29-1 



MAUSUPIALIA. 



hut the optic thalami (t, t, fig. 117) are im- 

 mediately brought into view, and instead of a 



Phascolomys fusca. 



broad corpus callosum, we perceive, situated 

 deeply at the bottom of the longitudinal 

 Hssure, a small commisstiral medullary band, 

 (>n, Jig. 117) passing in an arched form over 

 the anterior part of the thalami, and extending 

 beneath the overlapping internal or mesial sur- 

 faces of the hemispheres, (q, Jig. 117,) which 

 are thus, as in the Bird, disconnected with 

 each other. 



On gently raising the hemispheres from 

 above the commissure and pressing them out- 

 wards with the handle of a scalpel, the instru- 

 ment passes into the fissure upon which the 

 hippocampus (n,Jtg. 117) is folded; and, on 

 continuing the pressure, the hippocampus is 

 torn through, and the lateral ventricle is ex- 

 posed. The mesial wall of the hemisphere is 

 continued from the superior and internal border 

 of the hippocampus, and is composed in the 

 Wombat, as in the Bird, of a thin lamina of 

 medullary substance analogous to a detached 

 layer of the septum lucidum. In the Kan- 

 garoo the mesial parietes of the lateral ventricles 

 are stronger, being about two lines in thickness. 



The posterior transverse fibres of the com- 

 missure are continued outwards and backwards 

 beneath the more longitudinal fibres, which 

 overlap them as they pass from the trenice 

 hippocampi (o, fig. 117) forwards to the 

 anterior cerebral lobes. All the fibres of the 

 commissure pass along the floor of the lateral 

 ventricles into the substance of the hippocampi 

 majores, which are of proportionally very large 

 size. Thus the commissure, which is brought 

 into view on divaricating the cerebral hemi- 

 spheres in the Wombat, is seen to be partly 

 the bond of union of the two hippocampi 

 majores in the transverse direction, and partly 

 of the hippocampus and anterior lobe of the 

 same hemisphere in the longitudinal direction. 

 It id."-o fulfils the other function of the fornix 



by sending down from the inferior surface two 

 small nerve-like processes, which extend verti- 

 cally, behind the anterior commissure, through 

 the substance of the optic thalami, near their 

 mesial surfaces, to the corpus albicans, at the 

 base of the brain. 



The superior view of the connexions of the 

 hippocampal commissure of the Wombat is 

 given at in, n, o, Jig. 117. 



If in the Beaver's brain the posterior thick- 

 ened margin of the corpus callosurn be raised, 

 it is observed at the middle of its inferior sur- 

 face to be closely connected with the centre of 

 a commissural band of fibres, arching over the 

 anterior part of the optic thalami, and passing 

 outwards and backwards along the floor of the 

 lateral ventricles into the substance of the hip- 

 pocampi, Avhich arc as largely developed as in 

 the Wombat. The anterior part of the corpus 

 callosum is bent downwards, and is attached 

 along the middle line of its inferior surface by 

 a uniting medium of medullary substance, re- 

 presenting the septum lucidum, to the hippo- 

 campal commissure or fornix ; the ta?nis hip- 

 pocampi extend forwards, as in the Wombat, 

 into the anterior lobes. 



The corpus callosum being removed, and the 

 commissural fibres of the hippocampi being 

 left behind, the view of the Beaver's brain now 

 corresponds with that obtained in the previous 

 dissection of the brain of the Wombat, which 

 we regard, therefore, as wanting the principal 

 mass of thecorpuscallosum, the septum lucidum, 

 and consequently the fifth ventricle. The ar- 

 tery of the plexus choroides, (p,fig- 117,) in 

 both the Beaver and Wombat, enters the lateral 

 ventricle, where the hippocampus commences 

 at the base of the hemisphere, and the plexus 

 is continued along the under surface of the 

 tsenia hippocampi, and passes beneath the 

 fornix, through the usual foramen, to commu- 

 nicate with its fellow in the third ventricle im- 

 mediately behind the anterior crura of the 

 fornix, which are sent down in the Beaver, as 

 in the Wombat, from the centre of the inferior 

 surface of the hippocampal commissure. 



If we expose the lateral ventricle by removing 

 its outer parietes in a marsupial and placental 

 quadruped, as, for example, in the Kangaroo 

 and Ass, the hippocampus major, the tffinia 

 hippocampi, the plexus choroides, and the 

 foramen Monroianum are brought into view. If 

 a style be thrust transversely through the inter- 

 nal wall of the ventricle, immediately above 

 the hippocampus, in the placental quadruped, 

 it perforates the septum lucidum and enters 

 the opposite ventricle below the corpus callo- 

 sum. If the same be done in the marsupial 

 brain the style passes into the opposite ventricle, 

 but is immediately brought into view from 

 above by divaricating the hemispheres, and is 

 seen lying above the fornix or commissure of 

 the hippocampi. 



This commissure may, nevertheless, be re- 

 garded as representing, besides the fornix, the 

 rudimental commencement of the corpus callo- 

 sum ; but this determination does not invalidate 

 the fact that the great commissure which unites 

 the supra-ventricular masses in the Beaver, 

 the Bat, and all other placentally developed 



