292 



MARSUPIALIA. 



Fig. 114. 



hemispheres are 

 penetrated by an- 

 fractuous fissures. 

 In the Worn bat 

 a large longitudi- 

 nal fissure bounds 

 the outer side of 

 the natiform pro- 

 tuberance and ol- 

 factory tract at the 

 base of the brain ; 

 from the anterior 

 moiety of this fis- 

 sure three or four 

 smaller ones curve 

 upwards upon the 

 sides of the hemi- 

 spheres. On the 

 upper surface a 

 short transverse 

 fissure marks off 



what may be regarded as the anterior lobe 

 of the cerebrum, and behind this each hemi- 

 sphere exhibits a few detached shallow fissures. 

 In the Kangaroo (Jig. 115) these fissures be- 

 come continuous and are deeper ; a long 

 and nearly transverse anfractuosity divides the 

 upper surface of the hemisphere; behind the 

 shorter fissure which marks off the anterior 

 lobe, and between the two transverse fissures 

 there is a longitudinal one bounding a con- 

 volution that runs parallel with the median 

 interspace of the hemispheres. The ante- 

 rior lobes are also broken by small fissures; 

 two or three long and moderately deep 

 ones ascend upon the sides of the hemi- 

 spheres (), and the posterior portion (6) 

 presents occasionally small detached fissures. 

 So far therefore as the external surface is con- 



Brain of Dasyums wsinns. 



Fig. 115. 



Fig. 116. 



,--fs 



3, 



Bi'di/i uf Macroptts mnjur. 



cerned, the brains of the herbivorous Marsu- 

 pials are more complicated than those of any 

 of the Rodent Mammalia. The cerebellum 

 presents the usual close-set, sub-parallel, trans- 

 verse convolutions: it is remarkable for the 

 large proportional size of the median or vermi- 

 form lobe, as compared with the lateral lobes, 

 especially in the carnivorous and insectivorous 

 Marsupials, where this condition is associated 

 with a corresponding diminution of their com- 

 missural band or ' pons Varolii,' as is shown in 

 the view of the base of the brain of an Opos- 

 sum (fig. 116, 4). 



In the Kangaroos, 

 Perameles, Phalangers, 

 and Koala the hemis- 

 pheres or lateral lobes 

 of the cerebellum are 

 characterized by asmall 

 subspherical lateral 

 process or appendage 

 (< <>./& 115), which 

 is lodged in a peculiar 

 fossa of the petrous 

 bone above the internal 

 meatus : there are cor- 

 responding but less 

 produced processes in 

 the Dasyures and 

 , qjm ,_ Opossums, but they 



Brain of Didelphys Vir- are not developed in 

 giniana. the Wombat. On the 



upper surface of the 



cerebellum the medullary substance appears 

 superficially at a small tract between the ver- 

 miform processes, marked with an asterisk in 

 figures 115 and 117. The simple disposi- 

 tion of the arbor vitae is shown in Jig. 118, /. 

 Behind the pons Varolii are seen the two 

 trapezoid bodies (c, Jig. 116); and the corpora 

 pyramidalia (c/) are always clearly distin- 

 guishable from the corpora olivaria. The cruru 

 cerebri, which, in the Opossum (c,fig. \ 16) 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 nati- 

 form protuberances form a great proportion of 

 the under part of the cerebral hemispheres in 

 all the Marsupials ; their external boundary, 

 which is basial in the Wombat and Kangaroo, 

 runs along the side of the hemispheres to the 

 outer side of the olfactory lobe in the Opossum. 

 The inner root of the olfactory nerve forms a 

 bulbous or ganglionic enlargement (Jig. 116, 

 * b). Behind the commissure of the optic 

 nerves is seen a broad and short infundibulum 

 supporting the pituitary gland (d, fig.\l8\ 

 and posterior to this is the single corpus albi- 

 cans. The optic lobes are solid, and are each 

 divided by a transverse fissure, as in the Pla- 

 cental Mammalia; the anterior divisions or 

 'nates ' (B^g.ll?) have a greater longitudinal 

 diameter than the posterior ones or ' testes,' 

 which have a greater transverse development. 



The difference in the relative development of 

 the nates and testes is much less in the herbi- 

 vorous and carnivorous Marsupials than in the 

 corresponding Placental quadrupeds. 



