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They are called sensory neurones and their function is to carry messages inward to the brain. Thus, the brain represents, in great part, a central receiving station for impressions from the outside world. The nerve-cells carrying messages from the various parts of the body terminate in particular areas.
The nerve-cells in this group are called motor neurones and their function is to carry messages from the brain out to the muscles, for a muscle ordinarily does not act without a nervous current to set it off. So far we have seen that the brain has the two functions of receiving impressions from the sense-organs and of sending out orders to the muscles.
There is another large group of nerve-cells which, when traced out, are found to have one terminal in the brain and the other in the muscles throughout the body. The area in the brain, where these neurones emerge, is near the top of the brain in the area marked Motor on the diagram. From here the fibers travel down through the spinal cord and out to the muscles.
That there is a weakness of will power in the ticquer, with a lack of control or inhibition over the lower neurones normally regulated by the higher co-ordinating centres, so that certain automatic activities become dissociated and exist more or less independently, is generally acknowledged.
It is important to note that neurones never act singly; they always act in groups, the nervous current passing from neurone to neurone. It is thought that the most important changes in the nervous system do not occur within the individual neurones, but at the points where they join with each other.
The first factor, power of retention, is the most fundamental and to some extent limits the usefulness of the other two. It is determined by the character of the neurones and varies with different brains. Neurones which are easily impressed and retain their impression simply because they are so made are the gift of nature and the corner stone of a good memory.
The association neurones have the further function of connecting one sensory area in the brain with another. For example, when you see, smell, taste and touch an orange, the corresponding areas in the brain act in conjunction and are associated by means of the association neurones connecting them. The association neurones play a large part in the securing and organizing of knowledge.
It is a postulate of contemporary physiology that all the neurones taken together cannot spontaneously, that is, of themselves, give rise to any movement they receive from without, and expend their energy outwards. Nevertheless, between the two moments that, in reflex and instinctive actions, seem continuous, a third interposes, which, for the higher psychic acts, may be of long duration.
Each of these looks like a string frayed out at both ends, with a bulge somewhere along its length. The nervous system is made up of millions of these little cells packed together in various combinations and distributed throughout the body. Some of the neurones are as long as three feet; others measure but a fraction of an inch in length.
Just what this connection is which is found between the nerve cells is still open to question. It may be chemical or it may be electrical. We know it is not a growing together of the neurones, but further than that nothing is definitely known. That there are very definite pathways of discharge developed by the laws of inner growth and independent of individual learning, there can be no doubt.
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