THE NERVOUS SYSTEM. 37 



involuntary muscles which perform the work of the 

 heart, lungs, stomach and other vital organs, except 

 in cases of emergency. For example, when food 

 comes into the stomach, certain movements of the 

 walls of that organ are necessary. So the food makes 

 an impression on the nerves which report its pres- 

 ence, not to the brain or to the mind, as a sensation, 

 but to the origin center of those nerves, in the spinal 

 cord. Here the cord exercises its authority and 

 returns (reflects) an order over motor nerves to the 

 muscles of the stomach to perform the needed 

 service. 



In the same way, the presence of impure air or 

 the absence of air in the lungs causes impressions 

 which are carried to the cord, which returns orders 

 for the action of the breathing-out or breathing-in 

 muscles, as the case may be. All these performances 

 go on steadily, whether we are awake or asleep. 

 But when an emergency arises, as, for instance, if 

 the muscles of the chest are strongly resisted in their 

 efforts to expand it, by outside compression, the 

 news of the trouble is carried beyond the nerve cen- 

 ters of the cord up to the brain, where the mind 

 quickly grasps the situation and promptly issues 

 orders for the best possible measures of relief. A 

 familiar illustration of reflex action is found in the 

 flapping of a fowl whose head has been cut off. Its 

 muscles which produce its violent motions are not in 

 connection with the brain, and can not be controlled 

 by it. Each fall to the ground produces an impres- 



