Page:Structure and functions of the body; a hand-book of anatomy and physiology for nurses and others desiring a practical knowledge of the subject (IA structurefunctio00fiskrich).pdf/89

 The action of the nerves is similar to reflex action, only that an effort of will is needed to send an impulse from the brain. It is by the help of the brain along this line that an infinity of artificial reflexes or habits is acquired, for which volition is needed in the beginning but which are later done unconsciously. Herein lie the possibilities of all education.

The brain and spinal cord work together, the cord acting as a medium between the brain, in which all the higher psychical processes, such as will, thought, etc., originate, and the muscular apparatus. The cord, however, has some action entirely independent of the brain, as is seen in reflex action. This action is entirely involuntary, so that the cord is sometimes spoken of as the seat of involuntary action, commonly called reflex action. All unconscious acts are reflex acts, as when the hand is drawn away from a hot iron. If an impulse is sent along one of the sensory fibers, it enters the cord through the posterior horn, where its nerve cell is found. Then, through some connection between the nerve cell of the sensory fiber and that of the motor fiber the impulse is transmitted to the motor cell and another impulse is sent out of the cord along the motor fiber of the nerve to the muscle. One of the commonest reflexes is the knee-jerk. Reflex action is important because the reflexes are interfered with, delayed, destroyed, or increased in different diseases. The time normally required for a reflex act is very brief, that for the knee-jerk being about three one-hundredths of a second.

The nerves of the head, known as the cranial nerves, arise from the brain, while the rest of the body is supplied by the spinal nerves, which come off at intervals from the spinal cord. The cranial nerves consist of twelve pairs: (1) The olfactory or nerve of smell, (2) the optic or nerve of sight, (3) the motor oculi, (4) the patheticus, which controls the eye, (5) the trigeminus or trifacial, a nerve of general sensation, motion, and taste, (6) the abducens, a motor nerve, (7) the facial