The presence or absence of “sulci” or fissures on the surface of the hemisphere, dividing it into “convolutions” or “gyri,” and thus increasing the superficies of the cortical gray matter, as well as allowing the pia mater with its nutrient blood-vessels to penetrate into the cerebral substance, follow somewhat similar rules. The sulci are related partly to the high or low condition of organisation of the species, but also in a great degree to the size of the cerebral hemispheres. In very small species of all groups, even the Primates, they are absent, and in the largest species of groups so low in the scale as the Marsupials and Edentates they are found. They reach their maximum of development in the Cetacea. The accompanying woodcut (Fig. 23) shows the principal parts of a mammalian brain, as seen from the superior, lateral, and inner surfaces. The sylvian fissure (_sf_) is one of the most constant of the sulci found in the hemispheres. [Illustration: FIG. 23.—Brain of the Genet (_Genetta tigrina_). A, From above; B, from the right side; C, inner surface of right hemisphere; _cc_, corpus callosum; _c.m.s_, calloso-marginal sulcus; _c_, notch representing central sulcus of other forms; _d_, depression on superior lateral gyrus of hemisphere; _hg_, hippocampal gyrus; _i_, inferior lateral gyrus of hemisphere; _m_, middle lateral gyrus of do.; _s_, superior lateral gyrus of do.; _os_, supraorbital sulcus of do.; _sf_, sylvian fissure of do.; _ol_, olfactory lobes. The deeply convoluted part behind the cerebral hemisphere is the cerebellum, below which lies the medulla oblongata, or commencement of the spinal cord. (Mivart, _Proc. Zool. Soc._ 1882, p. 516.)] The researches of Palæontologists, founded upon studies of casts of the interior of the cranial cavity of extinct forms, have shown that, in many natural groups of mammals, if not in all, the brain has increased in size, and also in complexity of surface foldings, with the advance of time,—indicating in this, as in so many other respects, a gradual progress from a lower to a higher type of development. _Nerves._—The twelve pairs of cranial nerves generally recognised in vertebrates are usually all found in mammals, though the olfactory nerves are excessively rudimentary, if not altogether absent, in the Toothed Whales. The spinal cord, or continuation of the central nervous axis, lies in the canal formed by the neural arches of the vertebræ, and gives off the compound double-rooted nerves of the trunk and the extremities, corresponding in number to the vertebræ, through the interspaces between which they pass out to their destination. The cord is somewhat enlarged at the two points where it gives off the great nerves to the anterior and the posterior extremities, which, from their interlacements soon after their origin, are called respectively the brachial and lumbar plexuses. The ganglionic or sympathetic portion of the nervous system is well developed, and presents few modifications.