Regarded as the great central organ, the brain is divided into two halves or hemispheres, from each one of which goes forth supply of nerve fibres and nerve energy for the opposite side of the body. Its greatest depth is in the central part, the front and back being rounded down, the frontal region being, however, considerably more massive than the rear. Besides this great central body, there are several dependent subordinate bodies, placed underneath, and directly above the upper part of the spine. Most important of these is the _cerebellum_, or little brain, whose functions are now generally believed to be closely connected with the equilibrium of the body when moving. Somewhat nearer the centre, and quite under the brain proper is the _pons_ or bridge, providing for the interlacing of the fibres on their way out from the the central organ, and just below that are certain elongated bodies (_medulla oblongata_), consisting of masses of fibre just above the spinal cord. Before closing this very brief and hasty description of the nerve system, there is one peculiarly striking arrangement to which special reference may be made. The mass of nerve fibre which passes down within the back-bone constituting the spinal column, which is formed in two divisions equivalent to the hemispheres of the brain, gives out at each of the vertebræ or spinal joints a supply of nerve for the portion of the body contiguous. This supply is sent out from each side of the column, and issues in two roots, a posterior and anterior; the posterior root being a body of sensory nerves, the anterior root of motor nerves. Shortly after passing out, these two form into one, uniting to constitute a nerve trunk. Just after they have thus united, the trunk again opens up into two, and in each one of these two a share of the sensory and motor roots finds a place, and thus preparation is made for sending out towards both the front and back of the body suitable proportion of both sets of nerves. The two roots drawn together as if to bind them into one, are by some inexplicable process subdivided, and the two bands issuing from the united band are found to have each a share of the contents of each root. Of all the singular occurrences coming under scientific observation there is nothing more surprising. The fact is certain, but there is no scientific explanation of the contrivance by which such a singular result is secured. [Illustration: BRAIN OF THE CRAB.] [Illustration: BRAIN OF THE COD, THE TWO LARGER LOBES BEING THOSE OF VISION, THE BRAIN BEING IN FRONT OF THESE.]