Pain is due to abnormal action of sensory nerves, caused by disease, injury, or pressure, and the irritation made, being carried to the brain makes us conscious of the peculiar sensation we call pain. So the want of food or water makes an impression upon nerves, which being carried to the brain causes a peculiar sensation, and we say we feel hungry or thirsty.
The Motor Nerves arise in the cells of the brain and cord. Those which go to the voluntary muscles cause them to contract, and are under the control of the will. If the cells are diseased, if they do not get enough arterial blood, or are poisoned by carbonic acid, or if the nerves are diseased, injured, or cut, so that nerve impulses cannot be sent from the brain to the muscles, we have paralysis of a muscle or a group of muscles, according to the extent of the injury. Now we can appreciate the force of this teaching in the physiology of the muscular system, that “paralysis is a loss of power, either partial or complete, to contract muscles, due to disease of the nerves.”
By the ready action of our mind, the quick working of our will, we direct and control the action of our muscles, so as to perform with the utmost skill and ease the varied and innumerable movements of our body.
It seems very easy to do this, but watch a child learning to walk; it is educating its mind and will to control the muscles, and it is a slow and difficult education.
But all motor impulses and bodily activities are not under the control of the will. The heart is supplied with motor nerves, but we cannot by our will stop its beating or control its action. The taking of food makes a mental impression, and without the will being involved, impulses are sent to the glands of the mouth, setting them actively at work, and saliva flows. So the stomach begins to churn food when it is introduced, and the liver is kept at work making bile and sugar, and we breathe when we are asleep.
All the organs of the body are supplied with motor nerves, that regulate their action and give them the power to do their function or work, but with the exception of the muscles, this power is sent without the action of the will.
Our brains are very busy. While we are awake we are constantly receiving sensations, we are thinking, remembering, willing, and sending many messages every minute, and directing power to all parts of the body. The brain works and gets tired, just as the rest of the body gets tired, and, if abused, injured, or overworked, may become diseased as may any part. Its tissues wear out, are burned up, and require the same supply of material to repair them that any other part of the body requires. It needs then rest, good food, good blood, and plenty of oxygen.
No wonder some brains give out, and fail to do their work properly, and so cause insanity.