*Nature of the Nervous System.*—The nervous system represents the sum total of the neurons in the body. In some respects it may be compared to the modern telephone system. The neurons, like the electric wires, connect different places with a central station (the brain and spinal cord), and through the central station connections are established between the different places in the system. As the separate wires are massed together to form cables, the neurons are massed to form the gross structures of the nervous system. The nervous system, however, is so radically different from anything found outside of the animal body that no comparison can give an adequate idea of it. We now pass to a study of the gross structures observed in the nerve skeleton. *Divisions of the Nervous System.*—While all of the nervous structures are very closely blended, forming one complete system for the entire body, this system presents different divisions which may, for convenience, be studied separately. As physiologists have become better acquainted with the human nervous system, different schemes of classification have been proposed. The following outline, based upon the location of the different parts, presents perhaps the simplest view of the entire group of nervous structures: [Table] *The Central Division.*—This division of the nervous system lies within the cranial and spinal cavities, and consists of the brain and the spinal cord. The brain occupying the cranial cavity and the spinal cord in the spinal cavity connect with each other through the large opening at the base of the skull to form one continuous structure. The brain and cord are the most complicated portions of the nervous system, and the ones most difficult to understand. [Fig. 130] Fig. 130—*Diagram of divisions of brain.* *The Brain.*—The brain, which is the largest mass of nervous tissue in the body, weighs in the average sized man about 50 ounces, and in the average sized woman about 44 ounces.(98) It may be roughly divided into three parts, which are named from their positions (in lower animals) the forebrain, the midbrain, and the hindbrain (Fig. 130). The forebrain consists almost entirely of a single part, known as