578 



ZOOLOGY 



SECT. 



In tlic Monotremcs and Marsupials 

 no corpus callosum, wliile the anterior 



y.hab 



■^ =-\ hvp.com 



mt.a.coTn 



ant. com 



^^ ct^u ^ 



c.m.ct7rv 



-i/ent.3 



opt\ tub.oLf 



CO, 



Lforrt 



Fitt. 1-203.— Brain of Echidna aculeata, sagittal 

 section, ant. com. anterior ciimniissure ; chl. cere- 

 bellum ; c.mam. corpus manimillare ; col.r'orn. 

 column of the fornix ; c. qu. corpora quadrigeniina ; 

 crur. crura cerebri ; (jfiiici. hah. habenular ganglion ; 

 hip. com. hippocampal commissure ; hiipo. hypo- 

 physis ; med. medulla oblongata ; raid. com. middle 

 commissure ; olf. olfactory bulb ; opt. optic 

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

 third ventricle. 



(Figs. 1203, 1204) there is 

 commissure {ant. com.) is of 

 relatively large size, and, 

 unlike the corresponding 

 commissure in lower Ver- 

 tebrates, contains fibres 

 connecting together areas 

 of the non-olfactory regions 

 {nco-pallium) of the hemi- 

 spheres. The hippocampi 

 extend alonor 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 

 tcenia M^yiJOcamfi. In the 

 Eutheria, as we have seen 

 in the case of the Rabbit, 



the taeniae 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 



fibres, of the taenia 



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- 



■sitrc {hip. cam., cf. 



Figs. 935 and 971), 



the great develop- 

 ment of w h ic h 



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 



'tt,p.com 

 com] 



^.mon 



/ 



ant.com. 



j/L^po 



CTnain 



vertt.9 



Fin. 1204.— Sagittal section of brain cf Rock Wallaby (Petro- 

 fiale pcnicillata). ant. com. anterior commissure ; ilil. cere- 

 bellum ; c. iitam. corpus nianiniill:ire ; c. qu. corpora (Quadri- 

 geniina ; rrur. crura cerebri; (pi. epiphysis, with the pos- 

 terior commissure immediately behind \~f. Mon. position of 

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

 ing here of two layers continuous behind at the splenimn, 

 somewhat divergent in front where the septum lucidmn ex- 

 tends between them ; luipo. hypophysis ; mfd. medulla ob- 

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

 oi)(. optic chiasma ; vent. 3, third ventricle. 



