Finally, within the spaces of the arachnoid is a lymph-like liquid which completely envelops the brain and the cord, and which, by serving as a watery cushion, protects them from jars and shocks. Thus the brain and cord are directly shielded by bones, by membranes, and by the [pg 300]liquid which surrounds them. They are also protected from jars resulting from the movements of the body by the general elasticity of the skeleton.
Summary.—The nervous system establishes connections between all parts of the body, and provides a stimulus by means of which they are controlled. It is made up of a special form of cells, called neurons. The neurons form the different divisions of the nervous system, and also serve as the active agents in carrying on its work. Through a side-by-side method of joining they form the nerves, ganglia, spinal cord, and brain; and by a method of end-to-end joining they connect places remote from each other, and provide for nervous movements through the body. The nervous system, may in some respects be compared to a complicated system of telephony, in which the chains of neurons correspond to the wires, and the brain and spinal cord to the central station.
Exercises.—1. Give the meaning of the term "coördination." Supply illustrations.
2. What two general conditions are supplied in the body by the nervous system?
3. Compare the skeleton outline of the nervous system with the bony skeleton.
4. Sketch outlines of mon-axonic and di-axonic neurons.
5. Give two differences between the neurons and the other cells of the body.
6. Describe the two general methods of connecting neurons in the body. What purpose is accomplished by each method?
7. Name and locate the principal divisions of the nervous system.
8. Draw an outline of the brain (side view), locating each of its principal divisions.