ELEMENTS OF PHYSIOLOGY 



cut, the cells will be almost too inactive to absorb and 

 carry on growth and repair; hence unless continually 

 under the influence of motor nerves the cells dwindle 

 away. When the cells are much used, the impulses sent 

 cause them to take in more nourishment and grow in size. 

 Thus a muscle increases in size and strength when much 

 used. You learned that during the action of a muscle, the 

 vasomotor nerves going to the arteries in the muscle cause 

 what change ? Will this also aid in growth ? When a 

 nerve is cut, the ends, if placed together, will grow again, 

 the parts supplied by it being paralyzed in the meantime. 



536. Disease of Nerves and Effect of Alcohol. Nerves 

 may become inflamed, and the disease is called neuritis, 

 just as inflammation of the tonsils is called tonsilitis, or of 

 the stomach is called gastritis. Neuritis of the sciatic 

 nerve is called sciatica ; it is a very painful disease. One 

 of the many dangers of using alcohol is that it may pro- 

 duce neuritis. Either slow, steady drinking or occasional 

 sprees may cause it. The disease gives no warning before 

 it comes, and may remain a long while. Rheumatism or 

 malaria may also cause it, but alcohol produces the disease 

 as often as all other causes combined. 



537. The Rate of Transmission of the nerve impulse in 

 white fibers is about one hundred feet per second. In the 

 gray fibers the impulse travels much slower, probably about 

 twenty feet per second. 



THE SPINAL CORD 



538. The various white nerve fibers, both motor and 

 sensory, of the entire body unite and form forty-three 

 pairs of larger nerves ; twelve of these pairs, called cranial 

 nerves y go to the brain, and thirty-one pairs, called spinal 

 nerves, go to the spinal cord. Each one of the sixty-two 

 spinal nerves enters the spinal cord by two roots, one pos- 

 terior and one anterior (Fig. 199). It is significant that 



