PROSENCEFHALON OF MAMMALS. 107 



The crura cerebri, which, in the Opossum, <?, 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 

 Avhich 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 infimdibulum supporting the 

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

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

 lar but smaller ones rise behind, and form with them a ( bi^e- 



' O 



minal ' mass : the anterior divisions or f nates,' B, fig. 75, have a 

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

 which have a greater transverse developement. The difference 

 in the relative developement of the nates and testes between the 

 herbivorous and carnivorous Marsupials is less than in the corre- 

 sponding Placental quadrupeds. 



The posterior transverse fibres of the hippocampal commissure 

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

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

 Avard to the anterior cerebral lobes and pass into the substance 

 of the hippocampi, n. Thus the commissure, Avhich is brought 

 into vieAv on divaricating the cerebral hemispheres in the Wom- 

 bat, is seen to be partly the bond of union of the tAVO hippo- 

 campi majores in the transverse direction, like the ( lyra ' of 

 aiithropotomy, and partly of the hippocampus and the fore and 

 inner parts of the hemisphere in the longitudinal and vertical 

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

 as being the hippocampal commissure and continued backAvard 

 and downward as s posterior pillars ; ' but by sending down from 

 the inferior surface tAvo small nerve-like processes, Avhich extend 

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



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

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

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

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

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

 Pkascolomys might compare the appearances with those figured in the ' Philosophical 

 Transactions,' 1837, pi. vi. figs. 4 and G, and in the * Philosophical Transactions,' 

 1865, pi. xxxvi. fig. 4. 



