PROSENCEPHALON OF MAMMALS. 107 



The crura cerebri, which, in the Opossum, e, fig. 53, are left 

 exposed below, like the optic lobes above, by reason of the small 

 proportional size of the cerebrum, are more completely concealed 

 in the brain of the Kangaroo and Wombat. The natiform pro- 

 tuberances form a great proportion of the under part of the 

 cerebral hemispheres in all the Marsupials ; the ectorhinal fissure 

 which indents their base in the Wombat and Kangaroo, runs 

 along the side of the hemisphere to the outer side of the olfactory 

 lobe in the Opossum, indicating the large relative size of the 

 basirhinal fold or tract. Behind the commissure of the optic 

 nerves is seen a broad and short infundibulum supporting the 

 pituitary body, g y fig. 53, and posterior to this is the single corpus 

 albicans. The optic lobes, fig. 73, Z>, are solid; a pair of simi- 

 lar but smaller ones rise behind, and form with them a 'bigc- 

 al ' mass : the anterior divisions or ( nates,' B, fig. 75, have a 

 greater longitudinal diameter than the posterior ones or ' testes,' 

 Inch have a greater transverse developement. The difference 

 in the relative developement of the nates and testes between the 

 lerbivorous and carnivorous Marsupials is less than in the corre- 

 sponding Placental quadrupeds. 



The posterior transverse fibres of the hippocampal commissure 



ire continued, fig. 75, m, outward and backward beneath the 



lore longitudinal fibres, which overlap them as they pass for- 



r ard to the anterior cerebral lobes and pass into the substance 



>f the hippocampi, n. Thus the commissure, which is brought 



ito view on divaricating the cerebral hemispheres in the Wom- 



>at, is seen to be partly the bond of union of the two hippo- 



impi majores in the transverse direction, like the ' lyra ' of 



mthropotomy, and partly of the hippocampus and the fore and 



mer parts of the hemisphere in the longitudinal and vertical 



irections. It mainly fulfils the function of the fornix, not only 



being the hippocampal commissure and continued backward 



id downward as ' posterior pillars ; ' but by sending down from 



the inferior surface two small nerve-like processes, which extend 



vertically, behind the anterior commissure, to the corpus albicans, 



ricated hemispheres is left entire, and whether the fibres diverge into the substance 

 )f the roof of the ventricles is not shown. In lxx'. pi. vi. figs. 4 and 6, the requisite 

 lissection is made and figured, and the transverse fibres are shown to be lost beneath 



le ' labia hippocampi ' — that is, to be continued into the hippocampi, not into the 

 supraventricular substance. Other dissectors of the brain of a Maeropus Benettii and of 

 ^hascolomys might compare the appearances with those figured in the ' Philosophical 



'ransactions,' 1837, pi. vi. figs. 4 and 6, and in the ' Philosophical Transactions,' 

 1865, pi. xxxvi. fig. 4. 



