546 



ZOOLOGY 



SECT. 



FIG. 1134. Brain of Echidna aculeata, sagittal 

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

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

 quadrigcrnina ; crur. crura cerebri ; hip. com. hippo- 

 campal commissure ; hypo, hypophysis ; med. 

 medulla oblongata ; mid. com. middle commissure ; 

 olj. olfactory lobe ; opt. optic chiasma ; tub. olf. 

 tuberculum olfactorium ; vent. 3, third ventricle. 



mode of origin, to the fibres of the fornix, and they connect 

 together only the hippocampi, the fascia? dentata?, and an area 



of the hemisphere in front 

 of the anterior commissure 

 (pre-commissuml area). In 

 cinl.com, the Monotremes (Fig. 



1134) the hippocampal 

 commissure is only very 

 slightly bent dow^nwards 

 at its posterior extremity. 

 In most Marsupials (Fig. 



1135) it bends sharply 

 round posteriorly and runs 

 forward again, becoming 

 thus folded into two layers, 

 dorsal and ventral, con- 

 tinuous with one another 

 at a posterior bend or 

 splenium, similar to the 

 splenium of the corpus 

 callosum. The dorsal layer 

 of the hippocampal com- 

 missure becomes completely replaced in the Eutheria by the fibres 

 of the corpus callosum, and the ventral part alone persists in 

 the shape of the 



lyra. 



In Ornithorhyn- 

 chus (Fig. 1136) 

 the hemispheres 

 are smooth ; in 

 Echidna(Fig.ll37) 

 they are tolerably 

 richly convoluted. 

 In the lower Mar- 

 supials there are 

 no convolutions 

 (Notoryctes, Koala, 

 Phalangers), while 

 in the higher the 

 convolutions are 

 numerous, though 

 the sulci are not 

 very deep (Macro- 

 pus Fig. 1138). A- 

 mong the Eutheria 



there is a great range in the grade of development of the brain, 

 from the Rodents and lower Insectivores to the higher Primates. 



Gffl 



hip.com 

 tid.com\ 



f.771071 



cbl 



ctnl.com 



mecl 



if 



C-majn 



FIG. 1135. Sagittal section of brain of Bock Wallaby (P<t,-<>- 

 gale penicillatd). ant. com. anterior commissure ; cbl. cere- 

 bellum ; c. mam. corpus mammillare ; c. qu. corpora quadri- 

 gemina ; crur. crura cerebri ; epi. epiphysis, with the pos- 

 terior commissure immediately behind ; /. Mon. position of 

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

 ing here of two layers continuous behind at the splenium. 

 somewhat divergent in front where the septum lucidum ex- 

 tends between them ; hypo, hypophysis ; //</. medulla ob- 

 longata ; mid. com. middle commissure ; olf. olfactory lobe ; 

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



