152 BRAIN" OF THE OPOSSUM. 



The ventricle of the olfactory lobes nowhere communicates with the surface as, undoubt- 

 edly through the fliult of the engraver, the figures of Owen seem to indicate. 



Cerebral Zo5es.— The two together are somewhat pear shaped and considerably flattened 

 vertically. (Figs, i, iv, B.) They separate from each other behind, leaving a triangular 

 space in which the optic lobes are partly exposed. The surface is marked with very dis- 

 tinct vascular impressions which branch off from two principal trunks, the smaller one 

 beginning near the optic chiasma and extending forwards and upwards, and the larger ex- 

 tending horizontally along nearly the whole length of the side of the lobe. 



After the membranes are stripped off two infoldings or convolutions of the cortical sub- 

 stance are seen. The first (fig. iii) corresponds in position with the larger vessel referred 

 to above, and its sulcus is closed by this vessel superficially. Beginning just over the roots 

 of the olfiictory lobes it extends backwards in a curved line to the° hinder part of the 

 hemispheres. It divides each hemisphere on the surface into an upper and lower portion 

 the latter as already stated being continuous with the olfactory lobes. The second infold- 

 ing is quite small and crosses the upper and fore part of each lobe (figs, i, iii, &), forms a 

 crescentic furrow, extends from near the middle line above, on to the sides and Jnds near 

 the sulcus just described, but not connecting with it. On either side of the optic chiasma 

 an oval bulging of the surface is seen (fig. ii, h), having the appearance of a convolution 

 and surrounded by a deep groove. 



If a longitudinal section of the brain is made and the under part of the cerebral hemi- 

 sphere lifted from its place, two additional infoldings of the surface are brought into view; 

 the larger of them, beginning near the optic chiasma and winding around the inner surf^ice 

 of the hemisphere, is there seen on its vertical portion and ends near the union of the 

 hemisphere with the olfactory lobe. The second infolding is quite small and is found just 

 above the hinder part of the preceding. If the thin upper walls of the cerebral hemi- 

 spheres are removed, two convex foldings, corresponding with the infoldings just described, 

 will be seen, forming the oblique floor of the ventricle. The smaller and hindermost of 

 these (fig. iv), though a distinct and constant part, does not appear to have been noticed in 

 the descriptions of the brain of the opossum, but the larger {d) is known as the hippo- 

 campus. This last is of great size and fills nearly the whole cavity of the ventricle, and 

 may be compared in shape to an ox horn, its point being directed forwards. Behind, it 

 rests upon the nates and optic thalami, and in front it embraces the corpus striatum {g) in 

 its hollow. 



Commissures.— T\\Q^Q are represented in the brain of the opossum, chiefly by the corpus 

 callosum, fornix and anterior commissure. Owen, in his early and most valuable investiga- 

 tions of the brain of Marsupials and Monotremes, while he recognizes the existence of a 

 "rudimental commencement of the corpus callosum," in his zoological conclusions and 

 descriptions, leaves it out of the account as if it were wholly wanting, and assumes that 

 the absence of it is an especial characteristic by which the Placental and Implacental 

 Mammals are distinguished from each other, and this view has been largely accepted by 

 zoologists. ^ Leuret, Blainville, Pappenheim and other continental anatomists have ex- 

 pressed their dissent to this view, and Mr. Flower, by a series of comparative dissections, 

 has shown iha actual presence but gradual reduction of the corpus callosum in Placental 

 Mammals, and its still further reduction in Marsupials and Monotremes. Throughout the 

 whole of both series, this part retains its essential features and is simply reduced in size by 



