23. What do we learn from a comparison of brain weight and intelligence? 24. What is the relation between nervous pathology and mental abnormality? 25. Is phrenology admissible? 26. What view concerning the relation of brain and mind is suggested by the unity of mind? § 2. THE NERVOUS SYSTEM 1. _The Elements of the Nervous System_ [Illustration: FIG. 1.--MULTIPOLAR CELL BODY.] The number of elements making up the nervous system is estimated at about four thousand millions. It will help us to comprehend the significance of this number if we understand that a man’s life devoted to nothing but counting them would be too short to accomplish this task, for a hundred years contain little more than three thousand million seconds. These elements are stringlike bodies, so thin that they are invisible to the naked eye. They are generally called _neurons_. Within them different parts are to be distinguished. The part which is most important for the neuron’s life is a spherical, bobbin-shaped, pyramidal, or starlike body, called the ganglion cell or cell body, located usually near one of the ends of the long fiber of the neuron, but sometimes nearer the middle of the fiber. The length of the fiber varies from a fraction of an inch to several feet. The fiber may be compared with a telephone wire, inasmuch as its function consists in carrying a peculiar kind of excitatory process. [Illustration: FIG. 2--PYRAMIDAL CELL BODY. _a_, Nerve fiber with collaterals.] [Illustration: Fig. 3.--DENDRITES OF A NERVE CELL OF THE CEREBELLUM.] At both ends of the neuron are usually found treelike branches. When the cell body is located near one of the ends of the fiber, many of these branches take their origin from the cell body and give it the pyramidal or starlike appearance illustrated by figures 1, 2, and 4. These branches are called dendrites, from the Greek word for tree, _dendron_. How wonderfully complicated the branching of a neuron may be is illustrated by figure 3. In addition to the dendrites a neuron possesses another kind of branches, resembling in character the tributaries of a large river, entering into it at any point of its course. These are called collaterals (lowest part of figure 2). [Illustration: FIG. 4.--VARIOUS TYPES OF CELL BODIES. _1_ and _2_, Giant pyramidal cell bodies; _n_, nerve fiber.] [Illustration: FIG. 5.--LONGITUDINAL SECTION OF A NERVE FIBER WITH STAINED FIBRILS. _a_, Medullated sheath.]