of the opossum are separated from each other, the corpus cal- 

 losum is brought into view at (he fore part of the longitudinal fissure, just above and rest- 

 ing upon the fore part of the optic thalami, consisting of a transverse, nearly cylindrical 

 band, somewhat compressed from above downwards, and attached to the inner walls of 

 the two adjoining hemispheres. It consists of white fibres which enter the substance of 

 the hemisphere just below the hippocampal sulcus, where the fibres radiate in a fm-shaped 

 manner, but chiefly backwards, and are spread out over the convex surface of the hipito- 

 campus and end in the cerebral walls along the angle where these are reflected upwards in 

 the great longitudinal fissure. The remainder of the fibres are directed forwards and 

 downwards and were traced to the fore part of the cerebral lobe. It will be seen, therefore, 

 that the connections established by means of the corpus callosum are quite extensive. 



It is in the reduction of the corpus callosum to its diminutive proportions that the brain 

 of the Marsupials and Monotremes most closely resembles that of the oviparous classes, in 

 which last no trace of it has thus far been recognized. In all other respects, however, it 

 maintains its mammalian features, and shows nothing deviating from these to the same ex- 

 tent as do the organs of generation, from the usual structure of these parts in ordinary or 



placental mammals. 



The parts which are supposed to represent the fornix in the opossum, though quite 

 diminutive, are traced without much difficulty. They consist of two fibrous bands slightly 

 separated from each other, which at one end are connected with the foremost part of the 

 optic thalami, thence pass upwards over the "hard" or anterior commissure, restnig 

 closely on its surface and at the same place becoming attached to and apparently inter- 

 changing fibres with it, and descending in front of it are lost in the cerebral substance adjoin- 

 ing. ^Some of the fibres make a nearly complete circuit of the commissure. The portions 

 of the fornix which lie in front of, and behind, the anterior commissure, correspond with the 

 so called " pillars " ; and if we could suppose the corpus callosum gradually enlarged and its 

 hinder edge pushed backwards carrying the attachment of the fornix with it, these pillars 

 would be gradually lengthened so as assume the proportions they have in Placental Mam- 

 mals, those in front being the ones which Huxley calls precommissural. 



In the median longitudinal section, ih(i divided great commissure and the corpus cal- 

 losum are represented by two oval sections, the long axes of which incline towards each 

 other from opposite directions, and if prolonged would meet in front of them. The section 

 of the great commissure is nearly twice as large as the other, and has only the pillars of the 

 fornix interposed between it and the optic thalamus. Its fibres are easily traced, radiating 

 into the adjoining parts until they are lost in the fore part of the cerebral lobes and the 



corpora striata. 



The Optic lohes (tubercula quadrigemina) taken together, form a wedge-shaped mass 

 inserted between the cerebellum and the optic thalami, and in general resemble the same 

 parts in the rat. The testes issue vertically from the fourth ventricle, and have on their 

 hinder face a deep concavity which the middle lobe of the cerebellum fills. Above they 



MEMOIRS BOST. 800. NAT. HIST. VOL. II. 89 



