108 ANATOMY OF VERTEBRATES. 



at the base of the brain. The superior view of the connections of 

 the hippocampal commissure of the Wombat is given at m, n, o, 

 fig. 75. l 



The artery of the plexus choroides, entering with the fold of pia 

 mater at the lowest part of the hippocampus, is richly spread upon 

 the small production of that fold, ib. p, beneath the margin of the 

 ' tomia ' near the passage by which it is continued into the fold 

 and plexus of the opposite ventricle. This intercommunication 

 between the two prosencephalic cavities exists in all Mammals, 

 and is defined in anthropotomy as the ' foramen Monroianum.' 

 The pineal appendage, ib. u, is small compared with its ( crura,' 

 which, as in all other Mammals, are continued backward from 

 the fore and inner parts of the thalami, t. 



A well-marked ectorhinal fissure extends from the natiform 

 protuberance, defining externally its ( basirhinal tract ' 2 and the 

 forward continuation of the ' crus rhiiiencephali.' A longitudinal 

 white streak 3 divides the outer and inner portions of that crus. 

 The prosencephalic cavity is continued into the large rhinen- 

 cephalon, figs. 73 and 46, R, d. 



The characteristics of the Marsupial brain are, its small rela- 

 tive size, small proportion of cerebrum, convolutions wanting, or 

 few and symmetrical in those Marsupials possessing them, large 

 proportional anterior commissure, and still larger hippocampi ; 

 some fibres arch across from one to the other hippocampus, 

 answering to the 6 lyra,' and forming the beginning of the great 

 transverse commissure or ( corpus callosum ' of higher Mam- 

 mals ; the fibres radiating upon the fore and inner wall of the 

 ventricle are the anterior terminations of the great longitudi- 

 nal commissure answering to the ' fornix ' in anthropotomy. 

 The f corpus striatum,' fig. 75, r, is relatively small and in- 

 ferior in position to the hippocampus, being partially overlapped 

 thereby. 



B. Lissencephala. I demonstrated the characters differentiating 

 the first step in the developement of the ' corpus callosum ' of 

 the Hedgehog, in the longitudinal section of the brain 4 prepared 

 and added to the Hunterian series of Comparative Anatomy in 

 1834, by contrast with a similar section of the brain of the Opos- 

 sum 5 and Dasyure ; 6 placing a plate of mica in the fore part of 



1 Haller showed his appreciation of the essential nature of these ' fere fornicis 

 ipsins cruribus.' 



2 LXX'. pi. v. fig. 8, la. 3 Ib. lc. 



4 No. 1323 D, xx. vol. Hi. (1835), p. 29 ; and see XLIII". pi. xxxvii. fig. 7. 



5 xx. no. 1323 B. 6 xx. no. 1323 c. 



