560 



ZOOLOGY 



SECT. 



mid.com 

 cbl 



^ . com 



V 



Fio. 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. gw, corpora rjuadrigemina ; 

 I/any, hab. habenular ganglion ; hip. com. Hippo- 

 canipal commissure ; med. medulla oblongata ; 

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

 opt. optic chiasma ; tub. olf. tubercuhnn 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 

 o f opposite sides are con- 

 nected together across the 

 middle hne 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) of the hemispheres. 



7tip.com 

 idcom\ 



& 



man 



artt.com 



med 



c.mam 



FIG. 1216. Sagittal section of brain of Rock Wallaby 

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

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

 quadrigemina ; r.rur. crura cerebri ; epi. epiphysis, with the 

 posterior commissure immediately behind ; /. man. 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 ; wt'tl. 

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

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



