2?2 APPLIED PHYSIOLOGY 



Motor impulses are of three kinds, for motion, for 

 secretion, and for growth. 



491. Impulses producing motion. The action of every 

 muscle cell depends upon an impulse brought from the 

 central nerve cells by its motor nerve. When these influ- 

 ences are cut off, there is paralysis of the part, so that no 

 amount of willful effort can cause the muscles to move the 

 limb. The peristalsis of the intestine and the beating of 

 the heart are caused by influences brought to their muscle 

 cells by motor nerves. Orders for movements of which 

 we know nothing are far greater in amount than those 

 sent to voluntary muscles. 



492. Impulses producing secretion. Secretion is also 

 dependent upon orders brought to the glands by motor 

 nerves. For example, when food is taken into the mouth, 

 the sensory nerves carry the news to the nerve cells, which 

 at once send out an order along the motor nerves to the 

 salivary glands to produce more saliva. If the nerves are 

 cut, only a little saliva will be produced, while if the end 

 in connection with the gland is irritated, the gland will 

 respond with a greater quantity of saliva. In the same 

 way the secretion of all glands is controlled. 



493. Influences producing growth. When the motor 

 nerve to a part is cut, the cells will be inactive, and, as it 

 were, too lazy even to eat. So, unless continually under 

 the influence of motor nerves, the cells become weak and 

 waste away. When the cells of a part are much used, 

 impulses are sent causing them to take in more nourish- 

 ment, so that they increase in size and strength. Thus a 

 muscle becomes larger and stronger by use. During the 

 action of a muscle its motor nerves also bring orders for' 

 the arteries to dilate and carry more blood to feed the 

 working part. 



