Ol'TIC LOBES 



77 



merely selected cases, which do not indicate a wide applicability 

 of such a generalisation. Thus it is true that the brain of a Man 

 is more elaborate in its furrows and convolutions than is that of 

 a ( *at. The real fact of the matter is, that the complexity of the 

 brain from this point of view increases with the size of the animal 

 within the group. 



The Gorilla and the Chimpanzee have a more furrowed brain 

 than has the little Marmoset ; the Bear a more complicated 

 brain than the Weasel, etc. The most highly-convoluted brains 

 of all mammals are those of the Elephants, and there does not 



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Fig. 50. — Leptts cnaiculv.'i. Lougitudinal vertical section of the braiu. (Nat. size.) 

 a.co. Anterior commissure ; b.fo, body of the fornix ; ch, cerebelhmi, showing arbor 

 vitae ; c.c, erus cerebri ; c.h^, parencephalon or cerebral hemisphere ; cJi^, temporal 

 lobe ; c.ma, corpus niammillare ; cp.d, corpus callosum ; f.m, foramen of Monro ; 

 inf, infundibuluin ; l.t, lamina termiualis ; ly, lyra ; m.co, middle commissure ; 

 M.o, medulla oblongata ; o.ch, optic chiasma ; o./', o.P, corpora quadrigemina or 

 optic lobes ; olf, olfactory lobe ; jy.co, posterior commissure ; ^^(^.^^h, peduncle of the 

 pineal " gland," ^>i/ p.fo, anterior pillar of the fornix ; ptij, jiituitary body ; jn'-u, 

 pons Varolii ; sp.lu, septum lucidum ; v^, fourth ventricle ; vl.ip), velum interposi- 

 tum ; v.vn,. valve of Vieusseus ; //, optic nerve. (From Parker's Zootomy.) 



seem in the Ungulates to be so marked a relation between size 

 and abundance of fissures as there is among other mammals. 

 A regular plan of the fissures can be detected with certainty 

 for each group considered by itself; but it is not so easy to 

 homologise the details of arrangement from group to group. 

 This is so far in accord with the view that the existing groups 

 of mammals have diverged from each other cib initio. 



Another marked characteristic of the mammalian as opposed 

 to other brains is the relatively small importance in size and yet 

 the fourfold nature of the optic lobes. What was the case with 

 the optic lobes of the early Ungulates is difficult to understand, 

 on account of the fact that the casts are necessarily imperfect. 



