578 



ZOOLOGY 



cra.7t.Q.JLCtb 



TnicL.COTn 



ant. com 



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, 

 'hip.com unlike the corresponding 



commissure in loAver Ver- 

 tebrates, contains fibres 

 connecting together areas 

 of the non-olfactory regions 

 (neo-'pallmni) 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 



Tned 



tub.ol(f 

 eoLforn 



Fin. 1203.— Brain of Ecbidna aculeata, sagittal 

 section. a)it. com, anterior commissure ; chL cere- 

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

 column of the foi-nix ; c. qtt. corpora quadrigemina ; 

 cmr. crura cerebri ; gang, Jiah, h<abenular ganglion ; 

 hip, com, liippocampal commissure ; hypo, hypo- 

 physis ; med, medulla oblongata : mid, com, middle 

 commissure ; olf, olfactory bulb ; ojit. optic 

 clnasma ; tub. olf, tuberculum olfactorlum ; vent, 3, 

 third ventricle. 



arranged in a band — the 



tcenia 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 {sec p. 470). 



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



no such union ; the 



eioi /tip.oom 



cbl 



^.jnon 



a.nt.com 



fibres, of the tsenia 

 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- 

 pocamipal com^nis- 

 surc {hip. com,., cf. 

 Figs. 935 and 971), 

 the great develop- 

 ment of which 

 readily leads to its 

 being mistaken for 

 a corpus callosum. 



The fibres entering into the formation of this commissure corre- 

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



cinajn. 



vent.3 



Fir,. 1204— Sagittal section of brain of Rock Wallaby (Fdro- 

 gale jicnicitlaia), ant. covi, anterior commissure ; chl. cere- 

 bellum ; c, viavi, 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 ; /ij/jjo. hypophysis ; med, medulla ob- 

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

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



