THE BRAIN.--Inasmuch as Professor Wiedersheim has said with perfect truth that "the brain of the extinct Ungulate _Dinoceras_ shows so striking a likeness to that of a lizard that one would be compelled to explain it as that of a lizard without a knowledge of the skeleton," it is clear that to define the mammalian brain is a difficult matter. The existing Mammalia, however, all possess brains which can be readily distinguished from those of vertebrates lying lower in the scale. They are of relatively large size, brought about mainly by the dimensions of the cerebral hemispheres, which have an importance in this class of vertebrates that they have not elsewhere. Coupled with this large size of the hemispheres is a more elaborate system of transverse commissures uniting the two; and this culminates in the higher Mammalia, where the corpus callosum attains a large size and great physiological importance. A {76} very marked feature, moreover, of the mammal's brain is the development of regular fissures upon its surface, which fissures are only absent from _Ornithorhynchus_, various small Rodents, Bats, and Insectivores, among living mammals. It is sometimes, but erroneously, said that the more complicated the fissures of the brain are, the higher in intelligence and "zoological position" is the possessor of that brain. Instances can undoubtedly be quoted to support such a view; but they are {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 Cat. 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. [Illustration] FIG. 50.--_Lepus cuniculus._ Longitudinal vertical section of the brain. (Nat. size.) _a.co_, Anterior commissure; _b.fo_, body of the fornix; _cb_, cerebellum, showing arbor vitae; _c.c_, crus cerebri; _c.h_^1, parencephalon or cerebral hemisphere; _c.h_^2, temporal lobe; _c.ma_, corpus mammillare; _cp.cl_, corpus callosum; _f.m_, foramen of Monro; _inf_, infundibulum; _l.t_, lamina terminalis; _ly_, lyra; _m.co_, middle commissure; _m.o_, medulla oblongata; _o.ch_, optic chiasma; _o.l_^1, _o.l_^2, corpora quadrigemina or optic lobes; _olf_, olfactory lobe; _p.co_, posterior commissure; _pd.pn_, peduncle of the pineal "gland," _pn_; _p.fo_, anterior pillar of the fornix; _pty_, pituitary body; _pv.a_, pons Varolii; _sp.lu_, septum lucidum; _v_^4, fourth ventricle; _vl.ip_, velum interpositum; _v.vn_, valve of Vicussens; _II_, optic nerve. (From Parker's _Zootomy_.)