[Illustration: 10. CROSS-SECTION OF AXIAL SKELETON OF PETROMYZON. HERTWIG, FROM HIEDERSHEIM. _SS_, skeletogenous layer; _Ob_, _Ub_, dorsal and ventral processes of _SS_; _C_, notochord; _Cs_, sheath of notochord; _Ee_, elastic external layer of sheath; _F_, fatty tissue; _M_, spinal marrow; _P_, sheath of _M_.] The nervous system on the dorsal surface of the notochord consists of the brain in the head and the spinal marrow running down the back. The brain of all except the very lowest vertebrates consists of four portions: 1. The cerebrum, or cerebral lobes, or simply "forebrain," the seat of consciousness, thought, and will, and from which no nerves proceed. Whether the primitive vertebrate had any cerebrum is still uncertain. 2. The mid-brain, which sends nerves to the eyes, and in this respect reminds us of the brain of insects. Its anterior portion appears from embryology to be very primitive. 3. The small brain, or cerebellum, which in all higher forms is the centre for co-ordination of the motions of the body. 4. The medulla, which controls especially the internal organs. The spinal marrow, or that portion of the nervous system which lies outside of the head, is at the same time a great nerve-trunk and a centre for reflex action of the muscles of the body. But the development of these distinct portions and the division of labor between them must have been a long and gradual process. We have every reason to believe that here, as in insects, the head has been formed by annexation of segments from the rump and the fusion of their nervous matter with that of the brain. But here, instead of only three segments, from nine to fourteen have been fused in the head to furnish the material for the brain. Notochord and backbone may be the most striking and apparent characteristic of vertebrates, but their predominant characteristic is brain. On this system they lavished material, giving it from three to four times as much as any lower or earlier group had done. They very early set apart the cerebral lobes to be the commander-in-chief and centre of control for all other nerve-centres. To this all report, and from it all directly or indirectly receive orders. It can say to every other organ in the body, "Starve that I may live." It is the seat of thought and will. The other portions of the brain report to it what they have gathered of vision or sound; it explains the vision or song or parable. It is relieved as far as possible from all lower and routine work that it may think and remember and govern. The vertebrate built for mind, not neglecting the body.