. The human body and health : an elementary text-book of essential anatomy, applied physiology and practical hygiene for schools . acts in which the braincan have no part. Reflex action causes the hand stuck with a pin to bewithdrawn before the brain can act. The stimulus travelsup the nerve fibers of the arm and through the* sensorycells of the dorsal root, then out their processes to passover to the motor cells in the spinal cord. These cells arethus aroused to send the message to the muscles to con-tract. A twentieth of a second later the brain is awareof the pin, because the sensory fibers
. The human body and health : an elementary text-book of essential anatomy, applied physiology and practical hygiene for schools . acts in which the braincan have no part. Reflex action causes the hand stuck with a pin to bewithdrawn before the brain can act. The stimulus travelsup the nerve fibers of the arm and through the* sensorycells of the dorsal root, then out their processes to passover to the motor cells in the spinal cord. These cells arethus aroused to send the message to the muscles to con-tract. A twentieth of a second later the brain is awareof the pin, because the sensory fibers, upon entering thecord, split and send branches also up toward the brain(Fig. 153). The Use of the Sympathetic System.—The working of thissystem consists of a series of reflex actions. These nervescontrol the involuntary muscle in all parts of the help govern the movements of the intestines andthe heart beat, and determine the amount of blood sup-plied to various parts of the body by contracting orloosening the muscle in the walls of the small system regulates the activity of the sweat glands brain. po Fig. 153. — Diagram of reflex action. — Arrows sliow which way the impulseis traveling; n, spinal nerves. When the finger is pricked by a tack t, thenerve ending p receives the pain impulse and it is carried along the fibera through the sensory root around into the spinal cord. Here the impulseis given to the cell »i, and its fiber then conveys the impulse down the armto the muscle causing the finger to be withdrawn. The sensory fiberpassing in atpo splits inside of the cord, and one branch takes the messageto the brain. Here it passes to the motor cells, which may order thefinger held still or withdrawn. 230 HOW THE NERVOUS SYSTEM WOKKS and influences the secretion of the salivary glands, andcauses the flow of tears. Paralysis and Apoplexy. — Paralysis is inability of thenerves to act because of injury to the cell bodies or theprocesses. The bre
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