546 



ZOOLOGY 



SECT. 



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 



m id. com 



_ . 



/ivp.com, 



rned 



y 



-venla 



FIG. 1134. Brain of Echidna aculeata, sagittal 

 section, nut. com. anterior commissure ; cW. cere- 

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

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

 campal commissure ; hypo, hypophysis ; moil. 

 medulla oblongata ; mid. com. middle commissure ; 

 olf. olfactory lobe ; opt. optic chiasraa ; tub. olf. 

 tuberculum olfactorium ; rent. 3, third ventricle. 



of the anterior commissure 

 (pre-comimssural area). In 

 the Monotremes (Fig. 



1134) the hippocampal 

 commissure is only very 

 slightly bent downwards 

 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. epi Jiip.com 



In Ornithorhyn- 

 chus (Fig. 1136) 

 the hemispheres 

 are smooth ; in 

 Echidna (Fig. 1137) 

 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. 



TTLOTt 



cbl 



cml.com 



me 



FIG. 1135. Sagittal section of brain of Bock Wallaby (l'<.t,-<>- 

 gale penicillata). ant. com. anterior commissure ; <[>!. cere- 

 bellum ; c. mam. corpus mammillare; r. </c. L-orpnra quadri- 

 gemina ; crur. crura cerebri; epi. t-piphysis. with the pos- 

 terior commissure immediately behind; /. M<>,>. position of 

 foramen of Monro ; /> ''""' 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 ; /////>. hypophysis; pied, medulla ob- 

 longata ; w/'/. '". middle commissure ; olj. olfactory lobe ; 

 opt. optic chiasma ; y< ,il. 3, third ventricle. 



