578 



ZOOLOGY 



SECT. 



-venl.3 



FIG. 1203. 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 ; 

 crur. crura cerebri ; gang. liab. habenular ganglion ; 

 hip. com. hippocampal commissure ; liyj>o. hypo- 

 physis ; med. medulla oblongatn ; mid. com. middle 

 commissure ; olf. olfactory bulb ; opt. optic 

 chiasma ; tub. olf. tuberculum olfactorium ; vent. 3, 

 third ventricle. 



In the Monotremes and Marsupials (Figs. 1203, 1204) there is 



no corpus callosum, while the anterior commissure (ant. com.) is of 

 , , relatively large size, and, 



&- t hip.com unlike the corresponding 



7nid.com \ / commissure in lower Ver- 



tebrates, contains fibres 

 connecting together areas 

 of the non-olfactory regions 

 (neo-pallinm) of the hemi- 

 spheres. The hippocampi 

 extend along the whole 

 length of the lateral ven- 

 tricles. The layer of nerve- 

 cells in each hippocampus 

 gives origin, as in Eutheria, 

 to numerous fibres, which 

 form a layer on the sur- 

 face, the alveus, and become 

 arranged in a band the 

 tamia hippocampi. In the 

 Eutheria, as we have seen 

 in the case of the Rabbit, 



the tasnise unite mesially to form the body of the fornix (see p. 470). 



In the Monotremes and Marsupials, on the other hand, there is 



no such union ; the 



fibres, of the tasnia 7tip.com 



run towards the 



foramen of Monro, 



where they become 



divided into several 



sets. Of these one 



set, constituting the 



great majority of 



the fibres, pass into 



the hippocampus of 



the opposite side, 



giving rise to a hip- 

 pocampal commis- 

 sure (hip. corn,., cf. 



Figs. 935 and 971), 



the great develop- 

 ment of which 



readily leads to its 

 being mistaken for 

 a corpus eallosum. 



The fibres entering into the formation of this commissure corre- 

 spond, however, not to the fibres of the corpus callosum, which 



"771071 



cbl 



meet 



FIG. 1204. Sagittal section of brain of Rock Wallaby (Petro- 

 l/ale penicillatii). 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; \ip. 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 ; med. medulla ob- 

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

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



