560 



ZOOLOGY 



SECT. 



med 



c.mcim. 



tub.ol/f 

 eoLforn 



Fia. 1215. Brain of Echidna aculeata, sagittal 

 section, ant. com. anterior commissure ; cbl. cere- 

 bellum ; c. mam. corpus mammillare ; col. forn. 

 column of the fornix ; c. qu, corpora quadrigemina ; 

 gang. hab. habenular ganglion ; hip. com. hippo- 

 campal commissure ; med. medulla oblongata ; 

 mid. com. middle commissure ; olf. olfactory bulb ; 

 opt . optic chiasma ; tub. olf. tuberculum olfactorium ; 

 vent. 3, third ventricle. 



distinguished by its relatively large size, and by the large size and 

 complex structure of the cerebral hemispheres of the fore-brain. 



The cerebral hemispheres 



^ang.Tiab ^ . com of opposite sides are con- 



7Tzid.com \ nected together across the 



a.7tt.cont middle line in all Mammals, 

 except the Monotremes and 

 Marsupials, by a band of 

 nerve-tissue termed the 

 corpus callosum a struc- 

 ture not present in the 

 Sauropsida. The hemi- 

 spheres, in all but certain 

 of the lower and smaller 

 Mammals, are not smooth, 

 but marked by a number of 

 grooves or sulci separating 

 winding ridges or convolu- 

 tions. The lateral ventricles 

 in the interior of the hemi- 

 spheres are of large size and 

 somewhat complex form. 

 The optic lobes, which are relatively small, are divided into four 

 parts, and are hence called the corpora quadrigemina. The pineal 

 body is always a small gland-like structure. Connecting together 

 the lateral parts of 

 the cerebellum, 

 which, in the higher 

 Mammals, attains a 

 high degree of de- 

 velopment, is a trans- 

 verse flattened band 

 the pons Varolii 

 (Po.) crossing the 

 hind-brain on its ven- 

 tral aspect. 



In the Monotremes 

 and Marsupials (Figs. 

 1215, 1216) there is 

 no corpus callcsum, 

 while the anterior 

 commissure (ant. 

 com.) is of relatively 

 large size, and, unlike 

 the corresponding 

 commissure in lower Vertebrates, contains fibres connecting together 

 areas of the non-olfactory regions (neo-pallium) ofJJie hemispheres. 



i 7tip.com 

 \mid.com\ 



cbl 



anl.com 



meet 



& 



FIG. 1216. Sagittal section of brain of Rock 'Wallaby 

 (Petrogale penicillata). ant. com. anterior commissure; cbl. 

 cerebellum ; c. mam. corpus mammillare ; c. qu. corpora 

 quadrigemina ; crur. crura cerebri ; epi. epiphysis, with the 

 posterior commissure immediately behind ; /. won. position 

 of foramen of Monro ; hip. com. hippocampal commissure, 

 consisting here of two layers continuous behind at the 

 splenium, somewhat divergent in front where the septum 

 lucidum extends between them ; hypo, hypophysis ; med. 

 medulla oblongata ; mid. com. middle commissure ; off. 

 olfactory bulb ; opt. optic chiasma ; vent. 3, third ventricle. 



