5. The gray, cellular substance constitutes the larger portion of those important masses, which bear the name of _nervous centres_ and _ganglia_ (from _ganglion_, a knot), and in which all the nerve-fibres unite. These white nerve-fibres are found combined together in long and dense cords, called _nerves_ (from _neuron_, a cord), which serve to connect the nervous centres with each other, and to place them in communication with all the other parts of the body which have sensibility or power of motion. That part of the nervous system which is concerned in the animal functions, comprises the brain, the spinal cord, and the nerves which are derived therefrom; these are, together, called the _cerebro-spinal_ system (Fig. 40); while that other set of organs, which presides over, and regulates the vegetative functions, is called the sympathetic system of nerves. [Sidenote: 6. Location of the brain? Its weight? Its shape? Of what it consists? What organs at the base?] 6. THE BRAIN.--The brain is the great volume of nervous tissue that is lodged within the skull. It is the largest and most complex of the nervous centres, its weight, in the adult, being about fifty ounces, or one-fortieth of that of the whole body. The shape of the brain is oval, or egg-shaped, with one extremity larger than the other, which is placed posteriorly in the skull, to the concavity of which it very closely conforms. The brain consists chiefly of two parts; the _cerebrum_, or brain proper, and the _cerebellum_, or "little brain." In addition to these, there are several smaller organs at the base, among which is the commencement or expansion of the spinal cord, termed the _medulla oblongata_, or oblong marrow. {151} [Illustration: FIG. 40.--THE CEREBRO-SPINAL SYSTEM.] {152} [Sidenote: 7. The tissue of the brain? What, therefore, is required? Blows on the head? Membranes of the brain? Blood sent to the brain?] 7. The tissue of the brain is soft and easily altered in shape by pressure; it therefore requires to be placed in a well-protected position, such as is afforded by the skull, or _cranium_, which is strong without being cumbrous. In the course of an ordinary lifetime, this bony box sustains many blows, with little inconvenience; while, if they fell directly upon the brain, they would at once, and completely, disorganize that structure. Within the skull, the brain is enveloped by certain membranes, which at once protect it from friction, and furnish it with a supply of nutrient vessels; they are called the _arachnoid_, or "spider's web," the _dura mater_ and the _pia mater_, or the "tough" and "delicate coverings." The supply of blood sent to the brain is very liberal, amounting to one-fifth of all that the entire body possesses. The brain of man is heavier than that of any other animal, except the elephant and whale. [Sidenote: 8. Size of the brain proper? How divided? The exterior of the hemispheres? The interior?]