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The nerve is seen to be a bundle of axons, or nerve fibers, held together by connective tissue, while the ganglion is little more than a cluster of cell-bodies. *End-to-end Connections.*—These consist of loose end-to-end unions of the fiber branches of certain neurons with the dendrites of other neurons.
*Reflex Action in the Circulation of the Blood.*—On sudden exposure to cold, the small arteries going to the skin quickly diminish in size, check the flow of blood to the surface, and prevent too great a loss of heat. In this case, impulses starting at the surface of the body are transmitted to the bulb and then through the efferent neurons to the muscles in the walls of the arteries.
State the purpose of the nervous impulse. Show that the exciting cause of bodily action is outside of the nervous system and, to a large extent, outside of the body. Describe the arrangement that enables stimuli outside of the body to cause action within the body. Describe a reflex action and show how it is brought about. Distinguish between afferent, efferent, and intermediate neurons.
*Sight and Hearing.*—The sense organs of sight and hearing are highly complicated structures, and will be considered in the chapters following. *Summary.*—Sensations are certain activities of the mind that result from excitations within the body or at its surface. These cause the neurons to discharge impulses which on reaching the cerebrum cause the sensations.
*Arrangement of the Neurons of the Brain and Cord.*—The cell-bodies in the brain and spinal cord are collected into groups, and their fibers extend from these groups to places that may be near or remote.
Or we might fancy that by extremely subtle machinery the resistance is increased in those tissues which lie between the various neurons, or we might even think of toxic and antitoxic processes in the cerebral regions; and any day may open entirely new ways of explanation.
The substance of both the spinal cord and the brain is made up of millions of delicate, tiny cells, called neurons, most of which, with very long branches, are arranged in chains for carrying messages, forming the white matter; while the others lie in groups, or ganglia, for sorting and deciding upon messages, forming the gray matter.
In fact, the olfactory cells resemble closely the cell-bodies of neurons, and are thought to be such. *The Olfactory Stimulus.*—Only substances in the gaseous state can be smelled. From this it is inferred that the stimulus is supplied by gas particles. Solids and liquids are smelled because of the gas particles which separate from them.
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. What two general conditions are supplied in the body by the nervous system?
Pieces of cord of different colors and lengths are knotted to represent mon-axonic and di-axonic neurons. These are then pinned or tacked to the board in such a manner as to represent the connections in the different kinds of nerve pathways. *Study of the "Knee Jerk" Reflex.*—A boy is seated on a chair with the legs crossed.
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