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Its will is the agent, and its purpose is the action done. The second type assumes that 'ideas' struggling with one another are the agents, and that the prevalence of one set of them is the action. The third type believes that nerve-cells are the agents, and that resultant motor discharges are the acts achieved.
His theory that true character depends upon what he calls the reflex action of the nerve-cells; that acts of reason or conscience which have been put forth so many times that, in a sense, they perform themselves without any exercise of consciousness, are the best; that a man is an instinctive thief or liar, or a born poet, because the proper nervous structure has been fixed in his constitution by his ancestors; that any moral act, so long as it is conscious, is not ingrained in character, and the more conscious it is, the more dubious it is; and that "virtue itself is not safely lodged until it has become a habit" in other words, till it has become an automatic and unconscious operation of the nerve-cells, such a doctrine, in its extreme logical results, destroys all voluntary and conscious loyalty to principle, and renders man a mere automatic machine.
Now, so far as we can make out, the histological structure of our auditory centre is the same as that of our visual centre. The structure of the fibres and cells varies slightly in the motor and sensory regions, but no means have yet been discovered of perceiving a settled difference between the nerve-cells of the optic centre and those of the auditory centre.
The heart has its own little brains, so to speak, small collections of nervous substance which govern its rhythmical motions under ordinary conditions. But these lesser nervous centres are to a large extent dominated by influences transmitted from certain groups of nerve-cells in the brain and its immediate dependencies.
All the parts of the respiratory tract are supplied with nerves, which are of both kinds those which carry nervous impulses or messages from and those which convey them to the nervous centres concerned; in other words, to and from the bodies of the nerve-cells whose extensions are termed nerves.
It is quite inexplicable if consciousness is a function of the extended and divisible physical substratum which is built up of nerve-cells and nerve-fibres. And yet this unity is the fundamental condition of our whole inner life. Even the facts of association demonstrate it.
An earthworm avoids the light of day and digs its burrow and seeks its food by wonderfully coördinated activities of its muscles and other parts, which are controlled by a double chain of ganglia along its ventral side, connected with a similar pair of grouped nerve-cells above the anterior part of the digestive tract.
"There are two among the special groups of nerve-cells which produce directly opposite effects. One of these has the power of accelerating the action of the heart, while the other has the power of retarding or arresting this action. One acts as the spur, the other as the bridle. According as one or the other predominates, the action of the heart will be stimulated or restrained.
The second property of nerve-cells which is important in study is conductivity. As soon as a neurone is stimulated at one end, it communicates its excitement, by means of the nervous current, to the next neurone or to neighboring neurones. Just as an electric current might pass along one wire, thence to another, and along it to a third, so the nervous current passes from neurone to neurone.
Although to the naked eye the nervous system is apparently made up of a number of different kinds of material, still we see, when we turn our microscopes upon it, that its parts are structurally the same. Reduced to lowest terms, the nervous system is found to be composed of minute units of structure called nerve-cells or neurones.
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