PROSENCEPHALON OF MAMMALS. 



141 



124 



The anterior commissure — the most constant of the trans- 

 verse system — is relatively largest in Lyencephala, figs. 69, 

 73, c. In the human brain a similar transverse section of it 

 shows its insignificant dimensions, fig. 123, a. Traced tnm>- 

 versely, in them, it passes, as in a special canal, across the lower 

 part of the corpora striata, bends backward, and expands as it 

 radiates into the middle of each hemi- 

 sphere. It indicates the small part of 

 the human cerebrum which is homolo- 

 gous with the main part of that of birds 

 and marsupials. But the increase of 

 the mammalian over the avian brain 

 begets the added structures for asso- 

 ciation of added parts, already de- 

 scribed. In Man, each anterior pillar 

 of the fornix, after leaving the ( tha- 

 lamus,' descends and is bent upon itself 

 before ascending, the bend projecting 

 at the base of the brain, behind the 

 ( infundibulum,' as the ( corpus albi- 

 cans,' or 'mammillare,' fig. 128, m. 



In the Lissencephala, where a corpus callosum is first esta- 

 blished, it might seem, in a dissection from below, that the outer 

 fibres of the ( radiating cone ' curved over the lateral ventricle, 

 and were constricted lengthwise as they ran into each other 

 across the interhemispheral fissure, as in the dissection of the 

 Beaver's brain, fig. 78 : but it is deceptive. There is no actual 

 continuity of any of the ascending radiating fibres of the crus 

 cerebri with those which spread out in transverse curves from the 

 corpus callosum. The two systems are everywhere closely inter- 

 laced ; but the fibrous character of the commissural series is lost, 



Section of grey and white neurine of 

 prosencephala convolutions. Man. 



mammalian brains was summarised by the Eeporter for the ' Medical Times,' as fol- 

 lows : — 'A symmetrical arrangement, more or less regular or complex, can always be 

 traced between the foldings of the two hemispheres, and the more regular in propor- 

 tion to the simplicity of the convolutions : the foldings of the cerebral substance 

 iollow likewise, both in the embryonic development of a complex brain, and in the 

 progressive permanent stages presented by the mammalian series, a regular determi- 

 nate law: some convolutions being more constant than others, and these being trace- 

 able through the greatest number of brains, and recognisable even in the human 

 brain, where, at first sight, they are obscured by so many accessory convolutions.' 

 ' The Lecturer then demonstrated, in a considerable number of prepared brains of 

 different animals, and in a large series of diagrams, in which the corresponding con- 

 volutions in thu brains of different animals were marked by the same colours, the 

 facts establishing this important generalisation.' — The Medical Times, Nov. 12, 1842, 

 vol. vii. p. 101. Report of 13th Lecture, delivered May 16th, 1842. 



